The six metals: ruthenium, Ru, rhodium, Rh, palladium, Pd, osmium, Os, iridium, Ir, and platinum, Pt, are met with associated together in nature. Platinum always predominates over the others, and hence they are known as theplatinum metals. By their chemical character their position in the periodic system is in the eighth group, corresponding with iron, cobalt, and nickel.
The natural transition from titanium and vanadium to copper and zinc by means of the elements of the iron group is demonstrated by all the properties of these elements, and in exactly the same manner a transition from zirconium, niobium, and molybdenum to silver, cadmium, and indium, through ruthenium, rhodium, and palladium, is in perfect accordance with fact and with the magnitude of the atomic weights, as also is the position of osmium, iridium, and platinum between tantalum and tungsten on the one side, and gold and mercury on the other. In all these three cases the elements of smaller atomic weight (chromium, molybdenum, and tungsten) are able, in their higher grades of oxidation, to give acid oxides having the properties of distinct but feebly energetic acids (in the lower oxides they give bases), whilst the elements of greater atomic weight (zinc, cadmium, mercury), even in their higher grades of oxidation, only give bases, although with feebly developed basic properties. The platinum metals present the same intermediate properties such as we have already seen in iron and the elements of the eighth group.
In the platinum metals the intermediate propertiesof feebly acid and feebly basic metalsare developed with great clearness, so that there is not one sharply-defined acid anhydride among their oxides, although there is a great diversity in the grades of oxidation from the type RO4to R2O. The feebleness of the chemical forces observed in the platinum metals is connected with the ready decomposability of their compounds, with the small atomic volume of the metals themselves,and with their large atomic weight. The oxides of platinum, iridium, and osmium can scarcely be termed either basic or acid; they are capable of combinations of both kinds, each of which is feeble. They are all intermediate oxides.
The atomic weights of platinum, iridium, and osmium are nearly 191 to 196, and of palladium, rhodium, and ruthenium, 104 to 106. Thus, strictly speaking, we have here two series of metals, which are, moreover, perfectly parallel to each other; three members in the first series, and three members in the second—namely, platinum presents an analogy to palladium, iridium to rhodium, and osmium to ruthenium. As a matter of fact, however, the wholegroupof the platinum metals is characterised bya number of common properties, both physical and chemical, and, moreover, there are several points of resemblance between the members of this group and those of theirongroup (ChapterXXII.) The atomic volumes (Table III., column 18) of the elements of this group arenearly equalandvery small. The iron metals have atomic volumes of nearly 7, whilst that of the metals allied to palladium is nearly 9, and of those adjacent to platinum (Pt, Ir, Os) nearly 9·4. This comparatively small atomic volume corresponds with the great infusibility and tenacity proper to all the iron and platinum metals, and to their small chemical energy, which stands out very clearly in the heavy platinum metals. All the platinum metals are veryeasily reducedby ignition and by the action of various reducing agents, in which process oxygen, or a haloid group, is disengaged from their compounds and the metal left behind. This is a property of the platinum metals which determines many of their reactions, and the circumstance of their always being found in naturein a native state. In Russia in the Urals (discovered in 1819) and in Brazil (1735) platinum is obtained from alluvial deposits, but in 1892 Professor Inostrantseff discovered a vein deposit of platinum in serpentine near Tagil in the Urals.[1]The facility with which they are reduced is so great that their chlorides are even decomposed by gaseous hydrogen, especially when shaken up and heated under a certain pressure. Hence it will be readily understood that such metals as zinc, iron, &c., separate them from solutions with great ease, which fact is taken advantage of in practice and in the chemical treatment of the platinum metals.[1 bis]
All the platinum metals, like those of the iron group, are grey, with a comparatively feeble metallic lustre, and are very infusible. In this respect they stand in the same order as the metals of the iron series; nickel is more fusible and whiter than cobalt and iron, so also palladium is whiter and more fusible than rhodium and ruthenium, and platinum is comparatively more fusible and whiter than iridium or osmium. The saline compounds of these metals are red or yellow, like those of the majority of the metals of the iron series, and like the latter, the different forms of oxidation present different colours. Moreover, certain complex compounds of the platinum metals, like certain complex compounds of the iron series, either have particular characteristic tints or else are colourless.
The platinum metals are foundin nature associated together in alluvial depositsin a few localities, from which they are washed, owing to their very considerable density, which enables a stream of water to wash away the sand and clay with which they are mixed. Platinum deposits are chiefly known in the Urals, and also in Brazil and a few other localities. The platinum ore washed from these alluvial deposits presents the appearance of more or less coarse grains, and sometimes, as it were, of semi-fused nuggets.[2]
All the platinum metals give compounds with the halogens, and the highest haloid type of combination for all is RX4. For the majority of the platinum metals this type is exceedingly unstable; the lower compounds corresponding to the type RX2, which are formed by the separation of X2, are more stable. In the type RX2the platinum metals form more stable salts, which offer no little resemblance tothe kindred compounds of the iron series—for example, to nickelous chloride, NiCl2, cobaltous chloride, CoCl2, &c. This even expresses itself in a similarity of volume (platinous chloride, PtCl2, volume, 46; nickelous chloride, NiCl2= 50), although in the type RX2the true iron metals give very stable compounds, whilst the platinum metals frequently react after the manner of suboxides, decomposing into the metal and higher types, 2RX2= R + RX4. This probably depends on the facility with which RX2decomposes into R and X2, when X2combines with the remaining portion of RX2.
As in the series iron, cobalt, nickel, nickel gives NiO and Ni2O3, whilst cobalt and iron give higher and varied forms of oxidation, so also among the platinum metals, platinum and palladium only give the forms RX2and RX4, whilst rhodium and iridium form another and intermediate type, RX3, also met with in cobalt, corresponding with the oxide, having the composition R2O3, besides which they form an acid oxide, like ferric acid, which is also known in the form of salts, but is in every respect unstable.Osmiumandruthenium, like manganese, form still higher oxides, and in this respect exhibit the greatest diversity. They not only give RX2, RX3, RX4, and RX6, but also a stillhigher form of oxidation, RO4, which is not met with in any other series. This form is exceedingly characteristic, owing to the fact that the oxides, OsO4and RuO4, are volatile and have feebly acid properties. In this respect they most resemble permanganic anhydride, which is also somewhat volatile.[3]
When dissolved in aqua regia (PtCl4is formed) and liberated from the solution by sal-ammoniac ((NH4)2PtCl6is formed) and reduced by ignition (which may be done by Zn and other reducing agents, direct from a solution of PtCl4) platinum[3 bis]forms a powdery mass, knownas spongy platinum or platinum black. If this powder of platinum be heated and pressed, or hammered in a cylinder, the grains aggregate or forge together, and form a continuous, though of course not entirely homogeneous, mass. Platinum was formerly, and is even now, worked up in this manner. The platinum money formerly used in Russia was made in this way. Sainte-Claire Deville, in the fifties, for the first time melted platinum in considerable quantities by employing a special furnace made in the form of a small reverberatory furnace, and composed of two pieces of lime, on which the heat of the oxyhydrogen flame has no action. Into this furnace (shown in fig. 34, Vol. I. p.175)—or, more strictly speaking, into the cavity made in the pieces of lime—the platinum is introduced, and two orifices are made in the lime; through one, the upper, or side orifice, is introduced an oxyhydrogen gas burner, in which either detonating gas or a mixture of oxygen and coal-gas is burnt, whilst the other orifice serves for the escape of the products of combustion and certain impurities which are more volatile than the platinum, and especially the oxidised compounds of osmium, ruthenium, and palladium, which are comparatively easily volatilised by heat. In this manner the platinum is converted into a continuous metallic form by means of fusion, and this method is now used for melting considerable masses of platinum[4]and its alloys with iridium.
To obtain pure platinum, the ore is treated with aqua regia in which only the osmium and iridium are insoluble. The solution contains the platinum metals in the form RCl4, and in the lower forms of chlorination, RCl3and RCl2, because some of these metals—for instance, palladium and rhodium—form such unstable chlorides of the type RX4that they partially decompose even when diluted with water, and pass into the stable lower type of combination; in addition to which the chlorine is very easily disengaged if it comes in contact with substances on which it can act. In this respect platinum resists the action of heat and reducing agents better than any of its companions—that is, it passes with greater difficulty from PtCl4to the lower compound PtCl2. On this is based the method of preparation of more or less pure platinum. Lime or sodium hydroxide is added to the solution in aqua regia until neutralised, or only containing a very slight excess of alkali. It is best to first evaporate and slightly ignite the solution, in order to remove the excess of acid, and by heating it to partially convert the higher chlorides of the palladium, &c., into the lower. The addition of alkalis completes the reduction, because the chlorine held in the compounds RX4acts on the alkali like free chlorine, converting it into a hypochlorite. Thus palladium chloride, PdCl4, for example, is converted into palladious chloride, PdCl2, by this means, according to the equation PdCl4+ 2NaHO = PdCl2+ NaCl + NaClO + H2O. In a similar manner iridic chloride, IrCl4, is converted into the trichloride, IrCl3, by this method. When this conversion takes place the platinum still remains in the form of platinic chloride, PtCl4. It is then possible to take advantage of a certain difference in the properties of the higher and lower chlorides of the platinum metals. Thus lime precipitates the lower chlorides of the members of the platinum metals occurring in solution without acting on the platinic chloride, PtCl4, and hence the addition of a large proportion of lime immediately precipitates the associated metals, leaving the platinum itself in solution in the form of a soluble double salt, PtCl4,CaCl2. A far better and more perfectseparationis effectedby means of ammonium chloride, which gives, with platinic chloride, an insoluble yellow precipitate, PtCl4,2NH4Cl, whilst it forms soluble double salts with the lower chlorides RCl2and RCl3, so that ammonium chloride precipitates the platinum only from the solution obtained by the preceding method. These methods are employed for preparing the platinum which is used for the manufacture of platinum articles, because, having platinum in solution as calcium platinochloride, PtCaCl6, or as the insoluble ammonium platinochloride, Pt(NH4)2Cl6, the platinum compound in every case, after drying or ignition, loses all the chlorine from the platinic chloride and leaves finely-divided metallic platinum, which may be converted into homogeneous metal by compression and forging, or by fusion.[5]
Metallicplatinumin a fused state has a specific gravity of 21; it is grey, softer than iron but harder than copper, exceedingly ductile, and therefore easily drawn into wire and rolled into thin sheets, and may be hammered into crucibles and drawn into thin tubes, &c. In the state in which it is obtained by the ignition of its compounds, it forms a spongy mass, known as spongy platinum, or else as powder (platinum black).[6]In either case it is dull grey, and is characterised, as we already know, by the faculty of absorbing hydrogen and other gases. Platinum is not acted on by hydrochloric, hydriodic, nitric, and sulphuric acids, or a mixture of hydrofluoric and nitric acids. Aqua regia, and any liquid containing chlorine or able to evolve chlorine or bromine, dissolves platinum. Alkalis are decomposed by platinum at a red heat, owing to the faculty of the platinum oxide, PtO2, formed to combine with alkaline bases, inasmuch as it has a feebly-developed acid character (seeNote8). Sulphur, phosphorus (the phosphide, PtP2,is formed), arsenic and silicon all act more or less rapidly on platinum, under the influence of heat. Many of the metals form alloys with it. Even charcoal combines with platinum when it is ignited with it, and therefore carbonaceous matter cannot be subjected to prolonged and powerful ignition in platinum vessels. Hence a platinum crucible soon becomes dull on the surface in a smoky flame. Platinum also forms alloys with zinc, lead, tin, copper, gold, and silver.[7]Although mercury does not directly dissolve platinum, still it forms a solution or amalgam with spongy platinum in the presence of sodium amalgam; a similar amalgam is also formed by the action of sodium amalgam on a solution of platinum chloride, and is used for physical experiments.
There aretwo kindsofplatinum compounds, PtX4and PtX2. The former are produced by an excess of halogen in the cold, and the latter by the aid of heat or by the splitting up of the former. The starting-point for the platinum compounds isplatinum tetrachloride,platinic chloride, PtCl4, obtained by dissolving platinum in aqua regia.[7 bis]The solution crystallises in the cold, in a desiccator, in the form of reddish-brown deliquescent crystals which contain hydrochloric acid, PtCl4,2HCl,6H2O, and behave like a true acid whose salts correspond to the formula R2PtCl6—ammonium platinochloride, for example.[7 tri]The hydrochloric acid is liberated from these crystals by gently heating or evaporating the solution to dryness; or, better still, after treatment with silver nitrate a reddish-brown mass remains behind, which dissolves in water, and forms a yellowish-red solution which on cooling deposits crystals of the composition PtCl4,8H2O. Thetendencyof PtCl4to combinewith hydrochloric acid and water—that is,to form higher crystalline compounds—is evident in the platinum compounds, and must be taken into account in explaining the properties of platinum and the formation of many other of its complex compounds. Dilute solutions of platinic chloride are yellow, and are completely reduced by hydrogen, sulphurous anhydride, and many reducing agents, which first convert the platinic chloride intothe lower compound platinous chloride, PtCl2. That faculty which reveals itself in platinum tetrachloride of combining with water of crystallisation and hydrochloric acid is distinctly marked in its property, with which we are already acquainted, of giving precipitates with the salts of potassium, ammonium, rubidium, &c. In general itreadily forms double salts, R2PtCl6= PtCl4+ 2RCl, where R is a univalent metal such as potassium or NH4. Hence the addition of a solution of potassium or ammonium chloride to a solution of platinic chloride is followed by the formation of a yellow precipitate, which is sparingly soluble in water and almost entirely insoluble in alcohol and ether (platinic chloride is soluble in alcohol, potassium iridiochloride, IrK3Cl6,i.e.a compound of IrCl3, is soluble in water but not in alcohol). It is especially remarkable in this case, that the potassium compounds here, as in a number of other instances, separate in an anhydrous form, whilst the sodium compounds, which are soluble in water and alcohol, form red crystals containing water. The composition Na2PtCl6,6H2O exactly corresponds with the above-mentioned hydrochloric compound. The compounds with barium, BaPtCl6,4H2O, strontium, SrPtCl6,8H2O, calcium, magnesium, iron, manganese, and many other metals are all soluble in water.[8]
Platinous chloride, PtCl2, is formed when hydrogen platinochloride, PtH2Cl6, is ignited at 300°, or when potassium is heated at 230° in a stream of chlorine. The undecomposed tetrachloride is extracted from the residue by washing it with water, and a greenish-grey or brown insoluble mass of the dichloride (sp. gr. 5·9) is then obtained. It is soluble in hydrochloric acid, giving an acid solution of the composition PtCl2,2HCl, corresponding with the type of double salts PtR2Cl4. Although platinous chloride decomposes below 500°, still it is formed to a small extent at higher temperatures. Troost and Hautefeuille, and Seelheim observed that when platinum was strongly ignited in a stream of chlorine, the metal, as it were, slowly volatilised and was deposited in crystals; a volatile chloride, probably platinous chloride, was evidently formed in this case, and decomposed subsequently to its formation, depositing crystals of platinum.
The properties of platinum above-described are repeated more or less distinctly, or sometimes with certain modifications, in the above-mentioned associates and analogues of this metal. Thus although palladium forms PdCl4, this form passes into PdCl2with extreme ease.[9]Whilstrhodium and iridium in dissolving in aqua regia also form RhCl4and IrCl4, but they pass into RhCl3and IrCl3[9 bis]very easily when heated or when acted upon by substances capable of taking up chlorine (even alkalis, which form bleaching salts). Among the platinum metals, ruthenium and osmium have the most acid character, and although they give RuCl4and OsCl4they are easily oxidised to RuO4, and OsO4by the action of chlorine in the presence of water; the latter are volatile and may be distilled with the water and hydrochloric acid, from a solution containing other platinum metals.[9 tri]Thus with respect tothe types of combination, all the platinum metals, under certain circumstances, give compounds of the type RX4—for instance, RO2, RCl4, &c.But this is the highest form for only platinum and palladium. The remaining platinum metals further,like iron, give acidsof the typeRO3or hydrates, H2RO4= RO2(HO)2(the type of sulphuric acid); but they, like ferric and manganic acids, are chiefly known in the form of salts of the composition K2RO4or K2R2O7(like the dichromate). These salts are obtained, like the manganates and ferrates, by fusing the oxides, or even the metals themselves, with nitric, or, better still, with potassium peroxide. They are soluble in water, are easily deoxidised and do not yield the acid anhydrides under the action of acids, but break up, either (like the ferrate) forming oxygen and a basic oxide (iridium and rhodium react in this manner, as they do not give higher forms of oxidation), or passing into a lower and higher form of oxidation—that is, reacting like a manganate (or partly like nitrite or phosphite). Osmium and ruthenium react according to the latter form, as they are capable of givinghigher forms of oxidation, OsO4and RuO4, and therefore their reactions of decomposition may be essentially represented by the equation: 2OsO3= OsO2+ OsO4.[10]
Platinum and its analogues, like iron and its analogues, are able to form complex and comparatively stable cyanogen and ammonia compounds, corresponding with the ferrocyanides and the ammoniacal compounds of cobalt, which we have already considered in the preceding chapter.
If platinous chloride, PtCl2(insoluble in water), be added by degrees to a solution of potassium cyanide, it is completely dissolved (like silver chloride), and on evaporating the solution deposits rhombic prisms ofpotassium platinocyanide, PtK2(CN)4,3H2O. This salt, like all those corresponding with it, has a remarkable play of colours, due to the phenomena of dichromism, and even polychromism, natural to all the platinocyanides. Thus it is yellow and reflects a bright blue light. It is easily soluble in water, effloresces in air, then turns red, and at 100° orange, when it loses all its water. The loss of water does not destroy its stability—that is, it still remains unchanged, and its stability is further shown by the fact that it is formed when potassium ferrocyanide, K4Fe(CN)6, is heated with platinum black. This salt, first obtained by Gmelin, shows a neutral reaction with litmus; it is exceedingly stable under the action of air, like potassium ferrocyanide, which it resembles in many respects. Thus the platinum in it cannot be detected by reagents such as sulphuretted hydrogen; the potassium may be replaced by other metals by the action of their salts, so that it corresponds with a whole series of compounds, R2Pt(CN)4, and it is stable, although the potassium cyanide and platinous salts, of which it is composed, individually easily undergo change. When treated with oxidising agents it passes, like the ferrocyanide, into a higher form of combination of platinum. If salts of silver be added to its solution, it gives a heavy white precipitate of silver platinocyanide, PtAg2(CN)4, which when suspended in water and treated with sulphuretted hydrogen, enters into double decomposition with the latter and forms insoluble silver sulphide, Ag2S, and solublehydroplatinocyanic acid, H2Pt(CN)4. If potassium platinocyanide be mixed with an equivalent quantity of sulphuric acid, the hydroplatinocyanic acid liberated may be extracted by a mixture of alcohol and ether. The ethereal solution, when evaporated in a desiccator, deposits bright red crystals of the composition PtH2(CN)4,5H2O. This acid colours litmus paper, liberates carbonic anhydride from sodium carbonate, and saturates alkalis, so that it presents an analogy to hydroferrocyanic acid.[11]
Ammonia, like potassium cyanide, has the faculty of easily reacting with platinum dichloride, forming compounds similar to the platinocyanide and cobaltia compounds, which are comparatively stable. But as ammonia does not contain any hydrogen easily replaceable by metals, and as ammonia itself is able to combine with acids, the PtX2plays, as it were, the part of an acid with reference to the ammonia. Owing to the influence of the ammonia, the X2in the resultant compound will represent the same character as it has in ammoniacal salts; consequently, the ammoniacal compounds produced from PtX2will be salts in which X will be replaceable by various other haloids, just as the metal is replaced in the cyanogen salts; such is the nature of theplatino-ammonium compounds. PtX2forms compounds with 2NH3and with 4NH3, and so also PtX4gives (not directly from PtX4and ammonia, but from the compounds of PtX2by the action of chlorine, &c.) similar compounds with 2NH3and with 4NH3.[12]
If ammonia acts on a boiling solution of platinous chloride in hydrochloric acid, it produces the greensalt of Magnus(1829), PtCl2,2NH3, insoluble in water and hydrochloric acid. But, judging by its reactions, this salt has twice this formula. Thus, Gros (1837), on boiling Magnus's salt with nitric acid, observed that half the chlorine was replaced by the residue of nitric acid and half the platinum was disengaged: 2PtCl2(NH3)2+ 2HNO3= PtCl2(NO3)2(NH3)4+ 2PtCl2. The Gros's salt thus obtained, PtCl2(NO3)24NH3(if Magnus's saltbelongs to the type PtX2, then Gros's salt belongs to the type PtX4), is soluble in water, and the elements of nitric acid, but not the chlorine, contained in it are capable of easily submitting themselves to double saline decomposition. Thus silver nitrate does not enter into double decomposition with the chlorine of Gros's salt. Most instructive was the circumstance that Gros, by acting on his salt with hydrochloric acid, succeeded in substituting the residue of nitric acid in it by chlorine, and the chlorine thus introduced, easily reacted with silver nitrate. Thus it appeared that Gros's salt contained two varieties of chlorine—one which reacts readily, and the other which reacts with difficulty. The composition of Gros's first salt is PtCl2(NH3)4(NO3)2; it may be converted into PtCl2(NH3)4(SO4), and in general into PtCl2(NH3)4X2.[13]
The salt of Magnus when boiled with a solution of ammonia gives the salt (of Reiset's first base) PtCl2(NH3)4, and this, when treated with bromine, forms the salt PtCl2Br2(NH3)4, which has the same composition and reactions as Gros's salt. To Reiset's salts there corresponds a soluble, colourless, crystallinehydroxide, Pt(OH)2(NH3)4, having the properties of a powerful and very energeticalkali; it attracts carbonic anhydride from the atmosphere, precipitates metallic salts like potash, saturates active acids, even sulphuric, forming colourless (with nitric, carbonic, and hydrochloric acids), or yellow (with sulphuric acid), salts of the type PtX2(NH3)4.[14]The comparativestability (for instance, as compared with AgCl and NH3) of such compounds, and the existence of many other compounds analogousto them, endows them with a particular chemical interest. Thus Kournakoff (1889) obtained a series of corresponding compounds containing thiocarbamide, CSN2H4, in the place of ammonia, PtCl2,4CSN2H4, and others corresponding with Reiset's salts. Hydroxylamine, and other substances corresponding with ammonia, also give similar compounds. The common properties and composition of such compounds show their entire analogy to the cobaltia compounds (especially for ruthenium and iridium) and correspond to the fact that both the platinum metals and cobalt occur in the same, eighth, group.