[27]Phosphorus (Chapter XIX.) gives the hydride PH3, corresponding with ammonia, NH3, and forms phosphorous acid, PH3O3, which is analogous to nitrous acid, just as phosphoric acid is to nitric acid; but phosphoric (or, better, orthophosphoric) acid, PH3O4, is able to lose water and give pyro-and meta-phosphoric acids. The latter is equal to the ortho-acid minus water = PHO3, and therefore nitric acid, NHO3, is really meta-nitric acid. So also nitrous acid, HNO2, is meta-nitrous (anhydrous) acid, and thus the ortho-acid is NH3O3= N(OH)3. Hence for nitric acid we should expect to find, besides the ordinary or meta-nitric acid, HNO3(= ½N2O3,H2O), and ortho-nitric acid, H3NO4(= ½N2O3,3H2O), an intermediate pyro-nitric acid, N2H4O7, corresponding to pyrophosphoric acid, P2H4O7. We shall see (for instance, in Chapter XVI., Note 21) that in nitric acid there is indeed an inclination of the ordinary salts (of the meta-acid), MNO3, to combine with bases M2O, and to approximate to the composition of ortho-compounds which are equal to meta-compound and bases (MNO3+ M2O = M3NO4).[28]The formation of ammonia is observed in many cases of oxidation by means of nitric acid. This substance is even formed in the action of nitric acid on tin, especially if dilute acid be employed in the cold. A still more considerable amount of ammonia is obtained if, in the action of nitric acid, there are conditions directly tending to the evolution of hydrogen, which then reduces the acid to ammonia; for instance, in the action of zinc on a mixture of nitric and sulphuric acids.[28 bis]Curtius started with benzoylhydrazine, C6H5CONHNH2(hydrazine, see Note20 bis). (This substance is obtained by the action of hydrated hydrazine on the compound ether of benzoic acid). Benzoylhydrazine under the action of nitrous acid gives benzoylazoimide and water:C6H5CONHNH2+ NO2H = C6H5CON3+ 2H2O.Benzoylazoimide when treated with sodium alcoholate gives the sodium salt of hydronitrous acid:C6H5CON3+ C2H3ONa = C6H5O2C2H3+ NaN3.The addition of ether to the resultant solution precipitates the NaN3, and this salt when treated with sulphuric acid gives gaseous hydronitrous acid, HN3. It has an acrid smell, and is easily soluble in water. The aqueous solution exhibits a strongly acid reaction. Metals dissolve in this solution and give the corresponding salts. With hydronitrous acid gaseous ammonia forms a white cloud, consisting of the salt of ammonium, NH4N3. This salt separates out from an alcoholic solution in the form of white lustrous scales. The salts of hydronitrous acid are obtained by a reaction of substitution with the sodium or ammonium salts. In this manner Curtius obtained and studied the salts of silver (AgN3), mercury (HgN3), lead (PbN6), barium (BaN6). With hydrazine, N2H4, hydronitrous acid forms saline compounds in the composition of which there are one or two particles of N3H per one particle of hydrazine; thus N5H5and N8H6. The first was obtained in an almost pure form. It crystallises from an aqueous solution in dense, volatile, lustrous prisms (up to 1 in. long), which fuse at 50°, and deliquesce in the air; from a solution in boiling alcohol it separates out in bright crystalline plates. This salt, N5H5, has the same empirical composition, NH, as the ammonium salt of hydronitrous acid, N4H4, and imide; but their molecules and structure are different. Curtius also obtained (1893) hydronitrous acid by passing the vapour of N2O5(evolved by the action of HNO3on As2O3) into a solution of hydrazine, N2H4. Similarly Angeli, by acting upon a saturated solution of silver nitrite with a strong solution of hydrazine, obtained the explosive AgN3in the form of a precipitate, and this reaction, which is based upon the equation N2H4+ NHO2= HN3+ 2H2O, proceeds so easily that it forms an experiment for the lecture table. A thermal investigation of hydronitrous acid by Berthelot and Matignon gave the following figures for the heat of solution of the ammonium salt N3HNH3(1 grm. in 100 parts of water) -708 C., and for the heat of neutralisation by barium hydrate +10·0 C., and by ammonia +8·2 C. The heat of combustion of N4H4(+163·8 C. at a constant vol.) gives the heat of formation of the salt N4H4(solid) as - 25·3 C. and (solution) - 32·3 C.; this explains the explosive nature of this compound. In its heat of formation from the elements N3H = -62·6 C., this compound differs from all the hydrogen compounds of nitrogen in having a maximum absorption of heat, which explains its instability.[29]According to the thermochemical determinations of Favre, Thomsen, and more especially of Berthelot, it follows that, in the formation of such quantities of the oxides of nitrogen as express their formulæ, if gaseous nitrogen and oxygen be taken as the starting points, and if the compounds formed be also gaseous, the following amounts of heat, expressed in thousands of heat units, areabsorbed(hence a minus sign):—N2ON2O2N2O3N2O4N2O5-21-43-22-5-1-22+21+17+4The difference is given in the lower line. For example, if N2, or 28 grams of nitrogen, combine with O—that is, with 16 grams of oxygen—then 21,000 units of heat are absorbed, that is, sufficient heat to raise 21,000 grams of water through 1°. Naturally, direct observations are impossible in this case; but if charcoal, phosphorus, or similar substances are burnt both in nitrous oxide and in oxygen, and the heat evolved is observed in both cases, then the difference (more heat will be evolved in burning in nitrous oxide) gives the figures required. If N2O2, by combining with O2, gives N2O4, then, as is seen from the table, heat should be developed, namely, 38,000 units of heat, or NO + O = 19,000 units of heat. The differences given in the table show that the maximum absorption of heat corresponds with nitric oxide, and that the higher oxides are formed from it with evolution of heat. If liquid nitric acid, NHO3, were decomposed into N + O3+ H, then 41,000 heat units would be required; that is, an evolution of heat takes place in its formation from the gases. It should be observed that the formation of ammonia, NH3, from the gases N + H3evolves 12·2 thousand heat units.[29 bis]This often gives rise to the supposition that sulphuric acid possesses a considerable degree of affinity or energy compared with nitric acid, but we shall afterwards see that the idea of the relative degree of affinity of acids and bases is, in many cases, exceedingly unbiassed; it need not be accepted so long as it is possible to explain the observed phenomena without admitting any supposition whatever of the degree of the force of affinity, because the latter cannot be measured. The action of sulphuric acid upon nitre may be explained by the fact alone that the resultant nitric acid is volatile. The nitric acid is the only one of all the substances partaking in the reaction which is able to pass into vapour; it alone is volatile, while the remainder are non-volatile, or, more strictly speaking, exceedingly difficultly volatile substances. Let us imagine that the sulphuric acid is only able to set free a small quantity of nitric acid from its salt, and this will suffice to explain the decomposition of the whole of the nitre by the sulphuric acid, because once the nitric acid is separated it passes into vapour when heated, and passes away from the sphere of action of the remaining substances; then the free sulphuric acid will set free a fresh small quantity of nitric acid, and so on until it drives off the entire quantity. It is evident that, in this explanation, it is essential that the sulphuric acid should be in excess (although not greatly) throughout the reaction; according to the equation expressing the reaction, 98 parts of sulphuric acid are required per 85 parts of Chili nitre; but if this proportion be maintained in practice the nitric acid is not all disengaged by the sulphuric acid; an excess of the latter must be taken, and generally 80 parts of Chili nitre are taken per 98 parts of acid, so that a portion of the sulphuric acid remains free to the very end of the reaction.[30]It must be observed that sulphuric acid, at least when undiluted (60° Baumé), corrodes cast iron with difficulty, so that the acid may be heated in cast-iron retorts. Nevertheless, both sulphuric and nitric acids have a certain action on cast iron, and therefore the acid obtained will contain traces of iron. In practice sodium nitrate (Chili saltpetre) is usually employed because it is cheaper, but in the laboratory it is best to take potassium nitrate, because it is purer and does not froth up so much as sodium nitrate when heated with sulphuric acid. In the action of an excess of sulphuric acid on nitre and nitric acid a portion of the latter is decomposed, forming lower oxides of nitrogen, which are dissolved in the nitric acid. A portion of the sulphuric acid itself is also carried over as spray by the vapours of the nitric acid. Hence sulphuric acid occurs as an impurity in commercial nitric acid. A certain amount of hydrochloric acid will also be found to be present in it, because sodium chloride is generally found as an impurity in nitre, and under the action of sulphuric acid it forms hydrochloric acid. Commercial acid further contains a considerable excess of water above that necessary for the formation of the hydrate, because water is first poured into the earthenware vessels employed for condensing the nitric acid in order to facilitate its cooling and condensation. Further, the acid of composition HNO3decomposes with great ease, with the evolution of oxides of nitrogen. Thus the commercial acid contains a great number of impurities, and is frequently purified in the following manner:—Lead nitrate is first added to the acid because it forms non-volatile and almost insoluble (precipitated) substances with the free sulphuric and hydrochloric acids, and liberates nitric acid in so doing, according to the equations Pb(NO3)2+ 2HCl = PbCl2+ 2NHO3and Pb(NO3)2+ H2SO4= PbSO4+ 2NHO3. Potassium chromate is then added to the impure nitric acid, by which means oxygen is liberated from the chromic acid, and this oxygen, at the moment of its evolution, oxidises the lower oxides of nitrogen and converts them into nitric acid. A pure nitric acid, containing no impurities other than water, may be then obtained by carefully distilling the acid, treated as above described, and particularly if only the middle portions of the distillate are collected. Such acid should give no precipitate, either with a solution of barium chloride (a precipitate shows the presence of sulphuric acid) or with a solution of silver nitrate (a precipitate shows the presence of hydrochloric acid), nor should it, after being diluted with water, give a coloration with starch containing potassium iodide (a coloration shows the admixture of other oxides of nitrogen). The oxides of nitrogen may be most easily removed from impure nitric acid by heating for a certain time with a small quantity of pure charcoal. By the action of nitric acid on the charcoal carbonic anhydride is evolved, which carries off the lower oxides of nitrogen. On redistilling, pure acid is obtained. The oxides of nitrogen occurring in solution may also be removed by passing air through the nitric acid.[31]Dalton, Smith, Bineau, and others considered that the hydrate of constant boiling point (see Chapter I., Note60) for nitric acid was the compound 2HNO3,3H2O, but Roscoe showed that its composition changes with a variation of the pressure and temperature under which the distillation proceeds. Thus, at a pressure of 1 atmosphere the solution of constant boiling point contains 68·6 p.c., and at one-tenth of an atmosphere 66·8 p.c. Judging from what has been said concerning solutions of hydrochloric acid, and from the variation of specific gravity, I think that the comparatively large decrease in the tensions of the vapours depends on the formation of a hydrate, NHO3,2H2O (= 63·6 p.c.). Such a hydrate may be expressed by N(HO)5, that is, as NH4(HO), in which all the equivalents of hydrogen are replaced by hydroxyl. The constant boiling point will then be the temperature of the decomposition of this hydrate.The variation of the specific gravity at 15° from water (p= 0) to the hydrate NHO3,5H2O (41·2 p.c. HNO3) is expressed bys= 9992 + 57·4p+ 0·16p2, if water = 10,000 at 4°. For example, whenp= 30 p.c.,s= 11,860. For more concentrated solutions, at least, the above-mentioned hydrate, HNO3,2H2O, must be taken, up to which the specific gravitys= 9570 + 84·18p- 0·240p2; but perhaps (since the results of observations of the specific gravity of the solutions are not in sufficient agreement to arrive at a conclusion) the hydrate HNO3,3H2O should be recognised, as is indicated by many nitrates (Al, Mg, Co, &c.), which crystallise with this amount of water of crystallisation. From HNO3,2H2O to HNO3the specific gravity of the solutions (at 15°)s= 10,652 + 62·08p- 0·160p2. The hydrate HNO3,2H2O is recognised by Berthelot on the basis of the thermochemical data for solutions of nitric acid, because on approaching to this composition there is a rapid change in the amount of heat evolved by mixing nitric acid with water. Pickering (1892) by refrigeration obtained the crystalline hydrates: HNO3,H2O, melting at -37° and HNO3,3H2O, melting at -18°. A more detailed study of the reactions of hydrated nitric acid would no doubt show the existence of change in the process and rapidity of reaction in approaching these hydrates.[32]The normal hydrate HNO3, corresponding with the ordinary salts, may be termed the monohydrated acid, because the anhydride N2O5with water forms this normal nitric acid. In this sense the hydrate HNO3,2H2O is the pentahydrated acid.[33]For technical and laboratory purposes recourse is frequently had tored fuming nitric acid—that is, the normal nitric acid, HNO3, containing lower oxides of nitrogen in solution. This acid is prepared by decomposing nitre with half its weight of strong sulphuric acid, or by distilling nitric acid with an excess of sulphuric acid. The normal nitric acid is first obtained, but it partially decomposes, and gives the lower oxidation products of nitrogen, which are dissolved by the nitric acid, to which they impart its usual pale-brown or reddish colour. This acid fumes in the air, from which it attracts moisture, forming a less volatile hydrate. If carbonic anhydride be passed through the red-brown fuming nitric acid for a long period of time, especially if, assisted by a moderate heat, it expels all the lower oxides, and leaves a colourless acid free from these oxides. It is necessary, in the preparation of the red acid, that the receivers should be kept quite cool, because it is only when cold that nitric acid is able to dissolve a large proportion of the oxides of nitrogen. The strong red fuming acid has a specific gravity 1·56 at 20°, and has a suffocating smell of the oxides of nitrogen. When the red acid is mixed with water it turns green, and then of a bluish colour, and with an excess of water ultimately becomes colourless. This is owing to the fact that the oxides of nitrogen in the presence of water and nitric acid are changed, and give coloured solutions.Markleffsky (1892) showed that the green solutions contain (besides HNO3) HNO2and N2O4, whilst the blue solutions only contain HNO2(seeNote48).The action of red fuming nitric acid (or a mixture with sulphuric acid) is in many cases very powerful and rapid, and it sometimes acts differently from pure nitric acid. Thus iron becomes covered with a coating of oxides, and insoluble in acids; it becomes, as is said, passive. Thus chromic acid (and potassium dichromate) gives oxide of chromium in this red acid—that is, it is deoxidised. This is owing to the presence of the lower oxides of nitrogen, which are capable of being oxidised—that is, of passing into nitric acid like the higher oxides. But, generally, the action of fuming nitric acid, both red and colourless, is powerfully oxidising.[34]Hydrogen is not evolved in the action of nitric acid (especially strong) on metals, even with those metals which evolve hydrogen under the action of other acids. This is because the hydrogen at the moment of its separation reduces the nitric acid, with formation of the lower oxides of nitrogen, as we shall afterwards see.[35]Certain basic salts of nitric acid, however (for example, the basic salt of bismuth), are insoluble in water; whilst, on the other hand, all the normal salts are soluble, and this forms an exceptional phenomenon among acids, because all the ordinary acids form insoluble salts with one or another base. Thus, for sulphuric acid the salts of barium, lead, &c., for hydrochloric acid the salts of silver, &c., are insoluble in water. However, the normal salts of acetic and certain other acids are all soluble.[36]Ammonium nitrate, NH4NO3, is easily obtained by adding a solution of ammonia or of ammonium carbonate to nitric acid until it becomes neutral. On evaporating this solution, crystals of the salt are formed which contain no water of crystallisation. It crystallises in prisms like those formed by common nitre, and has a refreshing taste; 100 parts of water att° dissolve 54 + 0·61tparts by weight of the salt. It is soluble in alcohol, melts at 160°, and is decomposed at about 180°, forming water and nitrous oxide, NH4NO3= 2H2O + N2O. If ammonium nitrate be mixed with sulphuric acid, and the mixture be heated to about the boiling point of water, then nitric acid is evolved, and ammonium hydrogen sulphate remains in solution; but if the mixture be heated rapidly to 16O°, then nitrous oxide is evolved. In the first case the sulphuric acid takes up ammonia, and in the second place water. Ammonium nitrate is employed in practice for the artificial production of cold, because in dissolving in water it lowers the temperature very considerably. For this purpose it is best to take equal parts by weight of the salt and water. The salt must first be reduced to a powder and then rapidly stirred up in the water, when the temperature will fall from +15° to -10°, so that the water freezes.Ammonium nitrate absorbs ammonia, with which it forms unstable compounds resembling compounds containing water of crystallisation. (Divers 1872, Raoult 1873.) At -10° NH4NO3,2NH3is formed: it is a liquid of sp. gr. 1·15, which loses all its ammonia under the influence of heat. At +28° NH4NO3,NH3is formed: it is a solid which easily parts with its ammonia when heated, especially in solution.Troost (1882) investigated the tension of the dissociation of the compounds formed, and came to the conclusion that a definite compound corresponding to the formula 2NH4NO3,3NH3is formed, because the tension of dissociation remains constant in the decomposition of such a compound at 0°. Y. Kouriloff (1893), however, considers that the constancy of the tension of the ammonia evolved is due to the decomposition of a saturated solution, and not of a definite compound. During decomposition the system is composed of a liquid and a solid; the tension only becomes constant from the moment the solid falls down. The composition 2NH4NO3,3NH3corresponds to a saturated solution at 0°, and the solubility of NH4NO3in NH3increases with a rise of temperature.[36 bis]This is explained by saying that in true nitro-compounds the residue of nitric acid NO2takes the place of the hydrogen in the hydrocarbon group. For example, if C6H5OH be given, then C6H4(NO2)OH will be a true nitro-compound having the radical properties of C6H5OH. If, on the other hand, the NO2replace the hydrogen of the aqueous radicle (C6H5ONO2), then the chemical character varies, as in the passage of KOH into KONO2(nitre) (seeNote37and Organic Chemistry).[37]The compound ethers of nitric acid in which the hydrogen of the aqueous radicle (OH) is replaced by the residue of nitric acid (NO2) are frequently called nitro-compounds. But in their chemical character they differ from true nitro-compounds (for detailsseeOrganic Chemistry) and do not burn like them.The action of nitric acid on cellulose, C6H10O5, is an example. This substance, which forms the outer coating of all plant cells, occurs in an almost pure state in cotton, in common writing-paper, and in flax, &c.; under the action of nitric acid it forms water and nitrocellulose (like water and KNO3from KHO), which, although it has the same appearance as the cotton originally taken, differs from it entirely in properties. It explodes when struck, bursts into flame very easily under the action of sparks, and acts like gunpowder, whence its name of pyroxylin, or gun-cotton. The composition of gun-cotton is C6H7N3O11= C6H10O5+ 3NHO3- 3H2O. The proportion of the group NO2in nitrocellulose may be decreased by limiting the action of the nitric acid and compounds obtained with different properties; for instance, the (impure) well-knowncollodion cotton, containing from 11 to 12 per cent. of nitrogen, andpyro-collodion(Mendeléeff, 1890), containing 12·4 per cent. of nitrogen. Both these products are soluble in a mixture of alcohol and ether (in collodion a portion of the substance is soluble in alcohol), and the solution when evaporated gives a transparent film, which is insoluble in water. A solution of collodion is employed in medicine for covering wounds, and in wet-plate photography for giving on glass an even coating of a substance into which the various reagents employed in the process are introduced. Extremely fine threads (obtained by forcing a gelatinous mixture of collodion, ether, and alcohol through capillary tubes in water) of collodion form artificial silk.[37 bis]The property possessed by nitroglycerin (occurring in dynamite), nitrocellulose, and the other nitro-compounds, of burning with an explosion, and their employment for smokeless powder and as explosives in general, depends on the reasons in virtue of which a mixture of nitre and charcoal deflagrates and explodes; in both cases the elements of the nitric acid occurring in the compound are decomposed, the oxygen in burning unites with the carbon, and the nitrogen is set free; thus a very large volume of gaseous substances (nitrogen and oxides of carbon) is rapidly formed from the solid substances originally taken. These gases occupy an incomparably larger volume than the original substance, and therefore produce a powerful pressure and explosion. It is evident that in exploding with the development of heat (that is, in decomposing, not with the absorption of energy, as is generally the case, but with the evolution of energy) the nitro-compounds form stores of energy which are easily set free, and that consequently their elements occur in a state of particularly energetic motion, which is especially strong in the group NO2: this group is common to all nitro-compounds, and all the oxygen compounds of nitrogen are unstable, easily decomposable, and (Note29) absorb heat in their formation. On the other hand, the nitro-compounds are instructive as an example and proof of the fact that the elements and groups forming compounds are united in definite order in the molecules of a compound. A blow, concussion, or rise of temperature is necessary to bring the combustible elements C and H into the most intimate contact with NO2, and to distribute the elements in a new order in new compounds.As regards the composition of the nitro-compounds, it will be seen that the hydrogen of a given substance is replaced by the complex group NO2of the nitric acid. The same is observed in the passage of alkalis into nitrates, so that the reactions of substitution of nitric acid—that is, the formation of salts and nitro-compounds—may be expressed in the following manner. In these cases the hydrogen is replaced by the so-calledradicle of nitric acidNO2, as is evident from the following table:—Caustic potashKHO.GlycerinC3H5H3O3.NitreK(NO2)O.NitroglycerinC3H5(NO2)3O3.Hydrate of limeCaH2O2PhenolC6H5OH.Calcium nitrateCa(NO2)2O2.Picric acidC6H2(NO2)3OH, &c.The difference between the salts formed by nitric acid and the nitro-compounds consists in the fact that nitric acid is very easily separated from the salts of nitric acid by means of sulphuric acid (that is, by a method of double saline decomposition), whilst nitric acid is not displaced by sulphuric acid from true nitro-compounds; for instance, nitrobenzene, C6H5·NO2. As nitro-compounds are formed exclusively from hydrocarbons, they are described with them in organic chemistry.The group NO2of nitro-compounds in many cases (like all the oxidised compounds of nitrogen) passes into the ammonia group or into the ammonia radicle NH2. This requires the action of reducing substances evolving hydrogen: RNO2+ 6H = RNH3+ 2H2O. Thus Zinin converted nitrobenzene, C6H5·NO2, into aniline, C6H5·NH2, by the action of hydrogen sulphide.Admitting the existence of the group NO2, as replacing hydrogen in various compounds, then nitric acid may be considered as water in which half the hydrogen is replaced by the radical of nitric acid. In this sense nitric acid is nitro-water, NO2OH, and its anhydride dinitro-water, (NO2)2O. In nitric acid the radical of nitric acid is combined with hydroxyl, just as in nitrobenzene it is combined with the radical of benzene.It should here be remarked that the group NO3may be recognised in the salts of nitric acid, because the salts have the composition M(NO3)n, just as the metallic chlorides have the composition MCln. But the group NO3does not form any other compounds beyond the salts, and therefore it should he considered as hydroxyl, HO, in which H is replaced by NO2.[38]The nitro-compounds play a very important part in mining and artillery. Detailed accounts of them must be looked for in special works, among which the works of A. R. Shuliachenke and T. M. Chelletsoff occupy an important place in the Russian literature on this subject, although historically the scientific works of Abel in England and Berthelot in France stand pre-eminent. The latter elucidated much in connection with explosive compounds by a series of both experimental and theoretical researches. Among explosives a particularly important place from a practical point of view is occupied by ordinary or black gunpowder (Chapter XIII., Note16), fulminating mercury (Chapter XVI., Note 26), the different forms of gun-cotton (Chapter VI., Note37), and nitro-glycerine (Chapter VIII., Note45, and Chapter XII., Note33). The latter when mixed with solid pulverulent substances, like magnesia, tripoli, &c., forms dynamite, which is so largely used in quarries and mines in driving tunnels, &c. We may add that the simplest true nitro-compound, or marsh gas, CH4, in which all the hydrogens are replaced by NO2groups has been obtained by L. N. Shishkoff, C(NO2)4, as well as nitroform, CH(NO2)3.[39]see captionFig.49.—Decomposition of nitrous oxide by sodium.Nitric acid may be entirely decomposed by passing its vapour over highly incandescent copper, because the oxides of nitrogen first formed give up their oxygen to the red-hot metallic copper, so that water and nitrogen gas alone are obtained. This forms a means for determining the composition both of nitric acid and of all the other compounds of nitrogen with oxygen, because by collecting the gaseous nitrogen formed it is possible to calculate, from its volume, its weight and consequently its amount in a given quantity of a nitrogenous substance, and by weighing the copper before and after the decomposition it is possible to determine the amount of oxygen by the increase in weight. The complete decomposition of nitric acid is also accomplished by passing a mixture of hydrogen and nitric acid vapours through a red-hot tube. Sodium also decomposes the oxides of nitrogen at a red-heat, taking up all the oxygen. This method is sometimes used for determining the composition of the oxides of nitrogen.[40]The application of this acid for etching copper or steel in engraving is based on this fact. The copper is covered with a coating of wax, resin, &c. (etching ground), on which nitric acid does not act, and then the ground is removed in certain parts with a needle, and the whole is washed in nitric acid. The parts coated remain untouched, whilst the uncovered portions are eaten into by the acid. Copper plates for etchings, aquatints, &c., are prepared in this manner.[41]The formation of such complex equations as the above often presents some difficulty to the beginner. It should be observed that if the reacting and resultant substances be known, it is easy to form an equation for the reaction. Thus, if we wish to form an equation expressing the reaction that nitric acid acting on zinc gives nitrous oxide, N2O, and zinc nitrate, Zn(NO3)2, we must reason as follows:—Nitric acid contains hydrogen, whilst the salt and nitrous oxide do not; hence water is formed, and therefore it is as though anhydrous nitric acid, N2O5, were acting. For its conversion into nitrous oxide it parts with four equivalents of oxygen, and hence it is able to oxidise four equivalents of zinc and to convert it into zinc oxide, ZnO. These four equivalents of zinc oxide require for their conversion into the salt four more equivalents of nitric anhydride; consequently five equivalents in all of the latter are required, or ten equivalents of nitric acid. Thus ten equivalents of nitric acid are necessary for four equivalents of zinc in order to express the reaction in whole equivalents. It must not be forgotten, however, that there are very few such reactions which can be entirely expressed by simple equations. The majority of equations of reactions only express the chief and ultimate products of reaction, and thus none of the three preceding equations express all that in reality occurs in the action of metals on nitric acid. In no one of them is only one oxide of nitrogen formed, but always several together or consecutively—one after the other, according to the temperature and strength of the acid. And this is easily intelligible. The resulting oxide is itself capable of acting on metals and of being deoxidised, and in the presence of the nitric acid it may change the acid and be itself changed. The equations given must be looked on as a systematic expression of the main features of reactions, or as a limit towards which they tend, but to which they only attain in the absence of disturbing influences.[42]Montemartini endeavours to show that the products evolved in the action of nitric acid upon metals (and their amount) is in direct connection with both the concentration of the acid and the capacity of the metals to decompose water. Those metals which only decompose water at a high temperature give, under the action of nitric acid, NO2, N2O4, and NO; whilst those metals which decompose water at a lower temperature give, besides the above products, N2O, N, and NH3; and, lastly, the metals which decompose water at the ordinary temperature also evolve hydrogen. It is observed that concentrated nitric acid oxidises many metals with much greater difficulty than when diluted with water; iron, copper, and tin are very easily oxidised by dilute nitric acid, but remain unaltered under the influence of monohydrated nitric acid or of the pure hydrate NHO3. Nitric acid diluted with a large quantity of water does not oxidise copper, but it oxidises tin; dilute nitric acid also does not oxidise either silver or mercury; but, on the addition of nitrous acid, even dilute acid acts on the above metals. This naturally depends on the smaller stability of nitrous acid, and on the fact that after the commencement of the action the nitric acid is itself converted into nitrous acid, which continues to act on the silver and mercury. Veley (Oxford 1891) made detailed researches on the action of nitric acid upon Cu, Hg, and Bi, and showed that nitric acid of 30 p.c. strength does not act upon these metals at the ordinary temperature if nitrous acid (traces are destroyed by urea) and oxidising agents such as H2O2, KClO3, &c. be entirely absent; but in the presence of even a small amount of nitrous acid the metals form nitrites, which, with HNO3, form nitrates and the oxides of nitrogen, which re-form the nitrous acid necessary for starting the reaction, because the reaction 2NO + HNO3+ H2O = 3HNO2is reversible. The above metals are quickly dissolved in a 1 p.c. solution of nitrous acid. Moreover, Veley observed that nitric acid is partially converted into nitrous acid by gaseous hydrogen in the presence of the nitrates of Cu and Pb.[43]When nitric acid acts on many organic substances it often happens that not only is hydrogen removed, but also oxygen is combined; thus, for example, nitric acid converts toluene, C7H8, into benzoic acid, C7H6O2. In certain cases, also, a portion of the carbon contained in an organic substance burns at the expense of the oxygen of the nitric acid. So, for instance, phthalic acid, C8H6O4, is obtained from naphthalene,10H8. Thus the action of nitric acid on the hydrocarbons is often most complex; not only does nitrification take place, but also separation of carbon, displacement of hydrogen, and combination of oxygen. There are few organic substances which can withstand the action of nitric acid, and it causes fundamental changes in a number of them. It leaves a yellow stain on the skin, and in a large quantity causes a wound and entirely eats away the membranes of the body. The membranes of plants are eaten into with the greatest ease by strong nitric acid in just the same manner. One of the most durable blue vegetable dyes employed in dyeing tissues isindigo; yet it is easilyconverted into a yellow substanceby the action of nitric acid, and small traces of free nitric acid may be recognised by this means.[44]According to certain investigations, if a brown liquid is formed from the melted crystals by beating above -9°, then they no longer solidify at -10°, probably because a certain amount of N2O3(and oxygen) is formed, and this substance remains liquid at -30°, or it may be that the passage from 2NO2into N2O4is not so easily accomplished as the passage from N2O4into 2NO2.Liquid nitrogen peroxide (that is, a mixture of NO2and N2O4) is employed in admixture with hydrocarbons as an explosive.[45]Because ifxequal the amount by weight of N2O4, its volume will =x/46, and the amount of NO2will = 100 -x, and consequently its volume will = (100 -x)/23. But the mixture, having a density 38, will weigh 100; consequently its volume will = 100/38. Hencex/46 + (100 -x)/23 = 100/38, orx= 79·O.[46]The phenomena and laws of dissociation, which we shall consider only in particular instances, are discussed in detail in works on theoretical chemistry. Nevertheless, in respect to nitrogen peroxide, as an historically important example of dissociation in a homogeneous gaseous medium, we will cite the results of the careful investigations (1885–1880) of E. and L. Natanson, who determined the densities under various conditions of temperature and pressure. The degree of dissociation, expressed as above (it may also he expressed otherwise—for example, by the ratio of the quantity of substance decomposed to that unaltered), proves to increase at all temperatures as the pressure diminishes, which would he expected for a homogeneous gaseous medium, as a decreasing pressure aids the formation of the lightest product of dissociation (that having the least density or largest volume). Thus, in the Natansons' experiments the degree of dissociation at 0° increases from 10 p.c. to 30 p.c., with a decrease of pressure of from 251 to 38 mm.; at 49°·7 it increases from 49 p.c. to 93 p.c., with a fall of pressure of from 498 to 27 mm., and at 100° it increases from 89·2 p.c. to 99·7 p.c., with a fall of pressure of from 732·5 to 11·7 mm. At 130° and 150° the decomposition is complete—that is, only NO2remains at the low pressures (less than the atmospheric) at which the Natansons made their determinations; but it is probable that at higher pressures (of several atmospheres) molecules of N2O4would still be formed, and it would be exceedingly interesting to trace the phenomena under the conditions of both very considerable pressures and of relatively large volumes.[47]Liquid nitrogen peroxide is said by Geuther to boil at 22°–26°, and to have a sp. gr. at 0° = 1·494 and at 15° = 1·474. It is evident that, in the liquid as in the gaseous state, the variation of density with the temperature depends, not only on physical, but also on chemical changes, as the amount of N2O4decreases and the amount of NO2increases with the temperature, and they (as polymeric substances) should have different densities, as we find, for instance, in the hydrocarbons C5H10and C10H20.It may not be superfluous to mention here that the measurement of the specific heat of a mixture of the vapours of N2O4and NO2enabled Berthelot to determine that the transformation of 2NO2into N2O4is accompanied by the evolution of about 13,000 units of heat, and as the reaction proceeds with equal facility in either direction, it will be exothermal in the one direction and endothermal in the other; and this clearly demonstrates the possibility of reactions taking place in either direction, although, as a rule, reactions evolving heat proceed with greater ease.[48]Nitric acid of sp. gr. 1·51 in dissolving nitrogen peroxide becomes brown, whilst nitric acid of sp. gr. 1·32 is coloured greenish blue, and acid of sp. gr. below 1·15 remains colourless after absorbing nitrogen peroxide (Note33).[49]Nitrogen peroxide as a mixed substance has no corresponding independent salts, but Sabatier and Senderens (1892) showed that under certain conditions NO2combines directly with some metals—for instance, copper and cobalt—forming Cu2NO2and CoNO2as dark brown powders, which do not, however, exhibit the reactions of salts. Thus by passing gaseous nitrogen dioxide over freshly reduced (from the oxides by heating with hydrogen) copper at 25°–30°, Cu2NO2is directly formed. With water it partly gives off NO2and partly forms nitrite of copper, leaving metallic copper and its suboxide. The nature of these compounds has not yet been sufficiently investigated.
[27]Phosphorus (Chapter XIX.) gives the hydride PH3, corresponding with ammonia, NH3, and forms phosphorous acid, PH3O3, which is analogous to nitrous acid, just as phosphoric acid is to nitric acid; but phosphoric (or, better, orthophosphoric) acid, PH3O4, is able to lose water and give pyro-and meta-phosphoric acids. The latter is equal to the ortho-acid minus water = PHO3, and therefore nitric acid, NHO3, is really meta-nitric acid. So also nitrous acid, HNO2, is meta-nitrous (anhydrous) acid, and thus the ortho-acid is NH3O3= N(OH)3. Hence for nitric acid we should expect to find, besides the ordinary or meta-nitric acid, HNO3(= ½N2O3,H2O), and ortho-nitric acid, H3NO4(= ½N2O3,3H2O), an intermediate pyro-nitric acid, N2H4O7, corresponding to pyrophosphoric acid, P2H4O7. We shall see (for instance, in Chapter XVI., Note 21) that in nitric acid there is indeed an inclination of the ordinary salts (of the meta-acid), MNO3, to combine with bases M2O, and to approximate to the composition of ortho-compounds which are equal to meta-compound and bases (MNO3+ M2O = M3NO4).
[27]Phosphorus (Chapter XIX.) gives the hydride PH3, corresponding with ammonia, NH3, and forms phosphorous acid, PH3O3, which is analogous to nitrous acid, just as phosphoric acid is to nitric acid; but phosphoric (or, better, orthophosphoric) acid, PH3O4, is able to lose water and give pyro-and meta-phosphoric acids. The latter is equal to the ortho-acid minus water = PHO3, and therefore nitric acid, NHO3, is really meta-nitric acid. So also nitrous acid, HNO2, is meta-nitrous (anhydrous) acid, and thus the ortho-acid is NH3O3= N(OH)3. Hence for nitric acid we should expect to find, besides the ordinary or meta-nitric acid, HNO3(= ½N2O3,H2O), and ortho-nitric acid, H3NO4(= ½N2O3,3H2O), an intermediate pyro-nitric acid, N2H4O7, corresponding to pyrophosphoric acid, P2H4O7. We shall see (for instance, in Chapter XVI., Note 21) that in nitric acid there is indeed an inclination of the ordinary salts (of the meta-acid), MNO3, to combine with bases M2O, and to approximate to the composition of ortho-compounds which are equal to meta-compound and bases (MNO3+ M2O = M3NO4).
[28]The formation of ammonia is observed in many cases of oxidation by means of nitric acid. This substance is even formed in the action of nitric acid on tin, especially if dilute acid be employed in the cold. A still more considerable amount of ammonia is obtained if, in the action of nitric acid, there are conditions directly tending to the evolution of hydrogen, which then reduces the acid to ammonia; for instance, in the action of zinc on a mixture of nitric and sulphuric acids.
[28]The formation of ammonia is observed in many cases of oxidation by means of nitric acid. This substance is even formed in the action of nitric acid on tin, especially if dilute acid be employed in the cold. A still more considerable amount of ammonia is obtained if, in the action of nitric acid, there are conditions directly tending to the evolution of hydrogen, which then reduces the acid to ammonia; for instance, in the action of zinc on a mixture of nitric and sulphuric acids.
[28 bis]Curtius started with benzoylhydrazine, C6H5CONHNH2(hydrazine, see Note20 bis). (This substance is obtained by the action of hydrated hydrazine on the compound ether of benzoic acid). Benzoylhydrazine under the action of nitrous acid gives benzoylazoimide and water:C6H5CONHNH2+ NO2H = C6H5CON3+ 2H2O.Benzoylazoimide when treated with sodium alcoholate gives the sodium salt of hydronitrous acid:C6H5CON3+ C2H3ONa = C6H5O2C2H3+ NaN3.The addition of ether to the resultant solution precipitates the NaN3, and this salt when treated with sulphuric acid gives gaseous hydronitrous acid, HN3. It has an acrid smell, and is easily soluble in water. The aqueous solution exhibits a strongly acid reaction. Metals dissolve in this solution and give the corresponding salts. With hydronitrous acid gaseous ammonia forms a white cloud, consisting of the salt of ammonium, NH4N3. This salt separates out from an alcoholic solution in the form of white lustrous scales. The salts of hydronitrous acid are obtained by a reaction of substitution with the sodium or ammonium salts. In this manner Curtius obtained and studied the salts of silver (AgN3), mercury (HgN3), lead (PbN6), barium (BaN6). With hydrazine, N2H4, hydronitrous acid forms saline compounds in the composition of which there are one or two particles of N3H per one particle of hydrazine; thus N5H5and N8H6. The first was obtained in an almost pure form. It crystallises from an aqueous solution in dense, volatile, lustrous prisms (up to 1 in. long), which fuse at 50°, and deliquesce in the air; from a solution in boiling alcohol it separates out in bright crystalline plates. This salt, N5H5, has the same empirical composition, NH, as the ammonium salt of hydronitrous acid, N4H4, and imide; but their molecules and structure are different. Curtius also obtained (1893) hydronitrous acid by passing the vapour of N2O5(evolved by the action of HNO3on As2O3) into a solution of hydrazine, N2H4. Similarly Angeli, by acting upon a saturated solution of silver nitrite with a strong solution of hydrazine, obtained the explosive AgN3in the form of a precipitate, and this reaction, which is based upon the equation N2H4+ NHO2= HN3+ 2H2O, proceeds so easily that it forms an experiment for the lecture table. A thermal investigation of hydronitrous acid by Berthelot and Matignon gave the following figures for the heat of solution of the ammonium salt N3HNH3(1 grm. in 100 parts of water) -708 C., and for the heat of neutralisation by barium hydrate +10·0 C., and by ammonia +8·2 C. The heat of combustion of N4H4(+163·8 C. at a constant vol.) gives the heat of formation of the salt N4H4(solid) as - 25·3 C. and (solution) - 32·3 C.; this explains the explosive nature of this compound. In its heat of formation from the elements N3H = -62·6 C., this compound differs from all the hydrogen compounds of nitrogen in having a maximum absorption of heat, which explains its instability.
[28 bis]Curtius started with benzoylhydrazine, C6H5CONHNH2(hydrazine, see Note20 bis). (This substance is obtained by the action of hydrated hydrazine on the compound ether of benzoic acid). Benzoylhydrazine under the action of nitrous acid gives benzoylazoimide and water:
C6H5CONHNH2+ NO2H = C6H5CON3+ 2H2O.
Benzoylazoimide when treated with sodium alcoholate gives the sodium salt of hydronitrous acid:
C6H5CON3+ C2H3ONa = C6H5O2C2H3+ NaN3.
The addition of ether to the resultant solution precipitates the NaN3, and this salt when treated with sulphuric acid gives gaseous hydronitrous acid, HN3. It has an acrid smell, and is easily soluble in water. The aqueous solution exhibits a strongly acid reaction. Metals dissolve in this solution and give the corresponding salts. With hydronitrous acid gaseous ammonia forms a white cloud, consisting of the salt of ammonium, NH4N3. This salt separates out from an alcoholic solution in the form of white lustrous scales. The salts of hydronitrous acid are obtained by a reaction of substitution with the sodium or ammonium salts. In this manner Curtius obtained and studied the salts of silver (AgN3), mercury (HgN3), lead (PbN6), barium (BaN6). With hydrazine, N2H4, hydronitrous acid forms saline compounds in the composition of which there are one or two particles of N3H per one particle of hydrazine; thus N5H5and N8H6. The first was obtained in an almost pure form. It crystallises from an aqueous solution in dense, volatile, lustrous prisms (up to 1 in. long), which fuse at 50°, and deliquesce in the air; from a solution in boiling alcohol it separates out in bright crystalline plates. This salt, N5H5, has the same empirical composition, NH, as the ammonium salt of hydronitrous acid, N4H4, and imide; but their molecules and structure are different. Curtius also obtained (1893) hydronitrous acid by passing the vapour of N2O5(evolved by the action of HNO3on As2O3) into a solution of hydrazine, N2H4. Similarly Angeli, by acting upon a saturated solution of silver nitrite with a strong solution of hydrazine, obtained the explosive AgN3in the form of a precipitate, and this reaction, which is based upon the equation N2H4+ NHO2= HN3+ 2H2O, proceeds so easily that it forms an experiment for the lecture table. A thermal investigation of hydronitrous acid by Berthelot and Matignon gave the following figures for the heat of solution of the ammonium salt N3HNH3(1 grm. in 100 parts of water) -708 C., and for the heat of neutralisation by barium hydrate +10·0 C., and by ammonia +8·2 C. The heat of combustion of N4H4(+163·8 C. at a constant vol.) gives the heat of formation of the salt N4H4(solid) as - 25·3 C. and (solution) - 32·3 C.; this explains the explosive nature of this compound. In its heat of formation from the elements N3H = -62·6 C., this compound differs from all the hydrogen compounds of nitrogen in having a maximum absorption of heat, which explains its instability.
[29]According to the thermochemical determinations of Favre, Thomsen, and more especially of Berthelot, it follows that, in the formation of such quantities of the oxides of nitrogen as express their formulæ, if gaseous nitrogen and oxygen be taken as the starting points, and if the compounds formed be also gaseous, the following amounts of heat, expressed in thousands of heat units, areabsorbed(hence a minus sign):—N2ON2O2N2O3N2O4N2O5-21-43-22-5-1-22+21+17+4The difference is given in the lower line. For example, if N2, or 28 grams of nitrogen, combine with O—that is, with 16 grams of oxygen—then 21,000 units of heat are absorbed, that is, sufficient heat to raise 21,000 grams of water through 1°. Naturally, direct observations are impossible in this case; but if charcoal, phosphorus, or similar substances are burnt both in nitrous oxide and in oxygen, and the heat evolved is observed in both cases, then the difference (more heat will be evolved in burning in nitrous oxide) gives the figures required. If N2O2, by combining with O2, gives N2O4, then, as is seen from the table, heat should be developed, namely, 38,000 units of heat, or NO + O = 19,000 units of heat. The differences given in the table show that the maximum absorption of heat corresponds with nitric oxide, and that the higher oxides are formed from it with evolution of heat. If liquid nitric acid, NHO3, were decomposed into N + O3+ H, then 41,000 heat units would be required; that is, an evolution of heat takes place in its formation from the gases. It should be observed that the formation of ammonia, NH3, from the gases N + H3evolves 12·2 thousand heat units.
[29]According to the thermochemical determinations of Favre, Thomsen, and more especially of Berthelot, it follows that, in the formation of such quantities of the oxides of nitrogen as express their formulæ, if gaseous nitrogen and oxygen be taken as the starting points, and if the compounds formed be also gaseous, the following amounts of heat, expressed in thousands of heat units, areabsorbed(hence a minus sign):—
The difference is given in the lower line. For example, if N2, or 28 grams of nitrogen, combine with O—that is, with 16 grams of oxygen—then 21,000 units of heat are absorbed, that is, sufficient heat to raise 21,000 grams of water through 1°. Naturally, direct observations are impossible in this case; but if charcoal, phosphorus, or similar substances are burnt both in nitrous oxide and in oxygen, and the heat evolved is observed in both cases, then the difference (more heat will be evolved in burning in nitrous oxide) gives the figures required. If N2O2, by combining with O2, gives N2O4, then, as is seen from the table, heat should be developed, namely, 38,000 units of heat, or NO + O = 19,000 units of heat. The differences given in the table show that the maximum absorption of heat corresponds with nitric oxide, and that the higher oxides are formed from it with evolution of heat. If liquid nitric acid, NHO3, were decomposed into N + O3+ H, then 41,000 heat units would be required; that is, an evolution of heat takes place in its formation from the gases. It should be observed that the formation of ammonia, NH3, from the gases N + H3evolves 12·2 thousand heat units.
[29 bis]This often gives rise to the supposition that sulphuric acid possesses a considerable degree of affinity or energy compared with nitric acid, but we shall afterwards see that the idea of the relative degree of affinity of acids and bases is, in many cases, exceedingly unbiassed; it need not be accepted so long as it is possible to explain the observed phenomena without admitting any supposition whatever of the degree of the force of affinity, because the latter cannot be measured. The action of sulphuric acid upon nitre may be explained by the fact alone that the resultant nitric acid is volatile. The nitric acid is the only one of all the substances partaking in the reaction which is able to pass into vapour; it alone is volatile, while the remainder are non-volatile, or, more strictly speaking, exceedingly difficultly volatile substances. Let us imagine that the sulphuric acid is only able to set free a small quantity of nitric acid from its salt, and this will suffice to explain the decomposition of the whole of the nitre by the sulphuric acid, because once the nitric acid is separated it passes into vapour when heated, and passes away from the sphere of action of the remaining substances; then the free sulphuric acid will set free a fresh small quantity of nitric acid, and so on until it drives off the entire quantity. It is evident that, in this explanation, it is essential that the sulphuric acid should be in excess (although not greatly) throughout the reaction; according to the equation expressing the reaction, 98 parts of sulphuric acid are required per 85 parts of Chili nitre; but if this proportion be maintained in practice the nitric acid is not all disengaged by the sulphuric acid; an excess of the latter must be taken, and generally 80 parts of Chili nitre are taken per 98 parts of acid, so that a portion of the sulphuric acid remains free to the very end of the reaction.
[29 bis]This often gives rise to the supposition that sulphuric acid possesses a considerable degree of affinity or energy compared with nitric acid, but we shall afterwards see that the idea of the relative degree of affinity of acids and bases is, in many cases, exceedingly unbiassed; it need not be accepted so long as it is possible to explain the observed phenomena without admitting any supposition whatever of the degree of the force of affinity, because the latter cannot be measured. The action of sulphuric acid upon nitre may be explained by the fact alone that the resultant nitric acid is volatile. The nitric acid is the only one of all the substances partaking in the reaction which is able to pass into vapour; it alone is volatile, while the remainder are non-volatile, or, more strictly speaking, exceedingly difficultly volatile substances. Let us imagine that the sulphuric acid is only able to set free a small quantity of nitric acid from its salt, and this will suffice to explain the decomposition of the whole of the nitre by the sulphuric acid, because once the nitric acid is separated it passes into vapour when heated, and passes away from the sphere of action of the remaining substances; then the free sulphuric acid will set free a fresh small quantity of nitric acid, and so on until it drives off the entire quantity. It is evident that, in this explanation, it is essential that the sulphuric acid should be in excess (although not greatly) throughout the reaction; according to the equation expressing the reaction, 98 parts of sulphuric acid are required per 85 parts of Chili nitre; but if this proportion be maintained in practice the nitric acid is not all disengaged by the sulphuric acid; an excess of the latter must be taken, and generally 80 parts of Chili nitre are taken per 98 parts of acid, so that a portion of the sulphuric acid remains free to the very end of the reaction.
[30]It must be observed that sulphuric acid, at least when undiluted (60° Baumé), corrodes cast iron with difficulty, so that the acid may be heated in cast-iron retorts. Nevertheless, both sulphuric and nitric acids have a certain action on cast iron, and therefore the acid obtained will contain traces of iron. In practice sodium nitrate (Chili saltpetre) is usually employed because it is cheaper, but in the laboratory it is best to take potassium nitrate, because it is purer and does not froth up so much as sodium nitrate when heated with sulphuric acid. In the action of an excess of sulphuric acid on nitre and nitric acid a portion of the latter is decomposed, forming lower oxides of nitrogen, which are dissolved in the nitric acid. A portion of the sulphuric acid itself is also carried over as spray by the vapours of the nitric acid. Hence sulphuric acid occurs as an impurity in commercial nitric acid. A certain amount of hydrochloric acid will also be found to be present in it, because sodium chloride is generally found as an impurity in nitre, and under the action of sulphuric acid it forms hydrochloric acid. Commercial acid further contains a considerable excess of water above that necessary for the formation of the hydrate, because water is first poured into the earthenware vessels employed for condensing the nitric acid in order to facilitate its cooling and condensation. Further, the acid of composition HNO3decomposes with great ease, with the evolution of oxides of nitrogen. Thus the commercial acid contains a great number of impurities, and is frequently purified in the following manner:—Lead nitrate is first added to the acid because it forms non-volatile and almost insoluble (precipitated) substances with the free sulphuric and hydrochloric acids, and liberates nitric acid in so doing, according to the equations Pb(NO3)2+ 2HCl = PbCl2+ 2NHO3and Pb(NO3)2+ H2SO4= PbSO4+ 2NHO3. Potassium chromate is then added to the impure nitric acid, by which means oxygen is liberated from the chromic acid, and this oxygen, at the moment of its evolution, oxidises the lower oxides of nitrogen and converts them into nitric acid. A pure nitric acid, containing no impurities other than water, may be then obtained by carefully distilling the acid, treated as above described, and particularly if only the middle portions of the distillate are collected. Such acid should give no precipitate, either with a solution of barium chloride (a precipitate shows the presence of sulphuric acid) or with a solution of silver nitrate (a precipitate shows the presence of hydrochloric acid), nor should it, after being diluted with water, give a coloration with starch containing potassium iodide (a coloration shows the admixture of other oxides of nitrogen). The oxides of nitrogen may be most easily removed from impure nitric acid by heating for a certain time with a small quantity of pure charcoal. By the action of nitric acid on the charcoal carbonic anhydride is evolved, which carries off the lower oxides of nitrogen. On redistilling, pure acid is obtained. The oxides of nitrogen occurring in solution may also be removed by passing air through the nitric acid.
[30]It must be observed that sulphuric acid, at least when undiluted (60° Baumé), corrodes cast iron with difficulty, so that the acid may be heated in cast-iron retorts. Nevertheless, both sulphuric and nitric acids have a certain action on cast iron, and therefore the acid obtained will contain traces of iron. In practice sodium nitrate (Chili saltpetre) is usually employed because it is cheaper, but in the laboratory it is best to take potassium nitrate, because it is purer and does not froth up so much as sodium nitrate when heated with sulphuric acid. In the action of an excess of sulphuric acid on nitre and nitric acid a portion of the latter is decomposed, forming lower oxides of nitrogen, which are dissolved in the nitric acid. A portion of the sulphuric acid itself is also carried over as spray by the vapours of the nitric acid. Hence sulphuric acid occurs as an impurity in commercial nitric acid. A certain amount of hydrochloric acid will also be found to be present in it, because sodium chloride is generally found as an impurity in nitre, and under the action of sulphuric acid it forms hydrochloric acid. Commercial acid further contains a considerable excess of water above that necessary for the formation of the hydrate, because water is first poured into the earthenware vessels employed for condensing the nitric acid in order to facilitate its cooling and condensation. Further, the acid of composition HNO3decomposes with great ease, with the evolution of oxides of nitrogen. Thus the commercial acid contains a great number of impurities, and is frequently purified in the following manner:—Lead nitrate is first added to the acid because it forms non-volatile and almost insoluble (precipitated) substances with the free sulphuric and hydrochloric acids, and liberates nitric acid in so doing, according to the equations Pb(NO3)2+ 2HCl = PbCl2+ 2NHO3and Pb(NO3)2+ H2SO4= PbSO4+ 2NHO3. Potassium chromate is then added to the impure nitric acid, by which means oxygen is liberated from the chromic acid, and this oxygen, at the moment of its evolution, oxidises the lower oxides of nitrogen and converts them into nitric acid. A pure nitric acid, containing no impurities other than water, may be then obtained by carefully distilling the acid, treated as above described, and particularly if only the middle portions of the distillate are collected. Such acid should give no precipitate, either with a solution of barium chloride (a precipitate shows the presence of sulphuric acid) or with a solution of silver nitrate (a precipitate shows the presence of hydrochloric acid), nor should it, after being diluted with water, give a coloration with starch containing potassium iodide (a coloration shows the admixture of other oxides of nitrogen). The oxides of nitrogen may be most easily removed from impure nitric acid by heating for a certain time with a small quantity of pure charcoal. By the action of nitric acid on the charcoal carbonic anhydride is evolved, which carries off the lower oxides of nitrogen. On redistilling, pure acid is obtained. The oxides of nitrogen occurring in solution may also be removed by passing air through the nitric acid.
[31]Dalton, Smith, Bineau, and others considered that the hydrate of constant boiling point (see Chapter I., Note60) for nitric acid was the compound 2HNO3,3H2O, but Roscoe showed that its composition changes with a variation of the pressure and temperature under which the distillation proceeds. Thus, at a pressure of 1 atmosphere the solution of constant boiling point contains 68·6 p.c., and at one-tenth of an atmosphere 66·8 p.c. Judging from what has been said concerning solutions of hydrochloric acid, and from the variation of specific gravity, I think that the comparatively large decrease in the tensions of the vapours depends on the formation of a hydrate, NHO3,2H2O (= 63·6 p.c.). Such a hydrate may be expressed by N(HO)5, that is, as NH4(HO), in which all the equivalents of hydrogen are replaced by hydroxyl. The constant boiling point will then be the temperature of the decomposition of this hydrate.The variation of the specific gravity at 15° from water (p= 0) to the hydrate NHO3,5H2O (41·2 p.c. HNO3) is expressed bys= 9992 + 57·4p+ 0·16p2, if water = 10,000 at 4°. For example, whenp= 30 p.c.,s= 11,860. For more concentrated solutions, at least, the above-mentioned hydrate, HNO3,2H2O, must be taken, up to which the specific gravitys= 9570 + 84·18p- 0·240p2; but perhaps (since the results of observations of the specific gravity of the solutions are not in sufficient agreement to arrive at a conclusion) the hydrate HNO3,3H2O should be recognised, as is indicated by many nitrates (Al, Mg, Co, &c.), which crystallise with this amount of water of crystallisation. From HNO3,2H2O to HNO3the specific gravity of the solutions (at 15°)s= 10,652 + 62·08p- 0·160p2. The hydrate HNO3,2H2O is recognised by Berthelot on the basis of the thermochemical data for solutions of nitric acid, because on approaching to this composition there is a rapid change in the amount of heat evolved by mixing nitric acid with water. Pickering (1892) by refrigeration obtained the crystalline hydrates: HNO3,H2O, melting at -37° and HNO3,3H2O, melting at -18°. A more detailed study of the reactions of hydrated nitric acid would no doubt show the existence of change in the process and rapidity of reaction in approaching these hydrates.
[31]Dalton, Smith, Bineau, and others considered that the hydrate of constant boiling point (see Chapter I., Note60) for nitric acid was the compound 2HNO3,3H2O, but Roscoe showed that its composition changes with a variation of the pressure and temperature under which the distillation proceeds. Thus, at a pressure of 1 atmosphere the solution of constant boiling point contains 68·6 p.c., and at one-tenth of an atmosphere 66·8 p.c. Judging from what has been said concerning solutions of hydrochloric acid, and from the variation of specific gravity, I think that the comparatively large decrease in the tensions of the vapours depends on the formation of a hydrate, NHO3,2H2O (= 63·6 p.c.). Such a hydrate may be expressed by N(HO)5, that is, as NH4(HO), in which all the equivalents of hydrogen are replaced by hydroxyl. The constant boiling point will then be the temperature of the decomposition of this hydrate.
The variation of the specific gravity at 15° from water (p= 0) to the hydrate NHO3,5H2O (41·2 p.c. HNO3) is expressed bys= 9992 + 57·4p+ 0·16p2, if water = 10,000 at 4°. For example, whenp= 30 p.c.,s= 11,860. For more concentrated solutions, at least, the above-mentioned hydrate, HNO3,2H2O, must be taken, up to which the specific gravitys= 9570 + 84·18p- 0·240p2; but perhaps (since the results of observations of the specific gravity of the solutions are not in sufficient agreement to arrive at a conclusion) the hydrate HNO3,3H2O should be recognised, as is indicated by many nitrates (Al, Mg, Co, &c.), which crystallise with this amount of water of crystallisation. From HNO3,2H2O to HNO3the specific gravity of the solutions (at 15°)s= 10,652 + 62·08p- 0·160p2. The hydrate HNO3,2H2O is recognised by Berthelot on the basis of the thermochemical data for solutions of nitric acid, because on approaching to this composition there is a rapid change in the amount of heat evolved by mixing nitric acid with water. Pickering (1892) by refrigeration obtained the crystalline hydrates: HNO3,H2O, melting at -37° and HNO3,3H2O, melting at -18°. A more detailed study of the reactions of hydrated nitric acid would no doubt show the existence of change in the process and rapidity of reaction in approaching these hydrates.
[32]The normal hydrate HNO3, corresponding with the ordinary salts, may be termed the monohydrated acid, because the anhydride N2O5with water forms this normal nitric acid. In this sense the hydrate HNO3,2H2O is the pentahydrated acid.
[32]The normal hydrate HNO3, corresponding with the ordinary salts, may be termed the monohydrated acid, because the anhydride N2O5with water forms this normal nitric acid. In this sense the hydrate HNO3,2H2O is the pentahydrated acid.
[33]For technical and laboratory purposes recourse is frequently had tored fuming nitric acid—that is, the normal nitric acid, HNO3, containing lower oxides of nitrogen in solution. This acid is prepared by decomposing nitre with half its weight of strong sulphuric acid, or by distilling nitric acid with an excess of sulphuric acid. The normal nitric acid is first obtained, but it partially decomposes, and gives the lower oxidation products of nitrogen, which are dissolved by the nitric acid, to which they impart its usual pale-brown or reddish colour. This acid fumes in the air, from which it attracts moisture, forming a less volatile hydrate. If carbonic anhydride be passed through the red-brown fuming nitric acid for a long period of time, especially if, assisted by a moderate heat, it expels all the lower oxides, and leaves a colourless acid free from these oxides. It is necessary, in the preparation of the red acid, that the receivers should be kept quite cool, because it is only when cold that nitric acid is able to dissolve a large proportion of the oxides of nitrogen. The strong red fuming acid has a specific gravity 1·56 at 20°, and has a suffocating smell of the oxides of nitrogen. When the red acid is mixed with water it turns green, and then of a bluish colour, and with an excess of water ultimately becomes colourless. This is owing to the fact that the oxides of nitrogen in the presence of water and nitric acid are changed, and give coloured solutions.Markleffsky (1892) showed that the green solutions contain (besides HNO3) HNO2and N2O4, whilst the blue solutions only contain HNO2(seeNote48).The action of red fuming nitric acid (or a mixture with sulphuric acid) is in many cases very powerful and rapid, and it sometimes acts differently from pure nitric acid. Thus iron becomes covered with a coating of oxides, and insoluble in acids; it becomes, as is said, passive. Thus chromic acid (and potassium dichromate) gives oxide of chromium in this red acid—that is, it is deoxidised. This is owing to the presence of the lower oxides of nitrogen, which are capable of being oxidised—that is, of passing into nitric acid like the higher oxides. But, generally, the action of fuming nitric acid, both red and colourless, is powerfully oxidising.
[33]For technical and laboratory purposes recourse is frequently had tored fuming nitric acid—that is, the normal nitric acid, HNO3, containing lower oxides of nitrogen in solution. This acid is prepared by decomposing nitre with half its weight of strong sulphuric acid, or by distilling nitric acid with an excess of sulphuric acid. The normal nitric acid is first obtained, but it partially decomposes, and gives the lower oxidation products of nitrogen, which are dissolved by the nitric acid, to which they impart its usual pale-brown or reddish colour. This acid fumes in the air, from which it attracts moisture, forming a less volatile hydrate. If carbonic anhydride be passed through the red-brown fuming nitric acid for a long period of time, especially if, assisted by a moderate heat, it expels all the lower oxides, and leaves a colourless acid free from these oxides. It is necessary, in the preparation of the red acid, that the receivers should be kept quite cool, because it is only when cold that nitric acid is able to dissolve a large proportion of the oxides of nitrogen. The strong red fuming acid has a specific gravity 1·56 at 20°, and has a suffocating smell of the oxides of nitrogen. When the red acid is mixed with water it turns green, and then of a bluish colour, and with an excess of water ultimately becomes colourless. This is owing to the fact that the oxides of nitrogen in the presence of water and nitric acid are changed, and give coloured solutions.
Markleffsky (1892) showed that the green solutions contain (besides HNO3) HNO2and N2O4, whilst the blue solutions only contain HNO2(seeNote48).
The action of red fuming nitric acid (or a mixture with sulphuric acid) is in many cases very powerful and rapid, and it sometimes acts differently from pure nitric acid. Thus iron becomes covered with a coating of oxides, and insoluble in acids; it becomes, as is said, passive. Thus chromic acid (and potassium dichromate) gives oxide of chromium in this red acid—that is, it is deoxidised. This is owing to the presence of the lower oxides of nitrogen, which are capable of being oxidised—that is, of passing into nitric acid like the higher oxides. But, generally, the action of fuming nitric acid, both red and colourless, is powerfully oxidising.
[34]Hydrogen is not evolved in the action of nitric acid (especially strong) on metals, even with those metals which evolve hydrogen under the action of other acids. This is because the hydrogen at the moment of its separation reduces the nitric acid, with formation of the lower oxides of nitrogen, as we shall afterwards see.
[34]Hydrogen is not evolved in the action of nitric acid (especially strong) on metals, even with those metals which evolve hydrogen under the action of other acids. This is because the hydrogen at the moment of its separation reduces the nitric acid, with formation of the lower oxides of nitrogen, as we shall afterwards see.
[35]Certain basic salts of nitric acid, however (for example, the basic salt of bismuth), are insoluble in water; whilst, on the other hand, all the normal salts are soluble, and this forms an exceptional phenomenon among acids, because all the ordinary acids form insoluble salts with one or another base. Thus, for sulphuric acid the salts of barium, lead, &c., for hydrochloric acid the salts of silver, &c., are insoluble in water. However, the normal salts of acetic and certain other acids are all soluble.
[35]Certain basic salts of nitric acid, however (for example, the basic salt of bismuth), are insoluble in water; whilst, on the other hand, all the normal salts are soluble, and this forms an exceptional phenomenon among acids, because all the ordinary acids form insoluble salts with one or another base. Thus, for sulphuric acid the salts of barium, lead, &c., for hydrochloric acid the salts of silver, &c., are insoluble in water. However, the normal salts of acetic and certain other acids are all soluble.
[36]Ammonium nitrate, NH4NO3, is easily obtained by adding a solution of ammonia or of ammonium carbonate to nitric acid until it becomes neutral. On evaporating this solution, crystals of the salt are formed which contain no water of crystallisation. It crystallises in prisms like those formed by common nitre, and has a refreshing taste; 100 parts of water att° dissolve 54 + 0·61tparts by weight of the salt. It is soluble in alcohol, melts at 160°, and is decomposed at about 180°, forming water and nitrous oxide, NH4NO3= 2H2O + N2O. If ammonium nitrate be mixed with sulphuric acid, and the mixture be heated to about the boiling point of water, then nitric acid is evolved, and ammonium hydrogen sulphate remains in solution; but if the mixture be heated rapidly to 16O°, then nitrous oxide is evolved. In the first case the sulphuric acid takes up ammonia, and in the second place water. Ammonium nitrate is employed in practice for the artificial production of cold, because in dissolving in water it lowers the temperature very considerably. For this purpose it is best to take equal parts by weight of the salt and water. The salt must first be reduced to a powder and then rapidly stirred up in the water, when the temperature will fall from +15° to -10°, so that the water freezes.Ammonium nitrate absorbs ammonia, with which it forms unstable compounds resembling compounds containing water of crystallisation. (Divers 1872, Raoult 1873.) At -10° NH4NO3,2NH3is formed: it is a liquid of sp. gr. 1·15, which loses all its ammonia under the influence of heat. At +28° NH4NO3,NH3is formed: it is a solid which easily parts with its ammonia when heated, especially in solution.Troost (1882) investigated the tension of the dissociation of the compounds formed, and came to the conclusion that a definite compound corresponding to the formula 2NH4NO3,3NH3is formed, because the tension of dissociation remains constant in the decomposition of such a compound at 0°. Y. Kouriloff (1893), however, considers that the constancy of the tension of the ammonia evolved is due to the decomposition of a saturated solution, and not of a definite compound. During decomposition the system is composed of a liquid and a solid; the tension only becomes constant from the moment the solid falls down. The composition 2NH4NO3,3NH3corresponds to a saturated solution at 0°, and the solubility of NH4NO3in NH3increases with a rise of temperature.
[36]Ammonium nitrate, NH4NO3, is easily obtained by adding a solution of ammonia or of ammonium carbonate to nitric acid until it becomes neutral. On evaporating this solution, crystals of the salt are formed which contain no water of crystallisation. It crystallises in prisms like those formed by common nitre, and has a refreshing taste; 100 parts of water att° dissolve 54 + 0·61tparts by weight of the salt. It is soluble in alcohol, melts at 160°, and is decomposed at about 180°, forming water and nitrous oxide, NH4NO3= 2H2O + N2O. If ammonium nitrate be mixed with sulphuric acid, and the mixture be heated to about the boiling point of water, then nitric acid is evolved, and ammonium hydrogen sulphate remains in solution; but if the mixture be heated rapidly to 16O°, then nitrous oxide is evolved. In the first case the sulphuric acid takes up ammonia, and in the second place water. Ammonium nitrate is employed in practice for the artificial production of cold, because in dissolving in water it lowers the temperature very considerably. For this purpose it is best to take equal parts by weight of the salt and water. The salt must first be reduced to a powder and then rapidly stirred up in the water, when the temperature will fall from +15° to -10°, so that the water freezes.
Ammonium nitrate absorbs ammonia, with which it forms unstable compounds resembling compounds containing water of crystallisation. (Divers 1872, Raoult 1873.) At -10° NH4NO3,2NH3is formed: it is a liquid of sp. gr. 1·15, which loses all its ammonia under the influence of heat. At +28° NH4NO3,NH3is formed: it is a solid which easily parts with its ammonia when heated, especially in solution.
Troost (1882) investigated the tension of the dissociation of the compounds formed, and came to the conclusion that a definite compound corresponding to the formula 2NH4NO3,3NH3is formed, because the tension of dissociation remains constant in the decomposition of such a compound at 0°. Y. Kouriloff (1893), however, considers that the constancy of the tension of the ammonia evolved is due to the decomposition of a saturated solution, and not of a definite compound. During decomposition the system is composed of a liquid and a solid; the tension only becomes constant from the moment the solid falls down. The composition 2NH4NO3,3NH3corresponds to a saturated solution at 0°, and the solubility of NH4NO3in NH3increases with a rise of temperature.
[36 bis]This is explained by saying that in true nitro-compounds the residue of nitric acid NO2takes the place of the hydrogen in the hydrocarbon group. For example, if C6H5OH be given, then C6H4(NO2)OH will be a true nitro-compound having the radical properties of C6H5OH. If, on the other hand, the NO2replace the hydrogen of the aqueous radicle (C6H5ONO2), then the chemical character varies, as in the passage of KOH into KONO2(nitre) (seeNote37and Organic Chemistry).
[36 bis]This is explained by saying that in true nitro-compounds the residue of nitric acid NO2takes the place of the hydrogen in the hydrocarbon group. For example, if C6H5OH be given, then C6H4(NO2)OH will be a true nitro-compound having the radical properties of C6H5OH. If, on the other hand, the NO2replace the hydrogen of the aqueous radicle (C6H5ONO2), then the chemical character varies, as in the passage of KOH into KONO2(nitre) (seeNote37and Organic Chemistry).
[37]The compound ethers of nitric acid in which the hydrogen of the aqueous radicle (OH) is replaced by the residue of nitric acid (NO2) are frequently called nitro-compounds. But in their chemical character they differ from true nitro-compounds (for detailsseeOrganic Chemistry) and do not burn like them.The action of nitric acid on cellulose, C6H10O5, is an example. This substance, which forms the outer coating of all plant cells, occurs in an almost pure state in cotton, in common writing-paper, and in flax, &c.; under the action of nitric acid it forms water and nitrocellulose (like water and KNO3from KHO), which, although it has the same appearance as the cotton originally taken, differs from it entirely in properties. It explodes when struck, bursts into flame very easily under the action of sparks, and acts like gunpowder, whence its name of pyroxylin, or gun-cotton. The composition of gun-cotton is C6H7N3O11= C6H10O5+ 3NHO3- 3H2O. The proportion of the group NO2in nitrocellulose may be decreased by limiting the action of the nitric acid and compounds obtained with different properties; for instance, the (impure) well-knowncollodion cotton, containing from 11 to 12 per cent. of nitrogen, andpyro-collodion(Mendeléeff, 1890), containing 12·4 per cent. of nitrogen. Both these products are soluble in a mixture of alcohol and ether (in collodion a portion of the substance is soluble in alcohol), and the solution when evaporated gives a transparent film, which is insoluble in water. A solution of collodion is employed in medicine for covering wounds, and in wet-plate photography for giving on glass an even coating of a substance into which the various reagents employed in the process are introduced. Extremely fine threads (obtained by forcing a gelatinous mixture of collodion, ether, and alcohol through capillary tubes in water) of collodion form artificial silk.
[37]The compound ethers of nitric acid in which the hydrogen of the aqueous radicle (OH) is replaced by the residue of nitric acid (NO2) are frequently called nitro-compounds. But in their chemical character they differ from true nitro-compounds (for detailsseeOrganic Chemistry) and do not burn like them.
The action of nitric acid on cellulose, C6H10O5, is an example. This substance, which forms the outer coating of all plant cells, occurs in an almost pure state in cotton, in common writing-paper, and in flax, &c.; under the action of nitric acid it forms water and nitrocellulose (like water and KNO3from KHO), which, although it has the same appearance as the cotton originally taken, differs from it entirely in properties. It explodes when struck, bursts into flame very easily under the action of sparks, and acts like gunpowder, whence its name of pyroxylin, or gun-cotton. The composition of gun-cotton is C6H7N3O11= C6H10O5+ 3NHO3- 3H2O. The proportion of the group NO2in nitrocellulose may be decreased by limiting the action of the nitric acid and compounds obtained with different properties; for instance, the (impure) well-knowncollodion cotton, containing from 11 to 12 per cent. of nitrogen, andpyro-collodion(Mendeléeff, 1890), containing 12·4 per cent. of nitrogen. Both these products are soluble in a mixture of alcohol and ether (in collodion a portion of the substance is soluble in alcohol), and the solution when evaporated gives a transparent film, which is insoluble in water. A solution of collodion is employed in medicine for covering wounds, and in wet-plate photography for giving on glass an even coating of a substance into which the various reagents employed in the process are introduced. Extremely fine threads (obtained by forcing a gelatinous mixture of collodion, ether, and alcohol through capillary tubes in water) of collodion form artificial silk.
[37 bis]The property possessed by nitroglycerin (occurring in dynamite), nitrocellulose, and the other nitro-compounds, of burning with an explosion, and their employment for smokeless powder and as explosives in general, depends on the reasons in virtue of which a mixture of nitre and charcoal deflagrates and explodes; in both cases the elements of the nitric acid occurring in the compound are decomposed, the oxygen in burning unites with the carbon, and the nitrogen is set free; thus a very large volume of gaseous substances (nitrogen and oxides of carbon) is rapidly formed from the solid substances originally taken. These gases occupy an incomparably larger volume than the original substance, and therefore produce a powerful pressure and explosion. It is evident that in exploding with the development of heat (that is, in decomposing, not with the absorption of energy, as is generally the case, but with the evolution of energy) the nitro-compounds form stores of energy which are easily set free, and that consequently their elements occur in a state of particularly energetic motion, which is especially strong in the group NO2: this group is common to all nitro-compounds, and all the oxygen compounds of nitrogen are unstable, easily decomposable, and (Note29) absorb heat in their formation. On the other hand, the nitro-compounds are instructive as an example and proof of the fact that the elements and groups forming compounds are united in definite order in the molecules of a compound. A blow, concussion, or rise of temperature is necessary to bring the combustible elements C and H into the most intimate contact with NO2, and to distribute the elements in a new order in new compounds.As regards the composition of the nitro-compounds, it will be seen that the hydrogen of a given substance is replaced by the complex group NO2of the nitric acid. The same is observed in the passage of alkalis into nitrates, so that the reactions of substitution of nitric acid—that is, the formation of salts and nitro-compounds—may be expressed in the following manner. In these cases the hydrogen is replaced by the so-calledradicle of nitric acidNO2, as is evident from the following table:—Caustic potashKHO.GlycerinC3H5H3O3.NitreK(NO2)O.NitroglycerinC3H5(NO2)3O3.Hydrate of limeCaH2O2PhenolC6H5OH.Calcium nitrateCa(NO2)2O2.Picric acidC6H2(NO2)3OH, &c.The difference between the salts formed by nitric acid and the nitro-compounds consists in the fact that nitric acid is very easily separated from the salts of nitric acid by means of sulphuric acid (that is, by a method of double saline decomposition), whilst nitric acid is not displaced by sulphuric acid from true nitro-compounds; for instance, nitrobenzene, C6H5·NO2. As nitro-compounds are formed exclusively from hydrocarbons, they are described with them in organic chemistry.The group NO2of nitro-compounds in many cases (like all the oxidised compounds of nitrogen) passes into the ammonia group or into the ammonia radicle NH2. This requires the action of reducing substances evolving hydrogen: RNO2+ 6H = RNH3+ 2H2O. Thus Zinin converted nitrobenzene, C6H5·NO2, into aniline, C6H5·NH2, by the action of hydrogen sulphide.Admitting the existence of the group NO2, as replacing hydrogen in various compounds, then nitric acid may be considered as water in which half the hydrogen is replaced by the radical of nitric acid. In this sense nitric acid is nitro-water, NO2OH, and its anhydride dinitro-water, (NO2)2O. In nitric acid the radical of nitric acid is combined with hydroxyl, just as in nitrobenzene it is combined with the radical of benzene.It should here be remarked that the group NO3may be recognised in the salts of nitric acid, because the salts have the composition M(NO3)n, just as the metallic chlorides have the composition MCln. But the group NO3does not form any other compounds beyond the salts, and therefore it should he considered as hydroxyl, HO, in which H is replaced by NO2.
[37 bis]The property possessed by nitroglycerin (occurring in dynamite), nitrocellulose, and the other nitro-compounds, of burning with an explosion, and their employment for smokeless powder and as explosives in general, depends on the reasons in virtue of which a mixture of nitre and charcoal deflagrates and explodes; in both cases the elements of the nitric acid occurring in the compound are decomposed, the oxygen in burning unites with the carbon, and the nitrogen is set free; thus a very large volume of gaseous substances (nitrogen and oxides of carbon) is rapidly formed from the solid substances originally taken. These gases occupy an incomparably larger volume than the original substance, and therefore produce a powerful pressure and explosion. It is evident that in exploding with the development of heat (that is, in decomposing, not with the absorption of energy, as is generally the case, but with the evolution of energy) the nitro-compounds form stores of energy which are easily set free, and that consequently their elements occur in a state of particularly energetic motion, which is especially strong in the group NO2: this group is common to all nitro-compounds, and all the oxygen compounds of nitrogen are unstable, easily decomposable, and (Note29) absorb heat in their formation. On the other hand, the nitro-compounds are instructive as an example and proof of the fact that the elements and groups forming compounds are united in definite order in the molecules of a compound. A blow, concussion, or rise of temperature is necessary to bring the combustible elements C and H into the most intimate contact with NO2, and to distribute the elements in a new order in new compounds.
As regards the composition of the nitro-compounds, it will be seen that the hydrogen of a given substance is replaced by the complex group NO2of the nitric acid. The same is observed in the passage of alkalis into nitrates, so that the reactions of substitution of nitric acid—that is, the formation of salts and nitro-compounds—may be expressed in the following manner. In these cases the hydrogen is replaced by the so-calledradicle of nitric acidNO2, as is evident from the following table:—
The difference between the salts formed by nitric acid and the nitro-compounds consists in the fact that nitric acid is very easily separated from the salts of nitric acid by means of sulphuric acid (that is, by a method of double saline decomposition), whilst nitric acid is not displaced by sulphuric acid from true nitro-compounds; for instance, nitrobenzene, C6H5·NO2. As nitro-compounds are formed exclusively from hydrocarbons, they are described with them in organic chemistry.
The group NO2of nitro-compounds in many cases (like all the oxidised compounds of nitrogen) passes into the ammonia group or into the ammonia radicle NH2. This requires the action of reducing substances evolving hydrogen: RNO2+ 6H = RNH3+ 2H2O. Thus Zinin converted nitrobenzene, C6H5·NO2, into aniline, C6H5·NH2, by the action of hydrogen sulphide.
Admitting the existence of the group NO2, as replacing hydrogen in various compounds, then nitric acid may be considered as water in which half the hydrogen is replaced by the radical of nitric acid. In this sense nitric acid is nitro-water, NO2OH, and its anhydride dinitro-water, (NO2)2O. In nitric acid the radical of nitric acid is combined with hydroxyl, just as in nitrobenzene it is combined with the radical of benzene.
It should here be remarked that the group NO3may be recognised in the salts of nitric acid, because the salts have the composition M(NO3)n, just as the metallic chlorides have the composition MCln. But the group NO3does not form any other compounds beyond the salts, and therefore it should he considered as hydroxyl, HO, in which H is replaced by NO2.
[38]The nitro-compounds play a very important part in mining and artillery. Detailed accounts of them must be looked for in special works, among which the works of A. R. Shuliachenke and T. M. Chelletsoff occupy an important place in the Russian literature on this subject, although historically the scientific works of Abel in England and Berthelot in France stand pre-eminent. The latter elucidated much in connection with explosive compounds by a series of both experimental and theoretical researches. Among explosives a particularly important place from a practical point of view is occupied by ordinary or black gunpowder (Chapter XIII., Note16), fulminating mercury (Chapter XVI., Note 26), the different forms of gun-cotton (Chapter VI., Note37), and nitro-glycerine (Chapter VIII., Note45, and Chapter XII., Note33). The latter when mixed with solid pulverulent substances, like magnesia, tripoli, &c., forms dynamite, which is so largely used in quarries and mines in driving tunnels, &c. We may add that the simplest true nitro-compound, or marsh gas, CH4, in which all the hydrogens are replaced by NO2groups has been obtained by L. N. Shishkoff, C(NO2)4, as well as nitroform, CH(NO2)3.
[38]The nitro-compounds play a very important part in mining and artillery. Detailed accounts of them must be looked for in special works, among which the works of A. R. Shuliachenke and T. M. Chelletsoff occupy an important place in the Russian literature on this subject, although historically the scientific works of Abel in England and Berthelot in France stand pre-eminent. The latter elucidated much in connection with explosive compounds by a series of both experimental and theoretical researches. Among explosives a particularly important place from a practical point of view is occupied by ordinary or black gunpowder (Chapter XIII., Note16), fulminating mercury (Chapter XVI., Note 26), the different forms of gun-cotton (Chapter VI., Note37), and nitro-glycerine (Chapter VIII., Note45, and Chapter XII., Note33). The latter when mixed with solid pulverulent substances, like magnesia, tripoli, &c., forms dynamite, which is so largely used in quarries and mines in driving tunnels, &c. We may add that the simplest true nitro-compound, or marsh gas, CH4, in which all the hydrogens are replaced by NO2groups has been obtained by L. N. Shishkoff, C(NO2)4, as well as nitroform, CH(NO2)3.
[39]see captionFig.49.—Decomposition of nitrous oxide by sodium.Nitric acid may be entirely decomposed by passing its vapour over highly incandescent copper, because the oxides of nitrogen first formed give up their oxygen to the red-hot metallic copper, so that water and nitrogen gas alone are obtained. This forms a means for determining the composition both of nitric acid and of all the other compounds of nitrogen with oxygen, because by collecting the gaseous nitrogen formed it is possible to calculate, from its volume, its weight and consequently its amount in a given quantity of a nitrogenous substance, and by weighing the copper before and after the decomposition it is possible to determine the amount of oxygen by the increase in weight. The complete decomposition of nitric acid is also accomplished by passing a mixture of hydrogen and nitric acid vapours through a red-hot tube. Sodium also decomposes the oxides of nitrogen at a red-heat, taking up all the oxygen. This method is sometimes used for determining the composition of the oxides of nitrogen.
[39]
see captionFig.49.—Decomposition of nitrous oxide by sodium.
Fig.49.—Decomposition of nitrous oxide by sodium.
Nitric acid may be entirely decomposed by passing its vapour over highly incandescent copper, because the oxides of nitrogen first formed give up their oxygen to the red-hot metallic copper, so that water and nitrogen gas alone are obtained. This forms a means for determining the composition both of nitric acid and of all the other compounds of nitrogen with oxygen, because by collecting the gaseous nitrogen formed it is possible to calculate, from its volume, its weight and consequently its amount in a given quantity of a nitrogenous substance, and by weighing the copper before and after the decomposition it is possible to determine the amount of oxygen by the increase in weight. The complete decomposition of nitric acid is also accomplished by passing a mixture of hydrogen and nitric acid vapours through a red-hot tube. Sodium also decomposes the oxides of nitrogen at a red-heat, taking up all the oxygen. This method is sometimes used for determining the composition of the oxides of nitrogen.
[40]The application of this acid for etching copper or steel in engraving is based on this fact. The copper is covered with a coating of wax, resin, &c. (etching ground), on which nitric acid does not act, and then the ground is removed in certain parts with a needle, and the whole is washed in nitric acid. The parts coated remain untouched, whilst the uncovered portions are eaten into by the acid. Copper plates for etchings, aquatints, &c., are prepared in this manner.
[40]The application of this acid for etching copper or steel in engraving is based on this fact. The copper is covered with a coating of wax, resin, &c. (etching ground), on which nitric acid does not act, and then the ground is removed in certain parts with a needle, and the whole is washed in nitric acid. The parts coated remain untouched, whilst the uncovered portions are eaten into by the acid. Copper plates for etchings, aquatints, &c., are prepared in this manner.
[41]The formation of such complex equations as the above often presents some difficulty to the beginner. It should be observed that if the reacting and resultant substances be known, it is easy to form an equation for the reaction. Thus, if we wish to form an equation expressing the reaction that nitric acid acting on zinc gives nitrous oxide, N2O, and zinc nitrate, Zn(NO3)2, we must reason as follows:—Nitric acid contains hydrogen, whilst the salt and nitrous oxide do not; hence water is formed, and therefore it is as though anhydrous nitric acid, N2O5, were acting. For its conversion into nitrous oxide it parts with four equivalents of oxygen, and hence it is able to oxidise four equivalents of zinc and to convert it into zinc oxide, ZnO. These four equivalents of zinc oxide require for their conversion into the salt four more equivalents of nitric anhydride; consequently five equivalents in all of the latter are required, or ten equivalents of nitric acid. Thus ten equivalents of nitric acid are necessary for four equivalents of zinc in order to express the reaction in whole equivalents. It must not be forgotten, however, that there are very few such reactions which can be entirely expressed by simple equations. The majority of equations of reactions only express the chief and ultimate products of reaction, and thus none of the three preceding equations express all that in reality occurs in the action of metals on nitric acid. In no one of them is only one oxide of nitrogen formed, but always several together or consecutively—one after the other, according to the temperature and strength of the acid. And this is easily intelligible. The resulting oxide is itself capable of acting on metals and of being deoxidised, and in the presence of the nitric acid it may change the acid and be itself changed. The equations given must be looked on as a systematic expression of the main features of reactions, or as a limit towards which they tend, but to which they only attain in the absence of disturbing influences.
[41]The formation of such complex equations as the above often presents some difficulty to the beginner. It should be observed that if the reacting and resultant substances be known, it is easy to form an equation for the reaction. Thus, if we wish to form an equation expressing the reaction that nitric acid acting on zinc gives nitrous oxide, N2O, and zinc nitrate, Zn(NO3)2, we must reason as follows:—Nitric acid contains hydrogen, whilst the salt and nitrous oxide do not; hence water is formed, and therefore it is as though anhydrous nitric acid, N2O5, were acting. For its conversion into nitrous oxide it parts with four equivalents of oxygen, and hence it is able to oxidise four equivalents of zinc and to convert it into zinc oxide, ZnO. These four equivalents of zinc oxide require for their conversion into the salt four more equivalents of nitric anhydride; consequently five equivalents in all of the latter are required, or ten equivalents of nitric acid. Thus ten equivalents of nitric acid are necessary for four equivalents of zinc in order to express the reaction in whole equivalents. It must not be forgotten, however, that there are very few such reactions which can be entirely expressed by simple equations. The majority of equations of reactions only express the chief and ultimate products of reaction, and thus none of the three preceding equations express all that in reality occurs in the action of metals on nitric acid. In no one of them is only one oxide of nitrogen formed, but always several together or consecutively—one after the other, according to the temperature and strength of the acid. And this is easily intelligible. The resulting oxide is itself capable of acting on metals and of being deoxidised, and in the presence of the nitric acid it may change the acid and be itself changed. The equations given must be looked on as a systematic expression of the main features of reactions, or as a limit towards which they tend, but to which they only attain in the absence of disturbing influences.
[42]Montemartini endeavours to show that the products evolved in the action of nitric acid upon metals (and their amount) is in direct connection with both the concentration of the acid and the capacity of the metals to decompose water. Those metals which only decompose water at a high temperature give, under the action of nitric acid, NO2, N2O4, and NO; whilst those metals which decompose water at a lower temperature give, besides the above products, N2O, N, and NH3; and, lastly, the metals which decompose water at the ordinary temperature also evolve hydrogen. It is observed that concentrated nitric acid oxidises many metals with much greater difficulty than when diluted with water; iron, copper, and tin are very easily oxidised by dilute nitric acid, but remain unaltered under the influence of monohydrated nitric acid or of the pure hydrate NHO3. Nitric acid diluted with a large quantity of water does not oxidise copper, but it oxidises tin; dilute nitric acid also does not oxidise either silver or mercury; but, on the addition of nitrous acid, even dilute acid acts on the above metals. This naturally depends on the smaller stability of nitrous acid, and on the fact that after the commencement of the action the nitric acid is itself converted into nitrous acid, which continues to act on the silver and mercury. Veley (Oxford 1891) made detailed researches on the action of nitric acid upon Cu, Hg, and Bi, and showed that nitric acid of 30 p.c. strength does not act upon these metals at the ordinary temperature if nitrous acid (traces are destroyed by urea) and oxidising agents such as H2O2, KClO3, &c. be entirely absent; but in the presence of even a small amount of nitrous acid the metals form nitrites, which, with HNO3, form nitrates and the oxides of nitrogen, which re-form the nitrous acid necessary for starting the reaction, because the reaction 2NO + HNO3+ H2O = 3HNO2is reversible. The above metals are quickly dissolved in a 1 p.c. solution of nitrous acid. Moreover, Veley observed that nitric acid is partially converted into nitrous acid by gaseous hydrogen in the presence of the nitrates of Cu and Pb.
[42]Montemartini endeavours to show that the products evolved in the action of nitric acid upon metals (and their amount) is in direct connection with both the concentration of the acid and the capacity of the metals to decompose water. Those metals which only decompose water at a high temperature give, under the action of nitric acid, NO2, N2O4, and NO; whilst those metals which decompose water at a lower temperature give, besides the above products, N2O, N, and NH3; and, lastly, the metals which decompose water at the ordinary temperature also evolve hydrogen. It is observed that concentrated nitric acid oxidises many metals with much greater difficulty than when diluted with water; iron, copper, and tin are very easily oxidised by dilute nitric acid, but remain unaltered under the influence of monohydrated nitric acid or of the pure hydrate NHO3. Nitric acid diluted with a large quantity of water does not oxidise copper, but it oxidises tin; dilute nitric acid also does not oxidise either silver or mercury; but, on the addition of nitrous acid, even dilute acid acts on the above metals. This naturally depends on the smaller stability of nitrous acid, and on the fact that after the commencement of the action the nitric acid is itself converted into nitrous acid, which continues to act on the silver and mercury. Veley (Oxford 1891) made detailed researches on the action of nitric acid upon Cu, Hg, and Bi, and showed that nitric acid of 30 p.c. strength does not act upon these metals at the ordinary temperature if nitrous acid (traces are destroyed by urea) and oxidising agents such as H2O2, KClO3, &c. be entirely absent; but in the presence of even a small amount of nitrous acid the metals form nitrites, which, with HNO3, form nitrates and the oxides of nitrogen, which re-form the nitrous acid necessary for starting the reaction, because the reaction 2NO + HNO3+ H2O = 3HNO2is reversible. The above metals are quickly dissolved in a 1 p.c. solution of nitrous acid. Moreover, Veley observed that nitric acid is partially converted into nitrous acid by gaseous hydrogen in the presence of the nitrates of Cu and Pb.
[43]When nitric acid acts on many organic substances it often happens that not only is hydrogen removed, but also oxygen is combined; thus, for example, nitric acid converts toluene, C7H8, into benzoic acid, C7H6O2. In certain cases, also, a portion of the carbon contained in an organic substance burns at the expense of the oxygen of the nitric acid. So, for instance, phthalic acid, C8H6O4, is obtained from naphthalene,10H8. Thus the action of nitric acid on the hydrocarbons is often most complex; not only does nitrification take place, but also separation of carbon, displacement of hydrogen, and combination of oxygen. There are few organic substances which can withstand the action of nitric acid, and it causes fundamental changes in a number of them. It leaves a yellow stain on the skin, and in a large quantity causes a wound and entirely eats away the membranes of the body. The membranes of plants are eaten into with the greatest ease by strong nitric acid in just the same manner. One of the most durable blue vegetable dyes employed in dyeing tissues isindigo; yet it is easilyconverted into a yellow substanceby the action of nitric acid, and small traces of free nitric acid may be recognised by this means.
[43]When nitric acid acts on many organic substances it often happens that not only is hydrogen removed, but also oxygen is combined; thus, for example, nitric acid converts toluene, C7H8, into benzoic acid, C7H6O2. In certain cases, also, a portion of the carbon contained in an organic substance burns at the expense of the oxygen of the nitric acid. So, for instance, phthalic acid, C8H6O4, is obtained from naphthalene,10H8. Thus the action of nitric acid on the hydrocarbons is often most complex; not only does nitrification take place, but also separation of carbon, displacement of hydrogen, and combination of oxygen. There are few organic substances which can withstand the action of nitric acid, and it causes fundamental changes in a number of them. It leaves a yellow stain on the skin, and in a large quantity causes a wound and entirely eats away the membranes of the body. The membranes of plants are eaten into with the greatest ease by strong nitric acid in just the same manner. One of the most durable blue vegetable dyes employed in dyeing tissues isindigo; yet it is easilyconverted into a yellow substanceby the action of nitric acid, and small traces of free nitric acid may be recognised by this means.
[44]According to certain investigations, if a brown liquid is formed from the melted crystals by beating above -9°, then they no longer solidify at -10°, probably because a certain amount of N2O3(and oxygen) is formed, and this substance remains liquid at -30°, or it may be that the passage from 2NO2into N2O4is not so easily accomplished as the passage from N2O4into 2NO2.Liquid nitrogen peroxide (that is, a mixture of NO2and N2O4) is employed in admixture with hydrocarbons as an explosive.
[44]According to certain investigations, if a brown liquid is formed from the melted crystals by beating above -9°, then they no longer solidify at -10°, probably because a certain amount of N2O3(and oxygen) is formed, and this substance remains liquid at -30°, or it may be that the passage from 2NO2into N2O4is not so easily accomplished as the passage from N2O4into 2NO2.
Liquid nitrogen peroxide (that is, a mixture of NO2and N2O4) is employed in admixture with hydrocarbons as an explosive.
[45]Because ifxequal the amount by weight of N2O4, its volume will =x/46, and the amount of NO2will = 100 -x, and consequently its volume will = (100 -x)/23. But the mixture, having a density 38, will weigh 100; consequently its volume will = 100/38. Hencex/46 + (100 -x)/23 = 100/38, orx= 79·O.
[45]Because ifxequal the amount by weight of N2O4, its volume will =x/46, and the amount of NO2will = 100 -x, and consequently its volume will = (100 -x)/23. But the mixture, having a density 38, will weigh 100; consequently its volume will = 100/38. Hencex/46 + (100 -x)/23 = 100/38, orx= 79·O.
[46]The phenomena and laws of dissociation, which we shall consider only in particular instances, are discussed in detail in works on theoretical chemistry. Nevertheless, in respect to nitrogen peroxide, as an historically important example of dissociation in a homogeneous gaseous medium, we will cite the results of the careful investigations (1885–1880) of E. and L. Natanson, who determined the densities under various conditions of temperature and pressure. The degree of dissociation, expressed as above (it may also he expressed otherwise—for example, by the ratio of the quantity of substance decomposed to that unaltered), proves to increase at all temperatures as the pressure diminishes, which would he expected for a homogeneous gaseous medium, as a decreasing pressure aids the formation of the lightest product of dissociation (that having the least density or largest volume). Thus, in the Natansons' experiments the degree of dissociation at 0° increases from 10 p.c. to 30 p.c., with a decrease of pressure of from 251 to 38 mm.; at 49°·7 it increases from 49 p.c. to 93 p.c., with a fall of pressure of from 498 to 27 mm., and at 100° it increases from 89·2 p.c. to 99·7 p.c., with a fall of pressure of from 732·5 to 11·7 mm. At 130° and 150° the decomposition is complete—that is, only NO2remains at the low pressures (less than the atmospheric) at which the Natansons made their determinations; but it is probable that at higher pressures (of several atmospheres) molecules of N2O4would still be formed, and it would be exceedingly interesting to trace the phenomena under the conditions of both very considerable pressures and of relatively large volumes.
[46]The phenomena and laws of dissociation, which we shall consider only in particular instances, are discussed in detail in works on theoretical chemistry. Nevertheless, in respect to nitrogen peroxide, as an historically important example of dissociation in a homogeneous gaseous medium, we will cite the results of the careful investigations (1885–1880) of E. and L. Natanson, who determined the densities under various conditions of temperature and pressure. The degree of dissociation, expressed as above (it may also he expressed otherwise—for example, by the ratio of the quantity of substance decomposed to that unaltered), proves to increase at all temperatures as the pressure diminishes, which would he expected for a homogeneous gaseous medium, as a decreasing pressure aids the formation of the lightest product of dissociation (that having the least density or largest volume). Thus, in the Natansons' experiments the degree of dissociation at 0° increases from 10 p.c. to 30 p.c., with a decrease of pressure of from 251 to 38 mm.; at 49°·7 it increases from 49 p.c. to 93 p.c., with a fall of pressure of from 498 to 27 mm., and at 100° it increases from 89·2 p.c. to 99·7 p.c., with a fall of pressure of from 732·5 to 11·7 mm. At 130° and 150° the decomposition is complete—that is, only NO2remains at the low pressures (less than the atmospheric) at which the Natansons made their determinations; but it is probable that at higher pressures (of several atmospheres) molecules of N2O4would still be formed, and it would be exceedingly interesting to trace the phenomena under the conditions of both very considerable pressures and of relatively large volumes.
[47]Liquid nitrogen peroxide is said by Geuther to boil at 22°–26°, and to have a sp. gr. at 0° = 1·494 and at 15° = 1·474. It is evident that, in the liquid as in the gaseous state, the variation of density with the temperature depends, not only on physical, but also on chemical changes, as the amount of N2O4decreases and the amount of NO2increases with the temperature, and they (as polymeric substances) should have different densities, as we find, for instance, in the hydrocarbons C5H10and C10H20.It may not be superfluous to mention here that the measurement of the specific heat of a mixture of the vapours of N2O4and NO2enabled Berthelot to determine that the transformation of 2NO2into N2O4is accompanied by the evolution of about 13,000 units of heat, and as the reaction proceeds with equal facility in either direction, it will be exothermal in the one direction and endothermal in the other; and this clearly demonstrates the possibility of reactions taking place in either direction, although, as a rule, reactions evolving heat proceed with greater ease.
[47]Liquid nitrogen peroxide is said by Geuther to boil at 22°–26°, and to have a sp. gr. at 0° = 1·494 and at 15° = 1·474. It is evident that, in the liquid as in the gaseous state, the variation of density with the temperature depends, not only on physical, but also on chemical changes, as the amount of N2O4decreases and the amount of NO2increases with the temperature, and they (as polymeric substances) should have different densities, as we find, for instance, in the hydrocarbons C5H10and C10H20.
It may not be superfluous to mention here that the measurement of the specific heat of a mixture of the vapours of N2O4and NO2enabled Berthelot to determine that the transformation of 2NO2into N2O4is accompanied by the evolution of about 13,000 units of heat, and as the reaction proceeds with equal facility in either direction, it will be exothermal in the one direction and endothermal in the other; and this clearly demonstrates the possibility of reactions taking place in either direction, although, as a rule, reactions evolving heat proceed with greater ease.
[48]Nitric acid of sp. gr. 1·51 in dissolving nitrogen peroxide becomes brown, whilst nitric acid of sp. gr. 1·32 is coloured greenish blue, and acid of sp. gr. below 1·15 remains colourless after absorbing nitrogen peroxide (Note33).
[48]Nitric acid of sp. gr. 1·51 in dissolving nitrogen peroxide becomes brown, whilst nitric acid of sp. gr. 1·32 is coloured greenish blue, and acid of sp. gr. below 1·15 remains colourless after absorbing nitrogen peroxide (Note33).
[49]Nitrogen peroxide as a mixed substance has no corresponding independent salts, but Sabatier and Senderens (1892) showed that under certain conditions NO2combines directly with some metals—for instance, copper and cobalt—forming Cu2NO2and CoNO2as dark brown powders, which do not, however, exhibit the reactions of salts. Thus by passing gaseous nitrogen dioxide over freshly reduced (from the oxides by heating with hydrogen) copper at 25°–30°, Cu2NO2is directly formed. With water it partly gives off NO2and partly forms nitrite of copper, leaving metallic copper and its suboxide. The nature of these compounds has not yet been sufficiently investigated.
[49]Nitrogen peroxide as a mixed substance has no corresponding independent salts, but Sabatier and Senderens (1892) showed that under certain conditions NO2combines directly with some metals—for instance, copper and cobalt—forming Cu2NO2and CoNO2as dark brown powders, which do not, however, exhibit the reactions of salts. Thus by passing gaseous nitrogen dioxide over freshly reduced (from the oxides by heating with hydrogen) copper at 25°–30°, Cu2NO2is directly formed. With water it partly gives off NO2and partly forms nitrite of copper, leaving metallic copper and its suboxide. The nature of these compounds has not yet been sufficiently investigated.