[33]The change of colour is dependent in all probability on the combination with water, or according to others on polymeric transformation. It enables a solution of cobalt chloride to be used as sympathetic ink. If something be written with cobalt chloride on white paper, it will be invisible on account of the feeble colour of the solution, and when dry nothing can be distinguished. If, however, the paper be heated before the fire, the rose-coloured salt will be changed into a less hydrous blue salt, and the writing will become quite visible, but fade away when cool.The change of colour which takes place in solutions of CoCl2under the influence not only of solution in water or alcohol, but also of a change of temperature, is a characteristic of all the halogen salts of cobalt. Crystalline iodide of cobalt, CoI26H2O, gives a dark red solution between -22° and +20°; above +20° the solution turns brown and passes from olive to green, from +35° to 320° the solution remains green. According to Étard the change of colour is due to the fact that at first the solution contains the hydrate CoI2H2O, and that above 35° it contains CoI24H2O. These hydrates can be crystallised from the solutions; the former at ordinary temperature and the latter on heating the solution. The intermediate olive colour of the solutions corresponds to the incipient decomposition of the hexahydrated salt and its passage into CoI24H2O. A solution of the hexahydrated chloride of cobalt, CoCl26H2O, is rose-coloured between -22° and +25°; but the colour changes starting from +25°, and passes through all the tints between red and blue right up to 50°; a true blue solution is only obtained at 55° and remains up to 300°. This true blue solution contains another hydrate, CoCl22H2O.The dependence between the solubility of the iodide and chloride of cobalt and the temperature is expressed by two almost straight lines corresponding to the hexa- and di-hydrates; the passage of the one into the other hydrate being expressed by a curve. The same character of phenomena is seen also in the variation of the vapour tension of solutions of chloride of cobalt with the temperature. We have repeatedly seen that aqueous solutions (for instance, Chapter XXII., Note23for Fe2Cl6) deposit different crystallo-hydrates at different temperatures, and that the amount of water in the hydrate decreases as the temperaturetrises, so that it is not surprising that CoCl22H2O (or according to Potilitzin CoCl2H2O) should separate out above 55° and CoCl26H2O at 25° and below. Nor is it exceptional that the colour of a salt varies according as it contains different amounts of H2O. But in this instance it is characteristic that the change of colour takes place in solution in the presence of an excess of water. This apparently shows that the actual solution may contain either CoCl26H2O or CoCl22H2O. And as we know that a solution may contain both metaphosphoric PHO3and orthophosphoric acid H3PO4= HPO3+ H2O, as well as certain other anhydrides, the question of the state of substances in solutions becomes still more complicated.Nickel sulphate crystallises from neutral solutions at a temperature of from 15° to 20° inrhombiccrystals containing 7H2O. Its form approaches very closely to that of the salts of zinc and magnesium. The planes of a vertical prism for magnesium salts are inclined at an angle of 90° 30′, for zinc salts at an angle of 91° 7′, and for nickel salts at an angle of 91° 10′. Such is also the form of the zinc and magnesium selenates and chromates. Cobalt sulphate containing 7 molecules of water is deposited in crystals of themonoclinicsystem, like the corresponding salts of iron and manganese. The angle of a vertical prism for the iron salt = 82° 20′, for cobalt = 82° 22′, and the inclination of the horizontal pinacoid to the vertical prism for the iron salt = 99° 2′, and for the cobalt salt 99° 36′. All the isomorphous mixtures of the salts of magnesium, iron, cobalt, nickel and manganese have the same form if they contain 7 mol. H2O and iron or cobalt predominate, whilst if there is a preponderance of magnesium, zinc, or nickel, the crystals have a rhombic form like magnesium sulphate. Hence these sulphates aredimorphous, but for some the one form is more stable and for others the other. Brooke, Moss, Mitscherlich, Rammelsberg, and Marignac have explained these relations. Brooke and Mitscherlich also supposed that NiSO4,7H2O is not only capable of assuming these forms, but also that of thetetragonalsystem, because it is deposited in this form from acid, and especially from slightly-heated solutions (30° to 40°). But Marignac demonstrated that the tetragonal crystals do not contain 7, but 6, molecules of water, NiSO4,6H2O. He also observed that a solution evaporated at 50° to 70° deposits monoclinic crystals, but of a different form from ferrous sulphate, FeSO4,7H2O—namely, the angle of the prism is 71° 52′, that of the pinacoid 95° 6′. This salt appears to be the same with 6 molecules of water as the tetragonal. Marignac also obtained magnesium and zinc salts with 6 molecules of water by evaporating their solutions at a higher temperature, and these salts were found to be isomorphous with the monoclinic nickel salt. In addition to this it must be observed that the rhombic crystals of nickel sulphate with 7H2O become turbid under the influence of heat and light, lose water, and change into the tetragonal salt. The monoclinic crystals in time also become turbid, and change their structure, so that the tetragonal form of this salt is the most stable. Let us also add that nickel sulphate in all its shapes forms very beautiful emerald green crystals, which, when heated to 230°, assume a dirty greenish-yellow hue and then contain one molecule of water.Klobb (1891) and Langlot and Lenoir obtained anhydrous CoSO4and NiSO4by igniting the hydrated salt with (NH4)2SO4until the ammonium salt had completely volatilised and decomposed.We may add that when equivalent aqueous solutions of NiX2(green) and CoX2(red) are mixed together they give an almost colourless (grey) solution, in which the green and red colour of the component parts disappears owing to the combination of the complementary colours.A double salt NiKF3is obtained by heating NiCl2with KFHF in a platinum crucible; KCoF3is formed in a similar manner. The nickel salt occurs in fine green plates, easily soluble in water but scarcely soluble in ethyl and methyl alcohol. They decompose into green oxide of nickel and potassium fluoride when heated in a current of air. The analogous salt of cobalt crystallises in crimson flakes.If instead of potassium fluoride, CoCl2or NiCl2be fused with ammonium fluoride, they also form double salts with the latter. This gives the possibility of obtaining anhydrous fluorides NiF2and CoF2. Crystalline fluoride of nickel, obtained by heating the amorphous powder formed by decomposing the double ammonium salt in a stream of hydrofluoric acid, occurs in beautiful green prisms, sp. gr. 4·63, which are insoluble in water, alcohol, and ether; sulphuric, hydrochloric, and nitric acids also have no action upon them, even when heated; NiF2is decomposed by steam, with the formation of black oxide, which retains the crystalline structure of the salt. Fluoride of cobalt, obtained as a rose-coloured powder by decomposing the double ammonium salt with the aid of heat in a stream of hydrofluoric acid, fuses into a ruby-coloured mass which bears distinct signs of a crystalline structure; sp. gr. 4·43. The molten salt only volatilises at about 1400°, which forms a clear distinction between CoF2and the volatile NiF2. Hydrochloric, sulphuric, and nitric acids act upon CoF2even in the cold, although slowly, while when heated the reaction proceeds rapidly (Poulenc, 1892).[34]Hydrated suboxide of cobalt (de Schulten, 1889) is obtained in the following manner. A solution of 10 grams of CoCl26H2O in 60 c.c. of water is heated in a flask with 250 grams of caustic potash and a stream of coal gas is passed through the solution. When heated the hydrate of the suboxide of cobalt which separates out, dissolves in the caustic potash and forms a dark blue solution. This solution is allowed to stand for 24 hours in an atmosphere of coal gas (in order to prevent oxidation). The crystalline mass which separates out has a composition Co(OH)2, and to the naked eye appears as a violet powder, which is seen to be crystalline under the microscope. The specific gravity of this hydrate is 3·597 at 15°. It does not undergo change in the air; warm acetic acid dissolves it, but it is insoluble in warm and cold solutions of ammonia and sal-ammoniac.[34 bis]The following reaction may be added to those of the cobaltous and nickelous salts: potassium cyanide forms a precipitate with cobalt salts which is soluble in an excess of the reagent and forms a green solution. On heating this and adding a certain quantity of acid, a doublecobalt cyanideis formed which corresponds with potassium ferricyanide. Its formation is accompanied with the evolution of hydrogen, and is founded upon the property which cobalt has of oxidising in an alkaline solution, the development of which has been observed in such a considerable measure in the cobaltamine salts. The process which goes on here may be expressed by the following equation; CoC2N2+ 4KCN first forms CoK4C6N6, which salt with water, H2O, forms potassium hydroxide, KHO, hydrogen, H, and the salt, K3CoC6N6. Here naturally the presence of the acid is indispensable in consequence of its being required to combine with the alkali. From aqueous solutions this salt crystallises in transparent, hexagonal prisms of a yellow colour, easily soluble in water. The reactions of double decomposition, and even the formation of the corresponding acid, are here completely the same as in the case of the ferricyanide. If a nickelous salt be treated in precisely the same manner as that just described for a salt of cobalt, decomposition will occur.[35]The cobalt salts may be divided into at least the following classes, which repeat themselves for Cr, Ir, Rh (we shall not stop to consider the latter, particularly as they closely resemble the cobalt salts):—(a)Ammonium cobalt salts, which are simply direct compounds of the cobaltous salts CoX2with ammonia, similar to various other compounds of the salts of silver, copper, and even calcium and magnesium, with ammonia. They are easily crystallised from an ammoniacal solution, and have a pink colour. Thus, for instance, when cobaltous chloride in solution is mixed with sufficient ammonia to redissolve the precipitate first formed, octahedral crystals are deposited which have a composition CoCl2,H2O,6NH3. These salts are nothing else but combinations with ammonia of crystallisation—if it may be so termed—likening them in this way to combinations with water of crystallisation. This similarity is evident both from their composition and from their capability of giving off ammonia at various temperatures. The most important point to observe is that all these salts contain 6 molecules of ammonia to 1 atom of cobalt, and this ammonia is held in fairly stable connection. Water decomposes these salts. (Nickel behaves similarly without forming other compounds corresponding to the true cobaltic.)(b) The solutions of the above-mentioned salts are rendered turbid by the action of the air; they absorb oxygen and become covered with a crust ofoxycobaltamine salts. The latter are sparingly soluble in aqueous ammonia, have a brown colour, and are characterised by the fact that with warm waterthey evolve oxygen, forming salts of the following category: The nitrate may be taken as an example of this kind of salt; its composition is CoN2O7,5NH3,H2O. It differs from cobaltous nitrate, Co(NO3)2, in containing an extra atom of oxygen—that is, it corresponds with cobalt dioxide, CoO2, in the same way that the first salts correspond with cobaltous oxide; they contain 5, and not 6, molecules of ammonia, as if NH3had been replaced by O, but we shall afterwards meet compounds containing either 5NH3or 6NH3to each atom of cobalt.(c)The luteocobaltic saltsare thus called because they have a yellow (luteus) colour. They are obtained from the salts of the first kind by submitting them in dilute solution to the action of the air; in this case salts of the second kind are not formed, because they are decomposed by an excess of water, with the evolution of oxygen and the formation of luteocobaltic salts. By the action of ammonia the salts of the fifth kind (roseocobaltic) are also converted into luteocobaltic salts. These last-named salts generally crystallise readily, and have a yellow colour; they are comparatively much more stable than the preceding ones, and even for a certain time resist the action of boiling water. Boiling aqueous potash liberates ammonia and precipitates hydrated cobaltic oxide, Co2O3,3H2O, from them. This shows that the luteocobaltic salts correspond with cobaltic oxide, Co2O3, and those of the second kind with the dioxide. When a solution of luteocobaltic sulphate, Co2(SO4)3,12NH3,4H2O, is treated with baryta, barium sulphate is precipitated, and the solution contains luteocobaltic hydroxide, Co(OH)3,6NH3, which is soluble in water, is powerfully alkaline, absorbs the oxygen of the air, and when heated is decomposed with the evolution of ammonia. This compound therefore corresponds to a solution of cobaltic hydroxide in ammonia. The luteocobaltic salts contain 2 atoms of cobalt and 12 molecules of ammonia—that is, 6NH3to each atom of cobalt, like the salts of the first kind. The CoX2salts have a metallic taste, whilst those of luteocobalt and others have a purely saline taste, like the salts of the alkali metals. In the luteo-salts all the X's react (are ionised, as some chemists say) as in ordinary salts—for instance, all the Cl2is precipitated by a solution of AgNO3; all the (SO4)3gives a precipitate with BaX2, &c. The double salt formed with PtCl4is composed in the same manner as the potassium salt, K2PtCl4= 2KCl + PtCl4, that is, contains (CoCl3,6NH3)2,3PtCl4, or the amount of chlorine in the PtCl4is double that in the alkaline salt. In the rosepentamine (e), and rosetetramine (f), salts, also all the X's react or are ionised, but in the (g) and (h) salts only a portion of the X's react, and they are equal to the (e) and (f) salts minus water; this means that although the water dissolves them it is not combined with them, as PHO3differs from PH3O3; phenomena of this class correspond exactly to what has been already (Chapter XXI., Note7) mentioned respecting the green and violet salts of oxide of chromium.(d)The fuscocobaltic salts.An ammoniacal solution of cobalt salts acquires a brown colour in the air, due to the formation of these salts. They are also produced by the decomposition of salts of the second kind; they crystallise badly, and are separated from their solutions by addition of alcohol or an excess of ammonia. When boiled they give up the ammonia and cobaltic oxide which they contain. Hydrochloric and nitric acids give a yellow precipitate with these salts, which turns red when boiled, forming salts of the next category. The following is an example of the composition of two of the fuscocobaltic salts, Co2O(SO4)2,8NH3,4H2O and Co2OCl4,8NH3,3H2O. It is evident that the fuscocobaltic salts are ammoniacal compounds of basic cobaltic salts. The normal cobaltic sulphate ought to have the composition Co2(SO4)3= Co2O3,3SO3; the simplest basic salts will be Co2O(SO4)2= Co2O3)2SO3, and Co2O2(SO4) = Co2O3,SO3. The fuscocobaltic salts correspond with the first type of basic salts. They are changed (in concentrated solutions) into oxycobaltamine salts by absorption of one atom of oxygen, Co2O2(SO4)2. The whole process of oxidation will be as follows: first of all Co2X4, a cobaltous salt, is in the solution (X a univalent haloid, 2 molecules of the salt being taken), then Co2OX4, the basic cobaltic salt (4th series), then Co2O2X4, the salt of the dioxide (2nd series). The series of basic salts with an acid, 2HX, forms water and a normal salt, Co2X6(in 3, 5, 6 series). These salts are combined with various amounts of water and ammonia. Under many conditions the salts of fuscocobalt are easily transformed into salts of the next series. The salts of the series that has just been described contain 4 molecules of ammonia to 1 atom of cobalt.(e)The roseocobaltic(or rosepentamine), CoX2H2O,5NH3,salts, like the luteocobaltic, correspond with the normal cobaltic salts, but contain less ammonia, and an extra molecule of water. Thus the sulphate is obtained from cobaltous sulphate dissolved in ammonia and left exposed to the air until transformed into a brown solution of the fuscocobaltic salt; when this is treated with sulphuric acid a crystalline powder of the roseocobaltic salt, Co2(SO4)3,10NH3,5H2O, separates. The formation of this salt is easily understood: cobaltous sulphate in the presence of ammonia absorbs oxygen, and the solution of the fuscocobaltic salt will therefore contain, like cobaltous sulphate, one part of sulphuric acid to every part of cobalt, so that the whole process of formation may be expressed by the equation: 10NH3+ 2CoSO4+ H2SO4+ 4H2O + O = Co2(SO4)3,10NH3,5H2O. This salt forms tetragonal crystals of a red colour, slightly soluble in cold, but readily soluble in warm water. When the sulphate is treated with baryta, roseocobaltic hydroxide is formed in the solution, which absorbs the carbonic anhydride of the air. It is obtained from the next series by the action of alkalis.(f) Therosetetramine cobaltic saltsCoCl2,2H2O,4NH3were obtained by Jörgenson, and belong to the type of the luteo-salts, only with the substitution of 2NH3for H2O. Like the luteo- and roseo-salts they give double salts with PtCl4, similar to the alkaline double salts, for instance (Co2H2O,4NH3)2(SO4)2Cl2PtCl4. They are darker in colour than the preceding, but also crystallise well. They are formed by dissolving CoCO3in sulphuric acid (of a given strength), and after NH3and carbonate of ammonium have been added, air is passed through the solution (for oxidation) until the latter turns red. It is then evaporated with lumps of carbonate of ammonium, filtered from the precipitate and crystallised. A salt of the composition Co2(CO3)2(SO4),(2H2O,4NH3)2is thus obtained, from which the other salts may be easily prepared.(g) Thepurpureocobaltic salts, CoX3,5NH3, are also products of the direct oxidation of ammoniacal solutions of cobalt salts. They are easily obtained by heating the roseocobaltic and luteo-salts with strong acids. They are to all effects the same as the roseocobaltic salts, only anhydrous. Thus, for instance, the purpureocobaltic chloride, Co2Cl6,10NH3, or CoCl3,5NH3, is obtained by boiling the oxycobaltamine salts with ammonia. There is the same distinction between these salts and the preceding ones as between the various compounds of cobaltous chloride with water. In the purpureocobaltic only X2out of the X3react (are ionised). To the rosetetramine salts (f) there correspond thepurpureotetraminesalts, CoX3H2O,4NH3. The corresponding chromium purpureopentamine salt, CrCl3,5NH3is obtained with particular ease (Christensen, 1893). Dry anhydrous chromium chloride is treated with anhydrous liquid ammonia in a freezing mixture composed of liquid CO2and chlorine, and after some time the mixture is taken out of the freezing mixture, so that the excess of NH3boils away; the violet crystals then immediately acquire the red colour of the salt, CrCl3,5NH3, which is formed. The product is washed with water (to extract the luteo-salt, CrCl3,6NH3), which does not dissolve the salt, and it is then recrystallised from a hot solution of hydrochloric acid.(h) Thepraseocobaltic salts, CoX3,4NH3, are green, and form, with respect to the rosetetramine salts (f), the products of ultimate dehydration (for example, like metaphosphoric acid with respect to orthophosphoric acid, but in dissolving in water they give neither rosetetramine nor tetramine salts. (In my opinion one should expect salts with a still smaller amount of NH3, of the blue colour proper to the low hydrated compounds of cobalt; the green colour of the prazeo-salts already forms a step towards the blue.) Jörgenson obtained salts for ethylene-diamine, N2H4C2H4which replaces 2NH3. After being kept a long time in aqueous solution they give rosetetramine salts, just as metaphosphoric acid gives orthophosphoric acid, while the rosetetramine salts are converted into prazeo-salts by Ag2O and NaHO. Here only one X is ionised out of the X3. There are also basic salts of the same type; but the best known is the chromium salt called the rhodozochromic salt, Cr2(OH)3Cl3,6NH3,2H2O, which is formed by the prolonged action of water upon the corresponding roseo-salt.The cobaltamine compounds differ essentially but little from the ammoniacal compounds of other metals. The only difference is that here the cobaltic oxide is obtained from the cobaltous oxide in the presence of ammonia. In any case it is a simpler question than that of the double cyanides. Those forces in virtue of which such a considerable number of ammonia molecules are united with a molecule of a cobalt compound, appertain naturally to the series of those slightly investigated forces which exist even in the highest degrees of combination of the majority of elements. They are the same forces which lead to the formation of compounds containing water of crystallisation, double salts, isomorphous mixtures and complex acids (Chapter XXI., Note8 bis). The simplest conception, according to my opinion, of cobalt compounds (much more so than by assuming special complex radicles, with Schiff, Weltzien, Claus, and others), may be formed by comparing them with other ammoniacal products. Ammonia, like water, combines in various proportions with a multitude of molecules. Silver chloride and calcium chloride, just like cobalt chloride, absorb ammonia, forming compounds which are sometimes slightly stable, and easily dissociated, sometimes more stable, in exactly the same way as water combines with certain substances, forming fairly stable compounds called hydroxides or hydrates, or less stable compounds which are called compounds with water of crystallisation. Naturally the difference in the properties in both cases depends on the properties of those elements which enter into the composition of the given substance, and on those kinds of affinity towards which chemists have not as yet turned their attention. If boron fluoride, silicon fluoride, &c., combine with hydrofluoric acid, if platinic chloride, and even cadmium chloride, combine with hydrochloric acid, these compounds may be regarded as double salts, because acids are salts of hydrogen. But evidently water and ammonia have the same saline faculty, more especially as they, like haloid acids, contain hydrogen, and are both capable of further combination—for instance, ammonia with hydrochloric acid. Hence it is simpler to compare complex ammoniacal with double salts, hydrates, and similar compounds, butthe ammonio-metallic saltspresent a most complete qualitative and quantitative resemblance tothe hydrated salts of metals. The composition of the latter is MXnmH2O, where M = metal, X = the haloid, simple or complex, andnandmthe quantities of the haloid and so-called water of crystallisation respectively. The composition of the ammoniacal salts of metals is MXnmNH3. The water of crystallisation is held by the salt with more or less stability, and some salts even do not retain it at all; some part with water easily when exposed to the air, others when heated, and then with difficulty. In the case of some metals all the salts combine with water, whilst with others only a few, and the water so combined may then be easily disengaged. All this applies equally well to the ammoniacal salts, and therefore the combination of ammonia may be termedthe ammonia of crystallisation. Just as the water which is combined with a salt is held by it with different degrees of force, so it is with ammonia. In combining with 2NH3,PtCl2evolves 31,000 cals.; while CaCl2only evolves 14,000 cals.; and the former compound parts with its NH3(together with HCl in this case) with more difficulty, only above 200°, while the latter disengages ammonia at 180°. ZnCl2,2NH3in forming ZnCl2,4NH3evolves only 11,000 cals., and splits up again into its components at 80°. The amount of combined ammonia is as variable as the amount of water of crystallisation—for instance, SnI48NH3, CrCl28NH3, CrCl36NH3, CrCl35NH3,PtCl2,4NH3, &c. are known. Very often NH3is replaceable by OH2and conversely. A colourless, anhydrous cupric salt—for instance, cupric sulphate—when combined with water forms blue and green salts, and violet when combined with ammonia. If steam be passed through anhydrous copper sulphate the salt absorbs water and becomes heated; if ammonia be substituted for the water the heating becomes much more intense, and the salt breaks up into a fine violet powder. With water CuSO4,5H2O is formed, and with ammonia CuSO4,5NH3, the number of water and ammonia molecules retained by the salt being the same in each case, and as a proof of this, and that it is not an isolated coincidence, the remarkable fact must be borne in mind that water and ammonia consecutively, molecule for molecule, are capable of supplanting each other, and forming the compounds CuSO4,5H2O, CuSO4,4H2O,NH3; CuSO4,3H2O,2NH3; CuSO4,2H2O,3NH3; CuSO4,H2O,4NH3, and CuSO4,5NH3. The last of these compounds was obtained by Henry Rose, and my experiments have shown that more ammonia than this cannot be retained. By adding to a strong solution of cupric sulphate sufficient ammonia to dissolve the whole of the oxide precipitated, and then adding alcohol, Berzelius obtained the compound CuSO4,H2O,4NH3, &c. The law of substitution also assists in rendering these phenomena clearer, because a compound of ammonia with water forms ammonium hydroxide, NH4HO, and therefore these molecules combining with one another may also interchange, as being of equal value. In general, those salts form stable ammoniacal compounds which are capable of forming stable compounds with water of crystallisation; and as ammonia is capable of combining with acids, and as some of the salts formed by slightly energetic bases in their properties more closely resemble acids (that is, salts of hydrogen) than those salts containing more energetic bases, we might expect to find more stable and more easily-formed ammonio-metallic salts with metals and their oxides having weaker basic properties than with those which form energetic bases. This explains why the salts of potassium, barium, &c., do not form ammonio-metallic salts, whilst the salts of silver, copper, zinc, &c., easily form them, and the salts RX3still more easily and with greater stability. This consideration also accounts for the great stability of the ammoniacal compounds of cupric oxide compared with those of silver oxide, since the former is displaced by the latter. It also enables us to see clearly the distinction which exists in the stability of the cobaltamine salts containing salts corresponding with cobaltous oxide, and those corresponding with higher oxides of cobalt, for the latter are weaker bases than cobaltous oxides.The nature of the forces and quality of the phenomena occurring during the formation of the most stable substances, and of such compounds as crystallisable compounds, are one and the same, although perhaps exhibited in a different degree.This, in my opinion, may be best confirmed by examining the compounds of carbon, because for this element the nature of the forces acting during the formation of its compounds is well known. Let us take as an example two unstable compounds of carbon. Acetic acid, C2H4O2(specific gravity 1·06), with water forms the hydrate, C2H4O2,H2O, denser (1·07) than either of the components, but unstable and easily decomposed, generally simply referred to as a solution. Such also is the crystalline compound of oxalic acid, C2H2O4, with water, C2H2O4,2H2O. Their formation might be predicted as starting from the hydrocarbon C2H6, in which, as in any other, the hydrogen may be exchanged for chlorine, the water residue (hydroxyl), &c. The first substitution product with hydroxyl, C2H5(HO), is stable; it can be distilled without alteration, resists a temperature higher than 100°, and then does not give off water. This is ordinary alcohol. The second, C2H4(HO)2, can also be distilled without change, but can be decomposed into water and C2H4O (ethylene oxide or aldehyde); it boils at about 197°, whilst the first hydrate boils at 78°, a difference of about 100°. The compound C2H3(HO)3will be the third product of such substitution; it ought to boil at about 300°, but does not resist this temperature—it decomposes into H2O and C2H4O2, where only one hydroxyl group remains, and the other atom of oxygen is left in the same condition as in ethylene oxide, C2H4O. There is a proof of this. Glycol, C2H4(HO)2, boils at 197°, and forms water and ethylene oxide, which boils at 13° (aldehyde, its isomeride, boils at 21°); therefore the product disengaged by the splitting up of the hydrate boils at 184° lower than the hydrate C2H4(HO)2. Thus the hydrate C2H3(HO)3, which ought to boil at about 300°, splits up in exactly the same way into water and the product C2H4O2, which boils at 117°—that is, nearly 183° lower than the hydrate, C2H3(HO)3. But this hydrate splits up before distillation. The above-mentioned hydrate of acetic acid is such a decomposable hydrate—that is to say, what is called a solution. Still less stability may be expected from the following hydrates. C2H2(HO)4also splits up into water and a hydrate (it contains two hydroxyl groups) called glycolic acid, C2H2O(HO)2= C2H4O3. The next product of substitution will be C2H(HO)5; it splits up into water, H2O, and glyoxylic acid, C2H4O4(three hydroxyl groups). The last hydrate which ought to be obtained from C2H6, and ought to contain C2(HO)6, is the crystalline compound of oxalic acid, C2H2O4(two hydroxyl groups), and water, 2H2O, which has been already mentioned. The hydrate C2(HO)6= C2H2O4,2H2O, ought, according to the foregoing reasoning, to boil at about 600° (because the hydrate, C2H4(HO)2, boils at about 200°, and the substitution of 4 hydroxyl groups for 4 atoms of hydrogen will raise the boiling-point 400°). It does not resist this temperature, but at a much lower point splits up into water, 2H2O, and the hydrate C2O2(HO)2, which is also capable of yielding water. Without going into further discussion of this subject, it may be observed that the formation of the hydrates, or compounds with water of crystallisation, of acetic and oxalic acids has thus received an accurate explanation, illustrating the point we desired to prove in affirming that compounds with water of crystallisation are held together by the same forces as those which act in the formation of other complex substances, and that the easy displaceability of the water of crystallisation is only a peculiarity of a local character, and not a radical point of distinction. All the above-mentioned hydrates, C2X6, or products of their destruction, are actually obtained by the oxidation of the first hydrate, C2H3(HO), or common alcohol, by nitric acid (Sokoloff and others). Hence the forces which induce salts to combine withnH2O or with NH3are undoubtedly of the same order as the forces which govern the formation of ordinary ‘atomic’ and saline compounds. (A great impediment in the study of the former was caused by the conviction which reigned in the sixties and seventies, that ‘atomic’ were essentially different from ‘molecular’ compounds like crystallohydrates, in which it was assumed that there was a combination of entire molecules, as though without the participation of the atomic forces.) If the bond between chlorine and different metals is not equally strong, so also the bond unitingnH2O andnNH3is exceeding variable; there is nothing very surprising in this. And in the fact that the combination of different amounts of NH3and H2O alters the capacity of the haloids X of the salts RX2for reaction (for instance, in the luteo-salts all the X3, while in the purpureo, only 2 out of the 3, and in the prazeo-salts only 1 of the 3 X's reacts), we should see in the first place a phenomenon similar to what we met with in Cr2Cl6(Chapter XXI., Note7 bis), for in both instances the essence of the difference lies in the removal of water; a molecule RCl3,6H2O or RCl3,6NH3contains the halogen in a perfectly mobile (ionised) state, while in the molecule RCl3,5H2O or RCl3,5NH3a portion of the halogen has almost lost its faculty for reacting with AgNO3, just as metalepsical chlorine has lost this faculty which is fully developed in the chloranhydride. Until the reason of this difference be clear, we cannot expect that ordinary points of view and generalisation can give a clear answer. However, we may assume that here the explanation lies in the nature and kind of motion of the atoms in the molecules, although as yet it is not clear how. Nevertheless, I think it well to call attention again (ChapterI.) to the fact that the combination of water, and hence, also, of any other element, leads to most diverse consequences; the water in the gelatinous hydrate of alumina or in the decahydrated Glauber salt is very mobile, and easily reacts like water in a free state; but the same water combined with oxide of calcium, or C2H4(for instance, in C2H6O and in C4H10O), or with P2O5, has become quite different, and no longer acts like water in a free state. We see the same phenomenon in many other cases—for example, the chlorine in chlorates no longer gives a precipitate of chloride of silver with AgNO3. Thus, although the instance which is found in the difference between the roseo- and purpureo-salts deserves to be fully studied on account of its simplicity, still it is far from being exceptional, and we cannot expect it to be thoroughly explained unless a mass of similar instances, which are exceedingly common among chemical compounds, be conjointly explained. (Among the researches which add to our knowledge respecting the complex ammoniacal compounds, I think it indispensable to call the reader's attention to Prof. Kournakoff's dissertation ‘On complex metallic bases,’ 1893.)Kournakoff (1894) showed that the solubility of the luteo-salt, CoCl3,6NH3, at 0° = 4·30 (per 100 of water), at 20° = 7·7, that in passing into the roseo-salt, CoCl3H2O5NH3, the solubility rises considerably, and at 0° = 16·4, and at 20° = about 27, whilst the passage into the purpureo-salt, CoCl3,5NH3, is accompanied by a great fall in the solubility, namely, at 0° = 0·23, and at 20° = about 0·5. And as crystallohydrates with a smaller amount of water are usually more soluble than the higher crystallohydrates (Le Chatelier), whilst here we find that the solubility falls (in the purpureo-salt) with a loss of water, that water which is contained in the roseo-salt cannot be compared with the water of crystallisation. Kournakoff, therefore, connects the fall in solubility (in the passage of the roseo- into the purpureo-salts) with the accompanying loss in the reactive capacity of the chlorine.In conclusion, it may be observed that the elements of the eighth group—that is, the analogues of iron and platinum—according to my opinion, will yield most fruitful results when studied as to combinations with whole molecules, as already shown by the examples of complex ammoniacal, cyanogen, nitro-, and other compounds, which are easily formed in this eighth group, and are remarkable for their stability. This faculty of the elements of the eighth group for forming the complex compounds alluded to, is in all probability connected with the position which the eighth group occupies with regard to the others. Following the seventh, which forms the type RX7, it might be expected to contain the most complex type, RX8. This is met with in OsO4. The other elements of the eighth group, however, only form the lower types RX2, RX3, RX4… and these accordingly should be expected to aggregate themselves into the higher types, which is accomplished in the formation of the above-mentioned complex compounds.[35 bis]Marshall (1891) obtained cobaltic sulphate, Co2(SO4)3,18H2O, by the action of an electric current upon a strong solution of CoSO4.[36]The action of an alkaline hypochlorite or hypobromite upon a boiling solution of cobaltous salts, according to Schroederer (1889), produces oxides, whose composition varies between Co3O5(Rose's compound) and Co2O3, and also between Co5O8and Co12O19. If caustic potash and then bromine be added to the liquid, only Co2O3is formed. The action of alkaline hypochlorites or hypo-bromites, or of iodine, upon cobaltic salts, gives a highly-coloured precipitate which has a different colour to the hydrate of the oxide Co2(OH)6. According to Carnot the precipitate produced by the hypochlorites has a composition Co10O16, whilst that given by iodine in the presence of an alkali contains a larger amount of oxygen. Fortmann (1891) re-investigated the composition of the higher oxygen oxide obtained by iodine in the presence of alkali, and found that the greenish precipitate (which disengages oxygen when heated to 100°) corresponds to the formula CoO2. The reaction must be expressed by the equation: CoX2+ I2+ 4KHO = CoO2+ 2KX + 2KI + 2H2O.[37]Prior to Fortmann, Rousseau (1889) endeavoured to solve the question as to whether CoO2was able to combine with bases. He succeeded in obtaining a barium compound corresponding to this oxide. Fifteen grams of BaCl2or BaBr2are triturated with 5–6 grams of oxide of barium, and the mixture heated to redness in a closed platinum crucible; 1 gram of oxide of cobalt is then gradually added to the fused mass. Each addition of oxide is accompanied by a violent disengagement of oxygen. After a short time, however, the mass fuses quietly, and a salt settles at the bottom of the crucible, which, when freed from the residue, appears as black hexagonal, very brilliant crystals. In dissolving in water this substance evolves chlorine; its composition corresponds to the formula 2(CoO2)BaO. If the original mass be heated for a long time (40 hours), the amount of dioxide in the resultant mass decreases. The author obtained a neutral salt having the composition CoO2BaO (this compound = BaO2CoO) by breaking up the mass as it agglomerates together, and bringing the pieces into contact with the more heated surface of the crucible. This salt is formed between the somewhat narrow limits of temperature 1,000°-1,100°; above and below these limits compounds richer or poorer in CoO2are formed. The formation of CoO2by the action of BaO2, and the easy decomposition of CoO2with the evolution of oxygen, give reason for thinking that it belongs to the class of peroxides (like Cr2O7, CaO2, &c.); it is not yet known whether they give peroxide of hydrogen like the true peroxides. The fact that it is obtained by means of iodine (probably through HIO), and its great resemblance to MnO2, leads rather to the supposition that CoO2is a very feeble saline oxide. The form CoO2is repeated in the cobaltic compounds (Note35), and the existence of CoO2should have long ago been recognised upon this basis.[38]This compound is known as nickel tetra-carbonyl. It appears to me yet premature to judge of the structure of such an extraordinary compound as Ni(CO)4. It has long been known that potassium combines with CO forming Kn(CO)n(Chapter IX., Note31), but this substance is apparently saline and non-volatile, and has as little in common with Ni(CO)4as Na2H has with SbH3. However, Berthelot observed that when NiC4O4is kept in air, it oxidises and gives a colourless compound, Ni3C2O3,10H2O, having apparently saline properties. We may add that Schützenberger, on reducing NiCl2by heating it in a current of hydrogen, observed that a nickel compound partially volatilises with the HCl and gives metallic nickel when heated again. The platinum compound, PtCl2(CO)3(Chapter XXIII., Note11), offers the greatest analogy to Ni(CO)4. This compound was obtained as a volatile substance by Schützenberger by moderately heating (to 235°) metallic platinum in a mixture of chlorine and carbonic oxide. If we designate CO by Y, and an atom of chlorine by X, then taking into account that, according to the periodic system, Ni is an analogue of Pt, a certain degree of correspondence is seen in the composition NiY4and PtX2Y2. It would be interesting to compare the reactions of the two compounds.[39]According to its empirical formula oxalate of nickel also contains nickel and carbonic oxide.[40]The following are the thermo-chemical data (according to Thomsen, and referred to gram weights expressed by the formula, in large calories or thousand units of heat) for the formation of corresponding compounds of Mn, Fe, Co, Ni, and Cu (+ Aq signifies that the reaction proceeds in an excess of water):R = MnFeCoNiCuR + Cl2+ Aq128100959463R + Br2+ Aq10678737241R + I2+ Aq7648434132R + O + H2O9568636138R + O2+ SO2+nH2O193169163163130RCl2+ Aq+1618181911These examples show that for analogous reactions the amount of heat evolved in passing from Mn to Fe, Co, Ni, and Cu varies in regular sequences as the atomic weight increases. A similar difference is to be found in other groups and series, and proves that thermo-chemical phenomena are subject to the periodic law.
[33]The change of colour is dependent in all probability on the combination with water, or according to others on polymeric transformation. It enables a solution of cobalt chloride to be used as sympathetic ink. If something be written with cobalt chloride on white paper, it will be invisible on account of the feeble colour of the solution, and when dry nothing can be distinguished. If, however, the paper be heated before the fire, the rose-coloured salt will be changed into a less hydrous blue salt, and the writing will become quite visible, but fade away when cool.The change of colour which takes place in solutions of CoCl2under the influence not only of solution in water or alcohol, but also of a change of temperature, is a characteristic of all the halogen salts of cobalt. Crystalline iodide of cobalt, CoI26H2O, gives a dark red solution between -22° and +20°; above +20° the solution turns brown and passes from olive to green, from +35° to 320° the solution remains green. According to Étard the change of colour is due to the fact that at first the solution contains the hydrate CoI2H2O, and that above 35° it contains CoI24H2O. These hydrates can be crystallised from the solutions; the former at ordinary temperature and the latter on heating the solution. The intermediate olive colour of the solutions corresponds to the incipient decomposition of the hexahydrated salt and its passage into CoI24H2O. A solution of the hexahydrated chloride of cobalt, CoCl26H2O, is rose-coloured between -22° and +25°; but the colour changes starting from +25°, and passes through all the tints between red and blue right up to 50°; a true blue solution is only obtained at 55° and remains up to 300°. This true blue solution contains another hydrate, CoCl22H2O.The dependence between the solubility of the iodide and chloride of cobalt and the temperature is expressed by two almost straight lines corresponding to the hexa- and di-hydrates; the passage of the one into the other hydrate being expressed by a curve. The same character of phenomena is seen also in the variation of the vapour tension of solutions of chloride of cobalt with the temperature. We have repeatedly seen that aqueous solutions (for instance, Chapter XXII., Note23for Fe2Cl6) deposit different crystallo-hydrates at different temperatures, and that the amount of water in the hydrate decreases as the temperaturetrises, so that it is not surprising that CoCl22H2O (or according to Potilitzin CoCl2H2O) should separate out above 55° and CoCl26H2O at 25° and below. Nor is it exceptional that the colour of a salt varies according as it contains different amounts of H2O. But in this instance it is characteristic that the change of colour takes place in solution in the presence of an excess of water. This apparently shows that the actual solution may contain either CoCl26H2O or CoCl22H2O. And as we know that a solution may contain both metaphosphoric PHO3and orthophosphoric acid H3PO4= HPO3+ H2O, as well as certain other anhydrides, the question of the state of substances in solutions becomes still more complicated.Nickel sulphate crystallises from neutral solutions at a temperature of from 15° to 20° inrhombiccrystals containing 7H2O. Its form approaches very closely to that of the salts of zinc and magnesium. The planes of a vertical prism for magnesium salts are inclined at an angle of 90° 30′, for zinc salts at an angle of 91° 7′, and for nickel salts at an angle of 91° 10′. Such is also the form of the zinc and magnesium selenates and chromates. Cobalt sulphate containing 7 molecules of water is deposited in crystals of themonoclinicsystem, like the corresponding salts of iron and manganese. The angle of a vertical prism for the iron salt = 82° 20′, for cobalt = 82° 22′, and the inclination of the horizontal pinacoid to the vertical prism for the iron salt = 99° 2′, and for the cobalt salt 99° 36′. All the isomorphous mixtures of the salts of magnesium, iron, cobalt, nickel and manganese have the same form if they contain 7 mol. H2O and iron or cobalt predominate, whilst if there is a preponderance of magnesium, zinc, or nickel, the crystals have a rhombic form like magnesium sulphate. Hence these sulphates aredimorphous, but for some the one form is more stable and for others the other. Brooke, Moss, Mitscherlich, Rammelsberg, and Marignac have explained these relations. Brooke and Mitscherlich also supposed that NiSO4,7H2O is not only capable of assuming these forms, but also that of thetetragonalsystem, because it is deposited in this form from acid, and especially from slightly-heated solutions (30° to 40°). But Marignac demonstrated that the tetragonal crystals do not contain 7, but 6, molecules of water, NiSO4,6H2O. He also observed that a solution evaporated at 50° to 70° deposits monoclinic crystals, but of a different form from ferrous sulphate, FeSO4,7H2O—namely, the angle of the prism is 71° 52′, that of the pinacoid 95° 6′. This salt appears to be the same with 6 molecules of water as the tetragonal. Marignac also obtained magnesium and zinc salts with 6 molecules of water by evaporating their solutions at a higher temperature, and these salts were found to be isomorphous with the monoclinic nickel salt. In addition to this it must be observed that the rhombic crystals of nickel sulphate with 7H2O become turbid under the influence of heat and light, lose water, and change into the tetragonal salt. The monoclinic crystals in time also become turbid, and change their structure, so that the tetragonal form of this salt is the most stable. Let us also add that nickel sulphate in all its shapes forms very beautiful emerald green crystals, which, when heated to 230°, assume a dirty greenish-yellow hue and then contain one molecule of water.Klobb (1891) and Langlot and Lenoir obtained anhydrous CoSO4and NiSO4by igniting the hydrated salt with (NH4)2SO4until the ammonium salt had completely volatilised and decomposed.We may add that when equivalent aqueous solutions of NiX2(green) and CoX2(red) are mixed together they give an almost colourless (grey) solution, in which the green and red colour of the component parts disappears owing to the combination of the complementary colours.A double salt NiKF3is obtained by heating NiCl2with KFHF in a platinum crucible; KCoF3is formed in a similar manner. The nickel salt occurs in fine green plates, easily soluble in water but scarcely soluble in ethyl and methyl alcohol. They decompose into green oxide of nickel and potassium fluoride when heated in a current of air. The analogous salt of cobalt crystallises in crimson flakes.If instead of potassium fluoride, CoCl2or NiCl2be fused with ammonium fluoride, they also form double salts with the latter. This gives the possibility of obtaining anhydrous fluorides NiF2and CoF2. Crystalline fluoride of nickel, obtained by heating the amorphous powder formed by decomposing the double ammonium salt in a stream of hydrofluoric acid, occurs in beautiful green prisms, sp. gr. 4·63, which are insoluble in water, alcohol, and ether; sulphuric, hydrochloric, and nitric acids also have no action upon them, even when heated; NiF2is decomposed by steam, with the formation of black oxide, which retains the crystalline structure of the salt. Fluoride of cobalt, obtained as a rose-coloured powder by decomposing the double ammonium salt with the aid of heat in a stream of hydrofluoric acid, fuses into a ruby-coloured mass which bears distinct signs of a crystalline structure; sp. gr. 4·43. The molten salt only volatilises at about 1400°, which forms a clear distinction between CoF2and the volatile NiF2. Hydrochloric, sulphuric, and nitric acids act upon CoF2even in the cold, although slowly, while when heated the reaction proceeds rapidly (Poulenc, 1892).
[33]The change of colour is dependent in all probability on the combination with water, or according to others on polymeric transformation. It enables a solution of cobalt chloride to be used as sympathetic ink. If something be written with cobalt chloride on white paper, it will be invisible on account of the feeble colour of the solution, and when dry nothing can be distinguished. If, however, the paper be heated before the fire, the rose-coloured salt will be changed into a less hydrous blue salt, and the writing will become quite visible, but fade away when cool.
The change of colour which takes place in solutions of CoCl2under the influence not only of solution in water or alcohol, but also of a change of temperature, is a characteristic of all the halogen salts of cobalt. Crystalline iodide of cobalt, CoI26H2O, gives a dark red solution between -22° and +20°; above +20° the solution turns brown and passes from olive to green, from +35° to 320° the solution remains green. According to Étard the change of colour is due to the fact that at first the solution contains the hydrate CoI2H2O, and that above 35° it contains CoI24H2O. These hydrates can be crystallised from the solutions; the former at ordinary temperature and the latter on heating the solution. The intermediate olive colour of the solutions corresponds to the incipient decomposition of the hexahydrated salt and its passage into CoI24H2O. A solution of the hexahydrated chloride of cobalt, CoCl26H2O, is rose-coloured between -22° and +25°; but the colour changes starting from +25°, and passes through all the tints between red and blue right up to 50°; a true blue solution is only obtained at 55° and remains up to 300°. This true blue solution contains another hydrate, CoCl22H2O.
The dependence between the solubility of the iodide and chloride of cobalt and the temperature is expressed by two almost straight lines corresponding to the hexa- and di-hydrates; the passage of the one into the other hydrate being expressed by a curve. The same character of phenomena is seen also in the variation of the vapour tension of solutions of chloride of cobalt with the temperature. We have repeatedly seen that aqueous solutions (for instance, Chapter XXII., Note23for Fe2Cl6) deposit different crystallo-hydrates at different temperatures, and that the amount of water in the hydrate decreases as the temperaturetrises, so that it is not surprising that CoCl22H2O (or according to Potilitzin CoCl2H2O) should separate out above 55° and CoCl26H2O at 25° and below. Nor is it exceptional that the colour of a salt varies according as it contains different amounts of H2O. But in this instance it is characteristic that the change of colour takes place in solution in the presence of an excess of water. This apparently shows that the actual solution may contain either CoCl26H2O or CoCl22H2O. And as we know that a solution may contain both metaphosphoric PHO3and orthophosphoric acid H3PO4= HPO3+ H2O, as well as certain other anhydrides, the question of the state of substances in solutions becomes still more complicated.
Nickel sulphate crystallises from neutral solutions at a temperature of from 15° to 20° inrhombiccrystals containing 7H2O. Its form approaches very closely to that of the salts of zinc and magnesium. The planes of a vertical prism for magnesium salts are inclined at an angle of 90° 30′, for zinc salts at an angle of 91° 7′, and for nickel salts at an angle of 91° 10′. Such is also the form of the zinc and magnesium selenates and chromates. Cobalt sulphate containing 7 molecules of water is deposited in crystals of themonoclinicsystem, like the corresponding salts of iron and manganese. The angle of a vertical prism for the iron salt = 82° 20′, for cobalt = 82° 22′, and the inclination of the horizontal pinacoid to the vertical prism for the iron salt = 99° 2′, and for the cobalt salt 99° 36′. All the isomorphous mixtures of the salts of magnesium, iron, cobalt, nickel and manganese have the same form if they contain 7 mol. H2O and iron or cobalt predominate, whilst if there is a preponderance of magnesium, zinc, or nickel, the crystals have a rhombic form like magnesium sulphate. Hence these sulphates aredimorphous, but for some the one form is more stable and for others the other. Brooke, Moss, Mitscherlich, Rammelsberg, and Marignac have explained these relations. Brooke and Mitscherlich also supposed that NiSO4,7H2O is not only capable of assuming these forms, but also that of thetetragonalsystem, because it is deposited in this form from acid, and especially from slightly-heated solutions (30° to 40°). But Marignac demonstrated that the tetragonal crystals do not contain 7, but 6, molecules of water, NiSO4,6H2O. He also observed that a solution evaporated at 50° to 70° deposits monoclinic crystals, but of a different form from ferrous sulphate, FeSO4,7H2O—namely, the angle of the prism is 71° 52′, that of the pinacoid 95° 6′. This salt appears to be the same with 6 molecules of water as the tetragonal. Marignac also obtained magnesium and zinc salts with 6 molecules of water by evaporating their solutions at a higher temperature, and these salts were found to be isomorphous with the monoclinic nickel salt. In addition to this it must be observed that the rhombic crystals of nickel sulphate with 7H2O become turbid under the influence of heat and light, lose water, and change into the tetragonal salt. The monoclinic crystals in time also become turbid, and change their structure, so that the tetragonal form of this salt is the most stable. Let us also add that nickel sulphate in all its shapes forms very beautiful emerald green crystals, which, when heated to 230°, assume a dirty greenish-yellow hue and then contain one molecule of water.
Klobb (1891) and Langlot and Lenoir obtained anhydrous CoSO4and NiSO4by igniting the hydrated salt with (NH4)2SO4until the ammonium salt had completely volatilised and decomposed.
We may add that when equivalent aqueous solutions of NiX2(green) and CoX2(red) are mixed together they give an almost colourless (grey) solution, in which the green and red colour of the component parts disappears owing to the combination of the complementary colours.
A double salt NiKF3is obtained by heating NiCl2with KFHF in a platinum crucible; KCoF3is formed in a similar manner. The nickel salt occurs in fine green plates, easily soluble in water but scarcely soluble in ethyl and methyl alcohol. They decompose into green oxide of nickel and potassium fluoride when heated in a current of air. The analogous salt of cobalt crystallises in crimson flakes.
If instead of potassium fluoride, CoCl2or NiCl2be fused with ammonium fluoride, they also form double salts with the latter. This gives the possibility of obtaining anhydrous fluorides NiF2and CoF2. Crystalline fluoride of nickel, obtained by heating the amorphous powder formed by decomposing the double ammonium salt in a stream of hydrofluoric acid, occurs in beautiful green prisms, sp. gr. 4·63, which are insoluble in water, alcohol, and ether; sulphuric, hydrochloric, and nitric acids also have no action upon them, even when heated; NiF2is decomposed by steam, with the formation of black oxide, which retains the crystalline structure of the salt. Fluoride of cobalt, obtained as a rose-coloured powder by decomposing the double ammonium salt with the aid of heat in a stream of hydrofluoric acid, fuses into a ruby-coloured mass which bears distinct signs of a crystalline structure; sp. gr. 4·43. The molten salt only volatilises at about 1400°, which forms a clear distinction between CoF2and the volatile NiF2. Hydrochloric, sulphuric, and nitric acids act upon CoF2even in the cold, although slowly, while when heated the reaction proceeds rapidly (Poulenc, 1892).
[34]Hydrated suboxide of cobalt (de Schulten, 1889) is obtained in the following manner. A solution of 10 grams of CoCl26H2O in 60 c.c. of water is heated in a flask with 250 grams of caustic potash and a stream of coal gas is passed through the solution. When heated the hydrate of the suboxide of cobalt which separates out, dissolves in the caustic potash and forms a dark blue solution. This solution is allowed to stand for 24 hours in an atmosphere of coal gas (in order to prevent oxidation). The crystalline mass which separates out has a composition Co(OH)2, and to the naked eye appears as a violet powder, which is seen to be crystalline under the microscope. The specific gravity of this hydrate is 3·597 at 15°. It does not undergo change in the air; warm acetic acid dissolves it, but it is insoluble in warm and cold solutions of ammonia and sal-ammoniac.
[34]Hydrated suboxide of cobalt (de Schulten, 1889) is obtained in the following manner. A solution of 10 grams of CoCl26H2O in 60 c.c. of water is heated in a flask with 250 grams of caustic potash and a stream of coal gas is passed through the solution. When heated the hydrate of the suboxide of cobalt which separates out, dissolves in the caustic potash and forms a dark blue solution. This solution is allowed to stand for 24 hours in an atmosphere of coal gas (in order to prevent oxidation). The crystalline mass which separates out has a composition Co(OH)2, and to the naked eye appears as a violet powder, which is seen to be crystalline under the microscope. The specific gravity of this hydrate is 3·597 at 15°. It does not undergo change in the air; warm acetic acid dissolves it, but it is insoluble in warm and cold solutions of ammonia and sal-ammoniac.
[34 bis]The following reaction may be added to those of the cobaltous and nickelous salts: potassium cyanide forms a precipitate with cobalt salts which is soluble in an excess of the reagent and forms a green solution. On heating this and adding a certain quantity of acid, a doublecobalt cyanideis formed which corresponds with potassium ferricyanide. Its formation is accompanied with the evolution of hydrogen, and is founded upon the property which cobalt has of oxidising in an alkaline solution, the development of which has been observed in such a considerable measure in the cobaltamine salts. The process which goes on here may be expressed by the following equation; CoC2N2+ 4KCN first forms CoK4C6N6, which salt with water, H2O, forms potassium hydroxide, KHO, hydrogen, H, and the salt, K3CoC6N6. Here naturally the presence of the acid is indispensable in consequence of its being required to combine with the alkali. From aqueous solutions this salt crystallises in transparent, hexagonal prisms of a yellow colour, easily soluble in water. The reactions of double decomposition, and even the formation of the corresponding acid, are here completely the same as in the case of the ferricyanide. If a nickelous salt be treated in precisely the same manner as that just described for a salt of cobalt, decomposition will occur.
[34 bis]The following reaction may be added to those of the cobaltous and nickelous salts: potassium cyanide forms a precipitate with cobalt salts which is soluble in an excess of the reagent and forms a green solution. On heating this and adding a certain quantity of acid, a doublecobalt cyanideis formed which corresponds with potassium ferricyanide. Its formation is accompanied with the evolution of hydrogen, and is founded upon the property which cobalt has of oxidising in an alkaline solution, the development of which has been observed in such a considerable measure in the cobaltamine salts. The process which goes on here may be expressed by the following equation; CoC2N2+ 4KCN first forms CoK4C6N6, which salt with water, H2O, forms potassium hydroxide, KHO, hydrogen, H, and the salt, K3CoC6N6. Here naturally the presence of the acid is indispensable in consequence of its being required to combine with the alkali. From aqueous solutions this salt crystallises in transparent, hexagonal prisms of a yellow colour, easily soluble in water. The reactions of double decomposition, and even the formation of the corresponding acid, are here completely the same as in the case of the ferricyanide. If a nickelous salt be treated in precisely the same manner as that just described for a salt of cobalt, decomposition will occur.
[35]The cobalt salts may be divided into at least the following classes, which repeat themselves for Cr, Ir, Rh (we shall not stop to consider the latter, particularly as they closely resemble the cobalt salts):—(a)Ammonium cobalt salts, which are simply direct compounds of the cobaltous salts CoX2with ammonia, similar to various other compounds of the salts of silver, copper, and even calcium and magnesium, with ammonia. They are easily crystallised from an ammoniacal solution, and have a pink colour. Thus, for instance, when cobaltous chloride in solution is mixed with sufficient ammonia to redissolve the precipitate first formed, octahedral crystals are deposited which have a composition CoCl2,H2O,6NH3. These salts are nothing else but combinations with ammonia of crystallisation—if it may be so termed—likening them in this way to combinations with water of crystallisation. This similarity is evident both from their composition and from their capability of giving off ammonia at various temperatures. The most important point to observe is that all these salts contain 6 molecules of ammonia to 1 atom of cobalt, and this ammonia is held in fairly stable connection. Water decomposes these salts. (Nickel behaves similarly without forming other compounds corresponding to the true cobaltic.)(b) The solutions of the above-mentioned salts are rendered turbid by the action of the air; they absorb oxygen and become covered with a crust ofoxycobaltamine salts. The latter are sparingly soluble in aqueous ammonia, have a brown colour, and are characterised by the fact that with warm waterthey evolve oxygen, forming salts of the following category: The nitrate may be taken as an example of this kind of salt; its composition is CoN2O7,5NH3,H2O. It differs from cobaltous nitrate, Co(NO3)2, in containing an extra atom of oxygen—that is, it corresponds with cobalt dioxide, CoO2, in the same way that the first salts correspond with cobaltous oxide; they contain 5, and not 6, molecules of ammonia, as if NH3had been replaced by O, but we shall afterwards meet compounds containing either 5NH3or 6NH3to each atom of cobalt.(c)The luteocobaltic saltsare thus called because they have a yellow (luteus) colour. They are obtained from the salts of the first kind by submitting them in dilute solution to the action of the air; in this case salts of the second kind are not formed, because they are decomposed by an excess of water, with the evolution of oxygen and the formation of luteocobaltic salts. By the action of ammonia the salts of the fifth kind (roseocobaltic) are also converted into luteocobaltic salts. These last-named salts generally crystallise readily, and have a yellow colour; they are comparatively much more stable than the preceding ones, and even for a certain time resist the action of boiling water. Boiling aqueous potash liberates ammonia and precipitates hydrated cobaltic oxide, Co2O3,3H2O, from them. This shows that the luteocobaltic salts correspond with cobaltic oxide, Co2O3, and those of the second kind with the dioxide. When a solution of luteocobaltic sulphate, Co2(SO4)3,12NH3,4H2O, is treated with baryta, barium sulphate is precipitated, and the solution contains luteocobaltic hydroxide, Co(OH)3,6NH3, which is soluble in water, is powerfully alkaline, absorbs the oxygen of the air, and when heated is decomposed with the evolution of ammonia. This compound therefore corresponds to a solution of cobaltic hydroxide in ammonia. The luteocobaltic salts contain 2 atoms of cobalt and 12 molecules of ammonia—that is, 6NH3to each atom of cobalt, like the salts of the first kind. The CoX2salts have a metallic taste, whilst those of luteocobalt and others have a purely saline taste, like the salts of the alkali metals. In the luteo-salts all the X's react (are ionised, as some chemists say) as in ordinary salts—for instance, all the Cl2is precipitated by a solution of AgNO3; all the (SO4)3gives a precipitate with BaX2, &c. The double salt formed with PtCl4is composed in the same manner as the potassium salt, K2PtCl4= 2KCl + PtCl4, that is, contains (CoCl3,6NH3)2,3PtCl4, or the amount of chlorine in the PtCl4is double that in the alkaline salt. In the rosepentamine (e), and rosetetramine (f), salts, also all the X's react or are ionised, but in the (g) and (h) salts only a portion of the X's react, and they are equal to the (e) and (f) salts minus water; this means that although the water dissolves them it is not combined with them, as PHO3differs from PH3O3; phenomena of this class correspond exactly to what has been already (Chapter XXI., Note7) mentioned respecting the green and violet salts of oxide of chromium.(d)The fuscocobaltic salts.An ammoniacal solution of cobalt salts acquires a brown colour in the air, due to the formation of these salts. They are also produced by the decomposition of salts of the second kind; they crystallise badly, and are separated from their solutions by addition of alcohol or an excess of ammonia. When boiled they give up the ammonia and cobaltic oxide which they contain. Hydrochloric and nitric acids give a yellow precipitate with these salts, which turns red when boiled, forming salts of the next category. The following is an example of the composition of two of the fuscocobaltic salts, Co2O(SO4)2,8NH3,4H2O and Co2OCl4,8NH3,3H2O. It is evident that the fuscocobaltic salts are ammoniacal compounds of basic cobaltic salts. The normal cobaltic sulphate ought to have the composition Co2(SO4)3= Co2O3,3SO3; the simplest basic salts will be Co2O(SO4)2= Co2O3)2SO3, and Co2O2(SO4) = Co2O3,SO3. The fuscocobaltic salts correspond with the first type of basic salts. They are changed (in concentrated solutions) into oxycobaltamine salts by absorption of one atom of oxygen, Co2O2(SO4)2. The whole process of oxidation will be as follows: first of all Co2X4, a cobaltous salt, is in the solution (X a univalent haloid, 2 molecules of the salt being taken), then Co2OX4, the basic cobaltic salt (4th series), then Co2O2X4, the salt of the dioxide (2nd series). The series of basic salts with an acid, 2HX, forms water and a normal salt, Co2X6(in 3, 5, 6 series). These salts are combined with various amounts of water and ammonia. Under many conditions the salts of fuscocobalt are easily transformed into salts of the next series. The salts of the series that has just been described contain 4 molecules of ammonia to 1 atom of cobalt.(e)The roseocobaltic(or rosepentamine), CoX2H2O,5NH3,salts, like the luteocobaltic, correspond with the normal cobaltic salts, but contain less ammonia, and an extra molecule of water. Thus the sulphate is obtained from cobaltous sulphate dissolved in ammonia and left exposed to the air until transformed into a brown solution of the fuscocobaltic salt; when this is treated with sulphuric acid a crystalline powder of the roseocobaltic salt, Co2(SO4)3,10NH3,5H2O, separates. The formation of this salt is easily understood: cobaltous sulphate in the presence of ammonia absorbs oxygen, and the solution of the fuscocobaltic salt will therefore contain, like cobaltous sulphate, one part of sulphuric acid to every part of cobalt, so that the whole process of formation may be expressed by the equation: 10NH3+ 2CoSO4+ H2SO4+ 4H2O + O = Co2(SO4)3,10NH3,5H2O. This salt forms tetragonal crystals of a red colour, slightly soluble in cold, but readily soluble in warm water. When the sulphate is treated with baryta, roseocobaltic hydroxide is formed in the solution, which absorbs the carbonic anhydride of the air. It is obtained from the next series by the action of alkalis.(f) Therosetetramine cobaltic saltsCoCl2,2H2O,4NH3were obtained by Jörgenson, and belong to the type of the luteo-salts, only with the substitution of 2NH3for H2O. Like the luteo- and roseo-salts they give double salts with PtCl4, similar to the alkaline double salts, for instance (Co2H2O,4NH3)2(SO4)2Cl2PtCl4. They are darker in colour than the preceding, but also crystallise well. They are formed by dissolving CoCO3in sulphuric acid (of a given strength), and after NH3and carbonate of ammonium have been added, air is passed through the solution (for oxidation) until the latter turns red. It is then evaporated with lumps of carbonate of ammonium, filtered from the precipitate and crystallised. A salt of the composition Co2(CO3)2(SO4),(2H2O,4NH3)2is thus obtained, from which the other salts may be easily prepared.(g) Thepurpureocobaltic salts, CoX3,5NH3, are also products of the direct oxidation of ammoniacal solutions of cobalt salts. They are easily obtained by heating the roseocobaltic and luteo-salts with strong acids. They are to all effects the same as the roseocobaltic salts, only anhydrous. Thus, for instance, the purpureocobaltic chloride, Co2Cl6,10NH3, or CoCl3,5NH3, is obtained by boiling the oxycobaltamine salts with ammonia. There is the same distinction between these salts and the preceding ones as between the various compounds of cobaltous chloride with water. In the purpureocobaltic only X2out of the X3react (are ionised). To the rosetetramine salts (f) there correspond thepurpureotetraminesalts, CoX3H2O,4NH3. The corresponding chromium purpureopentamine salt, CrCl3,5NH3is obtained with particular ease (Christensen, 1893). Dry anhydrous chromium chloride is treated with anhydrous liquid ammonia in a freezing mixture composed of liquid CO2and chlorine, and after some time the mixture is taken out of the freezing mixture, so that the excess of NH3boils away; the violet crystals then immediately acquire the red colour of the salt, CrCl3,5NH3, which is formed. The product is washed with water (to extract the luteo-salt, CrCl3,6NH3), which does not dissolve the salt, and it is then recrystallised from a hot solution of hydrochloric acid.(h) Thepraseocobaltic salts, CoX3,4NH3, are green, and form, with respect to the rosetetramine salts (f), the products of ultimate dehydration (for example, like metaphosphoric acid with respect to orthophosphoric acid, but in dissolving in water they give neither rosetetramine nor tetramine salts. (In my opinion one should expect salts with a still smaller amount of NH3, of the blue colour proper to the low hydrated compounds of cobalt; the green colour of the prazeo-salts already forms a step towards the blue.) Jörgenson obtained salts for ethylene-diamine, N2H4C2H4which replaces 2NH3. After being kept a long time in aqueous solution they give rosetetramine salts, just as metaphosphoric acid gives orthophosphoric acid, while the rosetetramine salts are converted into prazeo-salts by Ag2O and NaHO. Here only one X is ionised out of the X3. There are also basic salts of the same type; but the best known is the chromium salt called the rhodozochromic salt, Cr2(OH)3Cl3,6NH3,2H2O, which is formed by the prolonged action of water upon the corresponding roseo-salt.The cobaltamine compounds differ essentially but little from the ammoniacal compounds of other metals. The only difference is that here the cobaltic oxide is obtained from the cobaltous oxide in the presence of ammonia. In any case it is a simpler question than that of the double cyanides. Those forces in virtue of which such a considerable number of ammonia molecules are united with a molecule of a cobalt compound, appertain naturally to the series of those slightly investigated forces which exist even in the highest degrees of combination of the majority of elements. They are the same forces which lead to the formation of compounds containing water of crystallisation, double salts, isomorphous mixtures and complex acids (Chapter XXI., Note8 bis). The simplest conception, according to my opinion, of cobalt compounds (much more so than by assuming special complex radicles, with Schiff, Weltzien, Claus, and others), may be formed by comparing them with other ammoniacal products. Ammonia, like water, combines in various proportions with a multitude of molecules. Silver chloride and calcium chloride, just like cobalt chloride, absorb ammonia, forming compounds which are sometimes slightly stable, and easily dissociated, sometimes more stable, in exactly the same way as water combines with certain substances, forming fairly stable compounds called hydroxides or hydrates, or less stable compounds which are called compounds with water of crystallisation. Naturally the difference in the properties in both cases depends on the properties of those elements which enter into the composition of the given substance, and on those kinds of affinity towards which chemists have not as yet turned their attention. If boron fluoride, silicon fluoride, &c., combine with hydrofluoric acid, if platinic chloride, and even cadmium chloride, combine with hydrochloric acid, these compounds may be regarded as double salts, because acids are salts of hydrogen. But evidently water and ammonia have the same saline faculty, more especially as they, like haloid acids, contain hydrogen, and are both capable of further combination—for instance, ammonia with hydrochloric acid. Hence it is simpler to compare complex ammoniacal with double salts, hydrates, and similar compounds, butthe ammonio-metallic saltspresent a most complete qualitative and quantitative resemblance tothe hydrated salts of metals. The composition of the latter is MXnmH2O, where M = metal, X = the haloid, simple or complex, andnandmthe quantities of the haloid and so-called water of crystallisation respectively. The composition of the ammoniacal salts of metals is MXnmNH3. The water of crystallisation is held by the salt with more or less stability, and some salts even do not retain it at all; some part with water easily when exposed to the air, others when heated, and then with difficulty. In the case of some metals all the salts combine with water, whilst with others only a few, and the water so combined may then be easily disengaged. All this applies equally well to the ammoniacal salts, and therefore the combination of ammonia may be termedthe ammonia of crystallisation. Just as the water which is combined with a salt is held by it with different degrees of force, so it is with ammonia. In combining with 2NH3,PtCl2evolves 31,000 cals.; while CaCl2only evolves 14,000 cals.; and the former compound parts with its NH3(together with HCl in this case) with more difficulty, only above 200°, while the latter disengages ammonia at 180°. ZnCl2,2NH3in forming ZnCl2,4NH3evolves only 11,000 cals., and splits up again into its components at 80°. The amount of combined ammonia is as variable as the amount of water of crystallisation—for instance, SnI48NH3, CrCl28NH3, CrCl36NH3, CrCl35NH3,PtCl2,4NH3, &c. are known. Very often NH3is replaceable by OH2and conversely. A colourless, anhydrous cupric salt—for instance, cupric sulphate—when combined with water forms blue and green salts, and violet when combined with ammonia. If steam be passed through anhydrous copper sulphate the salt absorbs water and becomes heated; if ammonia be substituted for the water the heating becomes much more intense, and the salt breaks up into a fine violet powder. With water CuSO4,5H2O is formed, and with ammonia CuSO4,5NH3, the number of water and ammonia molecules retained by the salt being the same in each case, and as a proof of this, and that it is not an isolated coincidence, the remarkable fact must be borne in mind that water and ammonia consecutively, molecule for molecule, are capable of supplanting each other, and forming the compounds CuSO4,5H2O, CuSO4,4H2O,NH3; CuSO4,3H2O,2NH3; CuSO4,2H2O,3NH3; CuSO4,H2O,4NH3, and CuSO4,5NH3. The last of these compounds was obtained by Henry Rose, and my experiments have shown that more ammonia than this cannot be retained. By adding to a strong solution of cupric sulphate sufficient ammonia to dissolve the whole of the oxide precipitated, and then adding alcohol, Berzelius obtained the compound CuSO4,H2O,4NH3, &c. The law of substitution also assists in rendering these phenomena clearer, because a compound of ammonia with water forms ammonium hydroxide, NH4HO, and therefore these molecules combining with one another may also interchange, as being of equal value. In general, those salts form stable ammoniacal compounds which are capable of forming stable compounds with water of crystallisation; and as ammonia is capable of combining with acids, and as some of the salts formed by slightly energetic bases in their properties more closely resemble acids (that is, salts of hydrogen) than those salts containing more energetic bases, we might expect to find more stable and more easily-formed ammonio-metallic salts with metals and their oxides having weaker basic properties than with those which form energetic bases. This explains why the salts of potassium, barium, &c., do not form ammonio-metallic salts, whilst the salts of silver, copper, zinc, &c., easily form them, and the salts RX3still more easily and with greater stability. This consideration also accounts for the great stability of the ammoniacal compounds of cupric oxide compared with those of silver oxide, since the former is displaced by the latter. It also enables us to see clearly the distinction which exists in the stability of the cobaltamine salts containing salts corresponding with cobaltous oxide, and those corresponding with higher oxides of cobalt, for the latter are weaker bases than cobaltous oxides.The nature of the forces and quality of the phenomena occurring during the formation of the most stable substances, and of such compounds as crystallisable compounds, are one and the same, although perhaps exhibited in a different degree.This, in my opinion, may be best confirmed by examining the compounds of carbon, because for this element the nature of the forces acting during the formation of its compounds is well known. Let us take as an example two unstable compounds of carbon. Acetic acid, C2H4O2(specific gravity 1·06), with water forms the hydrate, C2H4O2,H2O, denser (1·07) than either of the components, but unstable and easily decomposed, generally simply referred to as a solution. Such also is the crystalline compound of oxalic acid, C2H2O4, with water, C2H2O4,2H2O. Their formation might be predicted as starting from the hydrocarbon C2H6, in which, as in any other, the hydrogen may be exchanged for chlorine, the water residue (hydroxyl), &c. The first substitution product with hydroxyl, C2H5(HO), is stable; it can be distilled without alteration, resists a temperature higher than 100°, and then does not give off water. This is ordinary alcohol. The second, C2H4(HO)2, can also be distilled without change, but can be decomposed into water and C2H4O (ethylene oxide or aldehyde); it boils at about 197°, whilst the first hydrate boils at 78°, a difference of about 100°. The compound C2H3(HO)3will be the third product of such substitution; it ought to boil at about 300°, but does not resist this temperature—it decomposes into H2O and C2H4O2, where only one hydroxyl group remains, and the other atom of oxygen is left in the same condition as in ethylene oxide, C2H4O. There is a proof of this. Glycol, C2H4(HO)2, boils at 197°, and forms water and ethylene oxide, which boils at 13° (aldehyde, its isomeride, boils at 21°); therefore the product disengaged by the splitting up of the hydrate boils at 184° lower than the hydrate C2H4(HO)2. Thus the hydrate C2H3(HO)3, which ought to boil at about 300°, splits up in exactly the same way into water and the product C2H4O2, which boils at 117°—that is, nearly 183° lower than the hydrate, C2H3(HO)3. But this hydrate splits up before distillation. The above-mentioned hydrate of acetic acid is such a decomposable hydrate—that is to say, what is called a solution. Still less stability may be expected from the following hydrates. C2H2(HO)4also splits up into water and a hydrate (it contains two hydroxyl groups) called glycolic acid, C2H2O(HO)2= C2H4O3. The next product of substitution will be C2H(HO)5; it splits up into water, H2O, and glyoxylic acid, C2H4O4(three hydroxyl groups). The last hydrate which ought to be obtained from C2H6, and ought to contain C2(HO)6, is the crystalline compound of oxalic acid, C2H2O4(two hydroxyl groups), and water, 2H2O, which has been already mentioned. The hydrate C2(HO)6= C2H2O4,2H2O, ought, according to the foregoing reasoning, to boil at about 600° (because the hydrate, C2H4(HO)2, boils at about 200°, and the substitution of 4 hydroxyl groups for 4 atoms of hydrogen will raise the boiling-point 400°). It does not resist this temperature, but at a much lower point splits up into water, 2H2O, and the hydrate C2O2(HO)2, which is also capable of yielding water. Without going into further discussion of this subject, it may be observed that the formation of the hydrates, or compounds with water of crystallisation, of acetic and oxalic acids has thus received an accurate explanation, illustrating the point we desired to prove in affirming that compounds with water of crystallisation are held together by the same forces as those which act in the formation of other complex substances, and that the easy displaceability of the water of crystallisation is only a peculiarity of a local character, and not a radical point of distinction. All the above-mentioned hydrates, C2X6, or products of their destruction, are actually obtained by the oxidation of the first hydrate, C2H3(HO), or common alcohol, by nitric acid (Sokoloff and others). Hence the forces which induce salts to combine withnH2O or with NH3are undoubtedly of the same order as the forces which govern the formation of ordinary ‘atomic’ and saline compounds. (A great impediment in the study of the former was caused by the conviction which reigned in the sixties and seventies, that ‘atomic’ were essentially different from ‘molecular’ compounds like crystallohydrates, in which it was assumed that there was a combination of entire molecules, as though without the participation of the atomic forces.) If the bond between chlorine and different metals is not equally strong, so also the bond unitingnH2O andnNH3is exceeding variable; there is nothing very surprising in this. And in the fact that the combination of different amounts of NH3and H2O alters the capacity of the haloids X of the salts RX2for reaction (for instance, in the luteo-salts all the X3, while in the purpureo, only 2 out of the 3, and in the prazeo-salts only 1 of the 3 X's reacts), we should see in the first place a phenomenon similar to what we met with in Cr2Cl6(Chapter XXI., Note7 bis), for in both instances the essence of the difference lies in the removal of water; a molecule RCl3,6H2O or RCl3,6NH3contains the halogen in a perfectly mobile (ionised) state, while in the molecule RCl3,5H2O or RCl3,5NH3a portion of the halogen has almost lost its faculty for reacting with AgNO3, just as metalepsical chlorine has lost this faculty which is fully developed in the chloranhydride. Until the reason of this difference be clear, we cannot expect that ordinary points of view and generalisation can give a clear answer. However, we may assume that here the explanation lies in the nature and kind of motion of the atoms in the molecules, although as yet it is not clear how. Nevertheless, I think it well to call attention again (ChapterI.) to the fact that the combination of water, and hence, also, of any other element, leads to most diverse consequences; the water in the gelatinous hydrate of alumina or in the decahydrated Glauber salt is very mobile, and easily reacts like water in a free state; but the same water combined with oxide of calcium, or C2H4(for instance, in C2H6O and in C4H10O), or with P2O5, has become quite different, and no longer acts like water in a free state. We see the same phenomenon in many other cases—for example, the chlorine in chlorates no longer gives a precipitate of chloride of silver with AgNO3. Thus, although the instance which is found in the difference between the roseo- and purpureo-salts deserves to be fully studied on account of its simplicity, still it is far from being exceptional, and we cannot expect it to be thoroughly explained unless a mass of similar instances, which are exceedingly common among chemical compounds, be conjointly explained. (Among the researches which add to our knowledge respecting the complex ammoniacal compounds, I think it indispensable to call the reader's attention to Prof. Kournakoff's dissertation ‘On complex metallic bases,’ 1893.)Kournakoff (1894) showed that the solubility of the luteo-salt, CoCl3,6NH3, at 0° = 4·30 (per 100 of water), at 20° = 7·7, that in passing into the roseo-salt, CoCl3H2O5NH3, the solubility rises considerably, and at 0° = 16·4, and at 20° = about 27, whilst the passage into the purpureo-salt, CoCl3,5NH3, is accompanied by a great fall in the solubility, namely, at 0° = 0·23, and at 20° = about 0·5. And as crystallohydrates with a smaller amount of water are usually more soluble than the higher crystallohydrates (Le Chatelier), whilst here we find that the solubility falls (in the purpureo-salt) with a loss of water, that water which is contained in the roseo-salt cannot be compared with the water of crystallisation. Kournakoff, therefore, connects the fall in solubility (in the passage of the roseo- into the purpureo-salts) with the accompanying loss in the reactive capacity of the chlorine.In conclusion, it may be observed that the elements of the eighth group—that is, the analogues of iron and platinum—according to my opinion, will yield most fruitful results when studied as to combinations with whole molecules, as already shown by the examples of complex ammoniacal, cyanogen, nitro-, and other compounds, which are easily formed in this eighth group, and are remarkable for their stability. This faculty of the elements of the eighth group for forming the complex compounds alluded to, is in all probability connected with the position which the eighth group occupies with regard to the others. Following the seventh, which forms the type RX7, it might be expected to contain the most complex type, RX8. This is met with in OsO4. The other elements of the eighth group, however, only form the lower types RX2, RX3, RX4… and these accordingly should be expected to aggregate themselves into the higher types, which is accomplished in the formation of the above-mentioned complex compounds.
[35]The cobalt salts may be divided into at least the following classes, which repeat themselves for Cr, Ir, Rh (we shall not stop to consider the latter, particularly as they closely resemble the cobalt salts):—
(a)Ammonium cobalt salts, which are simply direct compounds of the cobaltous salts CoX2with ammonia, similar to various other compounds of the salts of silver, copper, and even calcium and magnesium, with ammonia. They are easily crystallised from an ammoniacal solution, and have a pink colour. Thus, for instance, when cobaltous chloride in solution is mixed with sufficient ammonia to redissolve the precipitate first formed, octahedral crystals are deposited which have a composition CoCl2,H2O,6NH3. These salts are nothing else but combinations with ammonia of crystallisation—if it may be so termed—likening them in this way to combinations with water of crystallisation. This similarity is evident both from their composition and from their capability of giving off ammonia at various temperatures. The most important point to observe is that all these salts contain 6 molecules of ammonia to 1 atom of cobalt, and this ammonia is held in fairly stable connection. Water decomposes these salts. (Nickel behaves similarly without forming other compounds corresponding to the true cobaltic.)
(b) The solutions of the above-mentioned salts are rendered turbid by the action of the air; they absorb oxygen and become covered with a crust ofoxycobaltamine salts. The latter are sparingly soluble in aqueous ammonia, have a brown colour, and are characterised by the fact that with warm waterthey evolve oxygen, forming salts of the following category: The nitrate may be taken as an example of this kind of salt; its composition is CoN2O7,5NH3,H2O. It differs from cobaltous nitrate, Co(NO3)2, in containing an extra atom of oxygen—that is, it corresponds with cobalt dioxide, CoO2, in the same way that the first salts correspond with cobaltous oxide; they contain 5, and not 6, molecules of ammonia, as if NH3had been replaced by O, but we shall afterwards meet compounds containing either 5NH3or 6NH3to each atom of cobalt.
(c)The luteocobaltic saltsare thus called because they have a yellow (luteus) colour. They are obtained from the salts of the first kind by submitting them in dilute solution to the action of the air; in this case salts of the second kind are not formed, because they are decomposed by an excess of water, with the evolution of oxygen and the formation of luteocobaltic salts. By the action of ammonia the salts of the fifth kind (roseocobaltic) are also converted into luteocobaltic salts. These last-named salts generally crystallise readily, and have a yellow colour; they are comparatively much more stable than the preceding ones, and even for a certain time resist the action of boiling water. Boiling aqueous potash liberates ammonia and precipitates hydrated cobaltic oxide, Co2O3,3H2O, from them. This shows that the luteocobaltic salts correspond with cobaltic oxide, Co2O3, and those of the second kind with the dioxide. When a solution of luteocobaltic sulphate, Co2(SO4)3,12NH3,4H2O, is treated with baryta, barium sulphate is precipitated, and the solution contains luteocobaltic hydroxide, Co(OH)3,6NH3, which is soluble in water, is powerfully alkaline, absorbs the oxygen of the air, and when heated is decomposed with the evolution of ammonia. This compound therefore corresponds to a solution of cobaltic hydroxide in ammonia. The luteocobaltic salts contain 2 atoms of cobalt and 12 molecules of ammonia—that is, 6NH3to each atom of cobalt, like the salts of the first kind. The CoX2salts have a metallic taste, whilst those of luteocobalt and others have a purely saline taste, like the salts of the alkali metals. In the luteo-salts all the X's react (are ionised, as some chemists say) as in ordinary salts—for instance, all the Cl2is precipitated by a solution of AgNO3; all the (SO4)3gives a precipitate with BaX2, &c. The double salt formed with PtCl4is composed in the same manner as the potassium salt, K2PtCl4= 2KCl + PtCl4, that is, contains (CoCl3,6NH3)2,3PtCl4, or the amount of chlorine in the PtCl4is double that in the alkaline salt. In the rosepentamine (e), and rosetetramine (f), salts, also all the X's react or are ionised, but in the (g) and (h) salts only a portion of the X's react, and they are equal to the (e) and (f) salts minus water; this means that although the water dissolves them it is not combined with them, as PHO3differs from PH3O3; phenomena of this class correspond exactly to what has been already (Chapter XXI., Note7) mentioned respecting the green and violet salts of oxide of chromium.
(d)The fuscocobaltic salts.An ammoniacal solution of cobalt salts acquires a brown colour in the air, due to the formation of these salts. They are also produced by the decomposition of salts of the second kind; they crystallise badly, and are separated from their solutions by addition of alcohol or an excess of ammonia. When boiled they give up the ammonia and cobaltic oxide which they contain. Hydrochloric and nitric acids give a yellow precipitate with these salts, which turns red when boiled, forming salts of the next category. The following is an example of the composition of two of the fuscocobaltic salts, Co2O(SO4)2,8NH3,4H2O and Co2OCl4,8NH3,3H2O. It is evident that the fuscocobaltic salts are ammoniacal compounds of basic cobaltic salts. The normal cobaltic sulphate ought to have the composition Co2(SO4)3= Co2O3,3SO3; the simplest basic salts will be Co2O(SO4)2= Co2O3)2SO3, and Co2O2(SO4) = Co2O3,SO3. The fuscocobaltic salts correspond with the first type of basic salts. They are changed (in concentrated solutions) into oxycobaltamine salts by absorption of one atom of oxygen, Co2O2(SO4)2. The whole process of oxidation will be as follows: first of all Co2X4, a cobaltous salt, is in the solution (X a univalent haloid, 2 molecules of the salt being taken), then Co2OX4, the basic cobaltic salt (4th series), then Co2O2X4, the salt of the dioxide (2nd series). The series of basic salts with an acid, 2HX, forms water and a normal salt, Co2X6(in 3, 5, 6 series). These salts are combined with various amounts of water and ammonia. Under many conditions the salts of fuscocobalt are easily transformed into salts of the next series. The salts of the series that has just been described contain 4 molecules of ammonia to 1 atom of cobalt.
(e)The roseocobaltic(or rosepentamine), CoX2H2O,5NH3,salts, like the luteocobaltic, correspond with the normal cobaltic salts, but contain less ammonia, and an extra molecule of water. Thus the sulphate is obtained from cobaltous sulphate dissolved in ammonia and left exposed to the air until transformed into a brown solution of the fuscocobaltic salt; when this is treated with sulphuric acid a crystalline powder of the roseocobaltic salt, Co2(SO4)3,10NH3,5H2O, separates. The formation of this salt is easily understood: cobaltous sulphate in the presence of ammonia absorbs oxygen, and the solution of the fuscocobaltic salt will therefore contain, like cobaltous sulphate, one part of sulphuric acid to every part of cobalt, so that the whole process of formation may be expressed by the equation: 10NH3+ 2CoSO4+ H2SO4+ 4H2O + O = Co2(SO4)3,10NH3,5H2O. This salt forms tetragonal crystals of a red colour, slightly soluble in cold, but readily soluble in warm water. When the sulphate is treated with baryta, roseocobaltic hydroxide is formed in the solution, which absorbs the carbonic anhydride of the air. It is obtained from the next series by the action of alkalis.
(f) Therosetetramine cobaltic saltsCoCl2,2H2O,4NH3were obtained by Jörgenson, and belong to the type of the luteo-salts, only with the substitution of 2NH3for H2O. Like the luteo- and roseo-salts they give double salts with PtCl4, similar to the alkaline double salts, for instance (Co2H2O,4NH3)2(SO4)2Cl2PtCl4. They are darker in colour than the preceding, but also crystallise well. They are formed by dissolving CoCO3in sulphuric acid (of a given strength), and after NH3and carbonate of ammonium have been added, air is passed through the solution (for oxidation) until the latter turns red. It is then evaporated with lumps of carbonate of ammonium, filtered from the precipitate and crystallised. A salt of the composition Co2(CO3)2(SO4),(2H2O,4NH3)2is thus obtained, from which the other salts may be easily prepared.
(g) Thepurpureocobaltic salts, CoX3,5NH3, are also products of the direct oxidation of ammoniacal solutions of cobalt salts. They are easily obtained by heating the roseocobaltic and luteo-salts with strong acids. They are to all effects the same as the roseocobaltic salts, only anhydrous. Thus, for instance, the purpureocobaltic chloride, Co2Cl6,10NH3, or CoCl3,5NH3, is obtained by boiling the oxycobaltamine salts with ammonia. There is the same distinction between these salts and the preceding ones as between the various compounds of cobaltous chloride with water. In the purpureocobaltic only X2out of the X3react (are ionised). To the rosetetramine salts (f) there correspond thepurpureotetraminesalts, CoX3H2O,4NH3. The corresponding chromium purpureopentamine salt, CrCl3,5NH3is obtained with particular ease (Christensen, 1893). Dry anhydrous chromium chloride is treated with anhydrous liquid ammonia in a freezing mixture composed of liquid CO2and chlorine, and after some time the mixture is taken out of the freezing mixture, so that the excess of NH3boils away; the violet crystals then immediately acquire the red colour of the salt, CrCl3,5NH3, which is formed. The product is washed with water (to extract the luteo-salt, CrCl3,6NH3), which does not dissolve the salt, and it is then recrystallised from a hot solution of hydrochloric acid.
(h) Thepraseocobaltic salts, CoX3,4NH3, are green, and form, with respect to the rosetetramine salts (f), the products of ultimate dehydration (for example, like metaphosphoric acid with respect to orthophosphoric acid, but in dissolving in water they give neither rosetetramine nor tetramine salts. (In my opinion one should expect salts with a still smaller amount of NH3, of the blue colour proper to the low hydrated compounds of cobalt; the green colour of the prazeo-salts already forms a step towards the blue.) Jörgenson obtained salts for ethylene-diamine, N2H4C2H4which replaces 2NH3. After being kept a long time in aqueous solution they give rosetetramine salts, just as metaphosphoric acid gives orthophosphoric acid, while the rosetetramine salts are converted into prazeo-salts by Ag2O and NaHO. Here only one X is ionised out of the X3. There are also basic salts of the same type; but the best known is the chromium salt called the rhodozochromic salt, Cr2(OH)3Cl3,6NH3,2H2O, which is formed by the prolonged action of water upon the corresponding roseo-salt.
The cobaltamine compounds differ essentially but little from the ammoniacal compounds of other metals. The only difference is that here the cobaltic oxide is obtained from the cobaltous oxide in the presence of ammonia. In any case it is a simpler question than that of the double cyanides. Those forces in virtue of which such a considerable number of ammonia molecules are united with a molecule of a cobalt compound, appertain naturally to the series of those slightly investigated forces which exist even in the highest degrees of combination of the majority of elements. They are the same forces which lead to the formation of compounds containing water of crystallisation, double salts, isomorphous mixtures and complex acids (Chapter XXI., Note8 bis). The simplest conception, according to my opinion, of cobalt compounds (much more so than by assuming special complex radicles, with Schiff, Weltzien, Claus, and others), may be formed by comparing them with other ammoniacal products. Ammonia, like water, combines in various proportions with a multitude of molecules. Silver chloride and calcium chloride, just like cobalt chloride, absorb ammonia, forming compounds which are sometimes slightly stable, and easily dissociated, sometimes more stable, in exactly the same way as water combines with certain substances, forming fairly stable compounds called hydroxides or hydrates, or less stable compounds which are called compounds with water of crystallisation. Naturally the difference in the properties in both cases depends on the properties of those elements which enter into the composition of the given substance, and on those kinds of affinity towards which chemists have not as yet turned their attention. If boron fluoride, silicon fluoride, &c., combine with hydrofluoric acid, if platinic chloride, and even cadmium chloride, combine with hydrochloric acid, these compounds may be regarded as double salts, because acids are salts of hydrogen. But evidently water and ammonia have the same saline faculty, more especially as they, like haloid acids, contain hydrogen, and are both capable of further combination—for instance, ammonia with hydrochloric acid. Hence it is simpler to compare complex ammoniacal with double salts, hydrates, and similar compounds, butthe ammonio-metallic saltspresent a most complete qualitative and quantitative resemblance tothe hydrated salts of metals. The composition of the latter is MXnmH2O, where M = metal, X = the haloid, simple or complex, andnandmthe quantities of the haloid and so-called water of crystallisation respectively. The composition of the ammoniacal salts of metals is MXnmNH3. The water of crystallisation is held by the salt with more or less stability, and some salts even do not retain it at all; some part with water easily when exposed to the air, others when heated, and then with difficulty. In the case of some metals all the salts combine with water, whilst with others only a few, and the water so combined may then be easily disengaged. All this applies equally well to the ammoniacal salts, and therefore the combination of ammonia may be termedthe ammonia of crystallisation. Just as the water which is combined with a salt is held by it with different degrees of force, so it is with ammonia. In combining with 2NH3,PtCl2evolves 31,000 cals.; while CaCl2only evolves 14,000 cals.; and the former compound parts with its NH3(together with HCl in this case) with more difficulty, only above 200°, while the latter disengages ammonia at 180°. ZnCl2,2NH3in forming ZnCl2,4NH3evolves only 11,000 cals., and splits up again into its components at 80°. The amount of combined ammonia is as variable as the amount of water of crystallisation—for instance, SnI48NH3, CrCl28NH3, CrCl36NH3, CrCl35NH3,PtCl2,4NH3, &c. are known. Very often NH3is replaceable by OH2and conversely. A colourless, anhydrous cupric salt—for instance, cupric sulphate—when combined with water forms blue and green salts, and violet when combined with ammonia. If steam be passed through anhydrous copper sulphate the salt absorbs water and becomes heated; if ammonia be substituted for the water the heating becomes much more intense, and the salt breaks up into a fine violet powder. With water CuSO4,5H2O is formed, and with ammonia CuSO4,5NH3, the number of water and ammonia molecules retained by the salt being the same in each case, and as a proof of this, and that it is not an isolated coincidence, the remarkable fact must be borne in mind that water and ammonia consecutively, molecule for molecule, are capable of supplanting each other, and forming the compounds CuSO4,5H2O, CuSO4,4H2O,NH3; CuSO4,3H2O,2NH3; CuSO4,2H2O,3NH3; CuSO4,H2O,4NH3, and CuSO4,5NH3. The last of these compounds was obtained by Henry Rose, and my experiments have shown that more ammonia than this cannot be retained. By adding to a strong solution of cupric sulphate sufficient ammonia to dissolve the whole of the oxide precipitated, and then adding alcohol, Berzelius obtained the compound CuSO4,H2O,4NH3, &c. The law of substitution also assists in rendering these phenomena clearer, because a compound of ammonia with water forms ammonium hydroxide, NH4HO, and therefore these molecules combining with one another may also interchange, as being of equal value. In general, those salts form stable ammoniacal compounds which are capable of forming stable compounds with water of crystallisation; and as ammonia is capable of combining with acids, and as some of the salts formed by slightly energetic bases in their properties more closely resemble acids (that is, salts of hydrogen) than those salts containing more energetic bases, we might expect to find more stable and more easily-formed ammonio-metallic salts with metals and their oxides having weaker basic properties than with those which form energetic bases. This explains why the salts of potassium, barium, &c., do not form ammonio-metallic salts, whilst the salts of silver, copper, zinc, &c., easily form them, and the salts RX3still more easily and with greater stability. This consideration also accounts for the great stability of the ammoniacal compounds of cupric oxide compared with those of silver oxide, since the former is displaced by the latter. It also enables us to see clearly the distinction which exists in the stability of the cobaltamine salts containing salts corresponding with cobaltous oxide, and those corresponding with higher oxides of cobalt, for the latter are weaker bases than cobaltous oxides.The nature of the forces and quality of the phenomena occurring during the formation of the most stable substances, and of such compounds as crystallisable compounds, are one and the same, although perhaps exhibited in a different degree.This, in my opinion, may be best confirmed by examining the compounds of carbon, because for this element the nature of the forces acting during the formation of its compounds is well known. Let us take as an example two unstable compounds of carbon. Acetic acid, C2H4O2(specific gravity 1·06), with water forms the hydrate, C2H4O2,H2O, denser (1·07) than either of the components, but unstable and easily decomposed, generally simply referred to as a solution. Such also is the crystalline compound of oxalic acid, C2H2O4, with water, C2H2O4,2H2O. Their formation might be predicted as starting from the hydrocarbon C2H6, in which, as in any other, the hydrogen may be exchanged for chlorine, the water residue (hydroxyl), &c. The first substitution product with hydroxyl, C2H5(HO), is stable; it can be distilled without alteration, resists a temperature higher than 100°, and then does not give off water. This is ordinary alcohol. The second, C2H4(HO)2, can also be distilled without change, but can be decomposed into water and C2H4O (ethylene oxide or aldehyde); it boils at about 197°, whilst the first hydrate boils at 78°, a difference of about 100°. The compound C2H3(HO)3will be the third product of such substitution; it ought to boil at about 300°, but does not resist this temperature—it decomposes into H2O and C2H4O2, where only one hydroxyl group remains, and the other atom of oxygen is left in the same condition as in ethylene oxide, C2H4O. There is a proof of this. Glycol, C2H4(HO)2, boils at 197°, and forms water and ethylene oxide, which boils at 13° (aldehyde, its isomeride, boils at 21°); therefore the product disengaged by the splitting up of the hydrate boils at 184° lower than the hydrate C2H4(HO)2. Thus the hydrate C2H3(HO)3, which ought to boil at about 300°, splits up in exactly the same way into water and the product C2H4O2, which boils at 117°—that is, nearly 183° lower than the hydrate, C2H3(HO)3. But this hydrate splits up before distillation. The above-mentioned hydrate of acetic acid is such a decomposable hydrate—that is to say, what is called a solution. Still less stability may be expected from the following hydrates. C2H2(HO)4also splits up into water and a hydrate (it contains two hydroxyl groups) called glycolic acid, C2H2O(HO)2= C2H4O3. The next product of substitution will be C2H(HO)5; it splits up into water, H2O, and glyoxylic acid, C2H4O4(three hydroxyl groups). The last hydrate which ought to be obtained from C2H6, and ought to contain C2(HO)6, is the crystalline compound of oxalic acid, C2H2O4(two hydroxyl groups), and water, 2H2O, which has been already mentioned. The hydrate C2(HO)6= C2H2O4,2H2O, ought, according to the foregoing reasoning, to boil at about 600° (because the hydrate, C2H4(HO)2, boils at about 200°, and the substitution of 4 hydroxyl groups for 4 atoms of hydrogen will raise the boiling-point 400°). It does not resist this temperature, but at a much lower point splits up into water, 2H2O, and the hydrate C2O2(HO)2, which is also capable of yielding water. Without going into further discussion of this subject, it may be observed that the formation of the hydrates, or compounds with water of crystallisation, of acetic and oxalic acids has thus received an accurate explanation, illustrating the point we desired to prove in affirming that compounds with water of crystallisation are held together by the same forces as those which act in the formation of other complex substances, and that the easy displaceability of the water of crystallisation is only a peculiarity of a local character, and not a radical point of distinction. All the above-mentioned hydrates, C2X6, or products of their destruction, are actually obtained by the oxidation of the first hydrate, C2H3(HO), or common alcohol, by nitric acid (Sokoloff and others). Hence the forces which induce salts to combine withnH2O or with NH3are undoubtedly of the same order as the forces which govern the formation of ordinary ‘atomic’ and saline compounds. (A great impediment in the study of the former was caused by the conviction which reigned in the sixties and seventies, that ‘atomic’ were essentially different from ‘molecular’ compounds like crystallohydrates, in which it was assumed that there was a combination of entire molecules, as though without the participation of the atomic forces.) If the bond between chlorine and different metals is not equally strong, so also the bond unitingnH2O andnNH3is exceeding variable; there is nothing very surprising in this. And in the fact that the combination of different amounts of NH3and H2O alters the capacity of the haloids X of the salts RX2for reaction (for instance, in the luteo-salts all the X3, while in the purpureo, only 2 out of the 3, and in the prazeo-salts only 1 of the 3 X's reacts), we should see in the first place a phenomenon similar to what we met with in Cr2Cl6(Chapter XXI., Note7 bis), for in both instances the essence of the difference lies in the removal of water; a molecule RCl3,6H2O or RCl3,6NH3contains the halogen in a perfectly mobile (ionised) state, while in the molecule RCl3,5H2O or RCl3,5NH3a portion of the halogen has almost lost its faculty for reacting with AgNO3, just as metalepsical chlorine has lost this faculty which is fully developed in the chloranhydride. Until the reason of this difference be clear, we cannot expect that ordinary points of view and generalisation can give a clear answer. However, we may assume that here the explanation lies in the nature and kind of motion of the atoms in the molecules, although as yet it is not clear how. Nevertheless, I think it well to call attention again (ChapterI.) to the fact that the combination of water, and hence, also, of any other element, leads to most diverse consequences; the water in the gelatinous hydrate of alumina or in the decahydrated Glauber salt is very mobile, and easily reacts like water in a free state; but the same water combined with oxide of calcium, or C2H4(for instance, in C2H6O and in C4H10O), or with P2O5, has become quite different, and no longer acts like water in a free state. We see the same phenomenon in many other cases—for example, the chlorine in chlorates no longer gives a precipitate of chloride of silver with AgNO3. Thus, although the instance which is found in the difference between the roseo- and purpureo-salts deserves to be fully studied on account of its simplicity, still it is far from being exceptional, and we cannot expect it to be thoroughly explained unless a mass of similar instances, which are exceedingly common among chemical compounds, be conjointly explained. (Among the researches which add to our knowledge respecting the complex ammoniacal compounds, I think it indispensable to call the reader's attention to Prof. Kournakoff's dissertation ‘On complex metallic bases,’ 1893.)
Kournakoff (1894) showed that the solubility of the luteo-salt, CoCl3,6NH3, at 0° = 4·30 (per 100 of water), at 20° = 7·7, that in passing into the roseo-salt, CoCl3H2O5NH3, the solubility rises considerably, and at 0° = 16·4, and at 20° = about 27, whilst the passage into the purpureo-salt, CoCl3,5NH3, is accompanied by a great fall in the solubility, namely, at 0° = 0·23, and at 20° = about 0·5. And as crystallohydrates with a smaller amount of water are usually more soluble than the higher crystallohydrates (Le Chatelier), whilst here we find that the solubility falls (in the purpureo-salt) with a loss of water, that water which is contained in the roseo-salt cannot be compared with the water of crystallisation. Kournakoff, therefore, connects the fall in solubility (in the passage of the roseo- into the purpureo-salts) with the accompanying loss in the reactive capacity of the chlorine.
In conclusion, it may be observed that the elements of the eighth group—that is, the analogues of iron and platinum—according to my opinion, will yield most fruitful results when studied as to combinations with whole molecules, as already shown by the examples of complex ammoniacal, cyanogen, nitro-, and other compounds, which are easily formed in this eighth group, and are remarkable for their stability. This faculty of the elements of the eighth group for forming the complex compounds alluded to, is in all probability connected with the position which the eighth group occupies with regard to the others. Following the seventh, which forms the type RX7, it might be expected to contain the most complex type, RX8. This is met with in OsO4. The other elements of the eighth group, however, only form the lower types RX2, RX3, RX4… and these accordingly should be expected to aggregate themselves into the higher types, which is accomplished in the formation of the above-mentioned complex compounds.
[35 bis]Marshall (1891) obtained cobaltic sulphate, Co2(SO4)3,18H2O, by the action of an electric current upon a strong solution of CoSO4.
[35 bis]Marshall (1891) obtained cobaltic sulphate, Co2(SO4)3,18H2O, by the action of an electric current upon a strong solution of CoSO4.
[36]The action of an alkaline hypochlorite or hypobromite upon a boiling solution of cobaltous salts, according to Schroederer (1889), produces oxides, whose composition varies between Co3O5(Rose's compound) and Co2O3, and also between Co5O8and Co12O19. If caustic potash and then bromine be added to the liquid, only Co2O3is formed. The action of alkaline hypochlorites or hypo-bromites, or of iodine, upon cobaltic salts, gives a highly-coloured precipitate which has a different colour to the hydrate of the oxide Co2(OH)6. According to Carnot the precipitate produced by the hypochlorites has a composition Co10O16, whilst that given by iodine in the presence of an alkali contains a larger amount of oxygen. Fortmann (1891) re-investigated the composition of the higher oxygen oxide obtained by iodine in the presence of alkali, and found that the greenish precipitate (which disengages oxygen when heated to 100°) corresponds to the formula CoO2. The reaction must be expressed by the equation: CoX2+ I2+ 4KHO = CoO2+ 2KX + 2KI + 2H2O.
[36]The action of an alkaline hypochlorite or hypobromite upon a boiling solution of cobaltous salts, according to Schroederer (1889), produces oxides, whose composition varies between Co3O5(Rose's compound) and Co2O3, and also between Co5O8and Co12O19. If caustic potash and then bromine be added to the liquid, only Co2O3is formed. The action of alkaline hypochlorites or hypo-bromites, or of iodine, upon cobaltic salts, gives a highly-coloured precipitate which has a different colour to the hydrate of the oxide Co2(OH)6. According to Carnot the precipitate produced by the hypochlorites has a composition Co10O16, whilst that given by iodine in the presence of an alkali contains a larger amount of oxygen. Fortmann (1891) re-investigated the composition of the higher oxygen oxide obtained by iodine in the presence of alkali, and found that the greenish precipitate (which disengages oxygen when heated to 100°) corresponds to the formula CoO2. The reaction must be expressed by the equation: CoX2+ I2+ 4KHO = CoO2+ 2KX + 2KI + 2H2O.
[37]Prior to Fortmann, Rousseau (1889) endeavoured to solve the question as to whether CoO2was able to combine with bases. He succeeded in obtaining a barium compound corresponding to this oxide. Fifteen grams of BaCl2or BaBr2are triturated with 5–6 grams of oxide of barium, and the mixture heated to redness in a closed platinum crucible; 1 gram of oxide of cobalt is then gradually added to the fused mass. Each addition of oxide is accompanied by a violent disengagement of oxygen. After a short time, however, the mass fuses quietly, and a salt settles at the bottom of the crucible, which, when freed from the residue, appears as black hexagonal, very brilliant crystals. In dissolving in water this substance evolves chlorine; its composition corresponds to the formula 2(CoO2)BaO. If the original mass be heated for a long time (40 hours), the amount of dioxide in the resultant mass decreases. The author obtained a neutral salt having the composition CoO2BaO (this compound = BaO2CoO) by breaking up the mass as it agglomerates together, and bringing the pieces into contact with the more heated surface of the crucible. This salt is formed between the somewhat narrow limits of temperature 1,000°-1,100°; above and below these limits compounds richer or poorer in CoO2are formed. The formation of CoO2by the action of BaO2, and the easy decomposition of CoO2with the evolution of oxygen, give reason for thinking that it belongs to the class of peroxides (like Cr2O7, CaO2, &c.); it is not yet known whether they give peroxide of hydrogen like the true peroxides. The fact that it is obtained by means of iodine (probably through HIO), and its great resemblance to MnO2, leads rather to the supposition that CoO2is a very feeble saline oxide. The form CoO2is repeated in the cobaltic compounds (Note35), and the existence of CoO2should have long ago been recognised upon this basis.
[37]Prior to Fortmann, Rousseau (1889) endeavoured to solve the question as to whether CoO2was able to combine with bases. He succeeded in obtaining a barium compound corresponding to this oxide. Fifteen grams of BaCl2or BaBr2are triturated with 5–6 grams of oxide of barium, and the mixture heated to redness in a closed platinum crucible; 1 gram of oxide of cobalt is then gradually added to the fused mass. Each addition of oxide is accompanied by a violent disengagement of oxygen. After a short time, however, the mass fuses quietly, and a salt settles at the bottom of the crucible, which, when freed from the residue, appears as black hexagonal, very brilliant crystals. In dissolving in water this substance evolves chlorine; its composition corresponds to the formula 2(CoO2)BaO. If the original mass be heated for a long time (40 hours), the amount of dioxide in the resultant mass decreases. The author obtained a neutral salt having the composition CoO2BaO (this compound = BaO2CoO) by breaking up the mass as it agglomerates together, and bringing the pieces into contact with the more heated surface of the crucible. This salt is formed between the somewhat narrow limits of temperature 1,000°-1,100°; above and below these limits compounds richer or poorer in CoO2are formed. The formation of CoO2by the action of BaO2, and the easy decomposition of CoO2with the evolution of oxygen, give reason for thinking that it belongs to the class of peroxides (like Cr2O7, CaO2, &c.); it is not yet known whether they give peroxide of hydrogen like the true peroxides. The fact that it is obtained by means of iodine (probably through HIO), and its great resemblance to MnO2, leads rather to the supposition that CoO2is a very feeble saline oxide. The form CoO2is repeated in the cobaltic compounds (Note35), and the existence of CoO2should have long ago been recognised upon this basis.
[38]This compound is known as nickel tetra-carbonyl. It appears to me yet premature to judge of the structure of such an extraordinary compound as Ni(CO)4. It has long been known that potassium combines with CO forming Kn(CO)n(Chapter IX., Note31), but this substance is apparently saline and non-volatile, and has as little in common with Ni(CO)4as Na2H has with SbH3. However, Berthelot observed that when NiC4O4is kept in air, it oxidises and gives a colourless compound, Ni3C2O3,10H2O, having apparently saline properties. We may add that Schützenberger, on reducing NiCl2by heating it in a current of hydrogen, observed that a nickel compound partially volatilises with the HCl and gives metallic nickel when heated again. The platinum compound, PtCl2(CO)3(Chapter XXIII., Note11), offers the greatest analogy to Ni(CO)4. This compound was obtained as a volatile substance by Schützenberger by moderately heating (to 235°) metallic platinum in a mixture of chlorine and carbonic oxide. If we designate CO by Y, and an atom of chlorine by X, then taking into account that, according to the periodic system, Ni is an analogue of Pt, a certain degree of correspondence is seen in the composition NiY4and PtX2Y2. It would be interesting to compare the reactions of the two compounds.
[38]This compound is known as nickel tetra-carbonyl. It appears to me yet premature to judge of the structure of such an extraordinary compound as Ni(CO)4. It has long been known that potassium combines with CO forming Kn(CO)n(Chapter IX., Note31), but this substance is apparently saline and non-volatile, and has as little in common with Ni(CO)4as Na2H has with SbH3. However, Berthelot observed that when NiC4O4is kept in air, it oxidises and gives a colourless compound, Ni3C2O3,10H2O, having apparently saline properties. We may add that Schützenberger, on reducing NiCl2by heating it in a current of hydrogen, observed that a nickel compound partially volatilises with the HCl and gives metallic nickel when heated again. The platinum compound, PtCl2(CO)3(Chapter XXIII., Note11), offers the greatest analogy to Ni(CO)4. This compound was obtained as a volatile substance by Schützenberger by moderately heating (to 235°) metallic platinum in a mixture of chlorine and carbonic oxide. If we designate CO by Y, and an atom of chlorine by X, then taking into account that, according to the periodic system, Ni is an analogue of Pt, a certain degree of correspondence is seen in the composition NiY4and PtX2Y2. It would be interesting to compare the reactions of the two compounds.
[39]According to its empirical formula oxalate of nickel also contains nickel and carbonic oxide.
[39]According to its empirical formula oxalate of nickel also contains nickel and carbonic oxide.
[40]The following are the thermo-chemical data (according to Thomsen, and referred to gram weights expressed by the formula, in large calories or thousand units of heat) for the formation of corresponding compounds of Mn, Fe, Co, Ni, and Cu (+ Aq signifies that the reaction proceeds in an excess of water):R = MnFeCoNiCuR + Cl2+ Aq128100959463R + Br2+ Aq10678737241R + I2+ Aq7648434132R + O + H2O9568636138R + O2+ SO2+nH2O193169163163130RCl2+ Aq+1618181911These examples show that for analogous reactions the amount of heat evolved in passing from Mn to Fe, Co, Ni, and Cu varies in regular sequences as the atomic weight increases. A similar difference is to be found in other groups and series, and proves that thermo-chemical phenomena are subject to the periodic law.
[40]The following are the thermo-chemical data (according to Thomsen, and referred to gram weights expressed by the formula, in large calories or thousand units of heat) for the formation of corresponding compounds of Mn, Fe, Co, Ni, and Cu (+ Aq signifies that the reaction proceeds in an excess of water):
These examples show that for analogous reactions the amount of heat evolved in passing from Mn to Fe, Co, Ni, and Cu varies in regular sequences as the atomic weight increases. A similar difference is to be found in other groups and series, and proves that thermo-chemical phenomena are subject to the periodic law.