  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 the platinum 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 properties of feebly acid and feebly basic metals are 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 RO4 to 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, 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 whole group of the platinum metals is characterised by a 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 the iron group (Chapter XXII.) The atomic volumes (Table III., column 18) of the elements of this group are nearly equal and very 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 very easily reduced by 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 nature in 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. 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 found in nature associated together in alluvial deposits in 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. 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 RX2 the platinum metals form more stable salts, which offer no little resemblance to 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 RX2 and 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. Osmium and ruthenium, 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 still higher 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, OsO4 and RuO4, are volatile and have feebly acid properties. In this respect they most resemble permanganic anhydride, which is also somewhat volatile. When dissolved in aqua regia (PtCl4 is formed) and liberated from the solution by sal-ammoniac ((NH4)2PtCl6 is formed) and reduced by ignition (which may be done by Zn and other reducing agents, direct from a solution of PtCl4) platinum 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, RCl3 and RCl2, because some of these metals—for instance, palladium and rhodium—form such unstable chlorides of the type RX4 that 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 PtCl4 to 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 RX4 acts 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 perfect Metallic platinum in 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). There are two kinds of platinum compounds, PtX4 and 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 is platinum tetrachloride, platinic chloride, PtCl4, obtained by dissolving platinum in aqua regia. 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 PdCl2 with extreme ease. 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 of potassium 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 soluble hydroplatinocyanic 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.
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 PtX2 plays, as it were, the part of an acid with reference to the ammonia. Owing to the influence of the ammonia, the X2 in the resultant compound will represent the same character as it has in ammoniacal salts; consequently, the ammoniacal compounds produced from PtX2 will 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 the platino-ammonium compounds. PtX2 forms compounds with 2NH3 and with 4NH3, and so also PtX4 gives (not directly from PtX4 and ammonia, but from the compounds of PtX2 by the action of chlorine, &c.) similar compounds with 2NH3 and with 4NH3.
If ammonia acts on a boiling solution of platinous chloride in hydrochloric acid, it produces the green salt 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 salt 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, crystalline hydroxide, Pt(OH)2(NH3)4, having the properties of a powerful and very energetic alkali; 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. Footnotes: The treatment of platinum ore is chiefly carried on for the extraction of the platinum itself and its alloys with iridium, because these metals offer a greater resistance to the action of chemical reagents and high temperatures than any of the other malleable and ductile metals, and therefore the wire so often used in the laboratory and for technical purposes is made from them, as also are various vessels used for chemical purposes in the laboratory and in works. Thus sulphuric acid is distilled in platinum retorts, and many substances are fused, ignited, and evaporated in the laboratory in platinum crucibles and on platinum foil. Gold and many other substances are dissolved in dishes made of iridium-platinum, because the alloys of platinum and iridium are but slightly attacked when subjected to the action of aqua regia. The comparatively high density (about 21·5), hardness, ductility, and infusibility (it does not melt at a furnace heat, but only in the oxyhydrogen flame or electric furnace), as well as the fact of its resisting the action of water, air, and other reagents, renders an alloy of 90 parts of platinum and 10 parts of iridium (Deville's platinum-iridium alloy) a most valuable material for making standard weights and measures, such as the metre, kilogram, and pound, and therefore all the newest standards of most countries are made of this alloy. Platinic bromide, PtBr4, and iodide, PtI4, are analogous to the tetrachloride, but the iodide is decomposed still more easily than the chloride. If sulphuric acid be added to platinic chloride, and the solution evaporated, it forms a black porous mass like charcoal, which deliquesces in the air, and has the composition Pt(SO4)2. But this, the only oxygen salt of the type PtX4, is exceedingly unstable. This is due to the fact that platinum oxide, the oxide of the type PtO2, has a feeble acid character. This is shown in a number of instances. Thus if a strong solution of platinic chloride treated with sodium carbonate be exposed to the action of light or evaporated to dryness and then washed with water, a sodium platinate, Pt3Na2O7,6H2O, remains. The composition of this salt, if we regard it in the same sense as we did the salts of silicic, titanic, molybdic and other acids, will be PtO(ONa)2,2PtO2,6H2O—that is, the same type is repeated as we saw in the crystalline compounds of platinum tetrachloride with sodium chloride, or with hydrochloric acid—namely, the type PtX48Y, where Y is the molecule H2O,HCl, &c. Similar compounds are also obtained with other alkalis. They will be platinates of the alkalis in which the platinic oxide, PtO2, plays the part of an acid oxide. Rousseau (1889) obtained different grades of combination BaOPtO2, 3(BaO)2PtO2, &c., by igniting a mixture of PtCl4 and caustic baryta. If such an alkaline compound of platinum be treated with acetic acid, the alkali combines with the latter, and a platinic hydroxide, Pt(OH)4, remains as a brown mass, which loses water and oxygen when ignited, and in so doing decomposes with a slight explosion. When slightly ignited this hydroxide first loses water and gives the very unstable oxide PtO2. Platinic sulphide, PtS2, belongs to the same type; it is precipitated by the action of sulphuretted hydrogen on a solution of platinum tetrachloride. The moist precipitate is capable of attracting oxygen, and is then converted into the sulphate above mentioned, which is soluble in water. This absorption of oxygen and conversion into sulphate is another illustration of the basic nature of PtO2, so that it clearly exhibits both basic and acid properties. The latter appear, for instance, in the fact that platinic sulphide, PtS2, gives crystalline compounds with the alkali sulphides. That portion of the platinum ore which dissolves in aqua regia and is precipitated by ammonium or potassium chloride does not contain palladium. It remains in solution, because the palladic chloride, PdCl4, is decomposed and the palladous chloride formed is not precipitated by ammonium chloride; the same holds good for all the other lower chlorides of the platinum metals. Zinc (and iron) separates out all the unprecipitated platinum metals (and also copper, &c.) from the solution. The palladium is found in these platinum residues precipitated by zinc. If this mixture of metals be treated with aqua regia, all the palladium will pass into solution as palladous chloride with some platinic chloride. By this treatment the main portion of the iridium, rhodium, &c. remains almost undissolved, the platinum is separated from the mixture of palladous and platinic chlorides by a solution of ammonium chloride, and the solution of palladium is precipitated by potassium iodide or mercuric cyanide. Wilm (1881) showed that palladium may be separated from an impure solution by saturating it with ammonia; all the iron present is thus precipitated, and, after filtering, the addition of hydrochloric acid to the filtrate gives a yellow precipitate of an ammonio-palladium compound, PdCl2,2NH3, whilst nearly all the other metals remain in solution. Metallic palladium is obtained by igniting the ammonio-compound or the cyanide, PdC2N2. It occurs native, although rarely, and is a metal of a whiter colour than platinum, sp. gr. 11·4, melts at about 1,500°; it is much more volatile than platinum, partially oxidises on the surface when heated (Wilm obtained spongy palladium by igniting PdCl2,2NH3, and observed that it gives PdO when ignited in oxygen, and that on further ignition this oxide forms a mixture of Pd2O and Pd), and loses its absorbed oxygen on a further rise of temperature. It does not blacken or tarnish (does not absorb sulphur) in the air at the ordinary temperature, and is therefore better suited than silver for astronomical and other instruments in which fine divisions have to be engraved on a white metal, in order that the fine lines should be clearly visible. The most remarkable property of palladium, discovered by Graham, consists in its capacity for absorbing a large amount of hydrogen. Ignited palladium absorbs as much as 940 volumes of hydrogen, or about 0·7 p.c. of its own weight, which closely approaches to the formation of the compound Pd3H2, and probably indicates the formation of palladium hydride, Pd2H. This absorption also takes place at the ordinary temperature—for example, when palladium serves as an electrode at which hydrogen is evolved. In absorbing the hydrogen, the palladium does not change in appearance, and retains all its metallic properties, only its volume increases by about 10 p.c.—that is, the hydrogen pushes out and separates the atoms of the palladium from each other, and is itself compressed to 1/900 of its volume. This compression indicates a great force of chemical attraction, and is accompanied by the evolution of heat (Chapter II., Note 38). The absorption of 1 grm. of hydrogen by metallic palladium (Favre) is accompanied by the evolution of 4·2 thousand calories (for Pt 20, for Na 13, for K 10 thousand units of heat). Troost showed that the dissociation pressure of palladium hydride is inconsiderable at the ordinary temperature, but reaches the atmospheric pressure at about 140°. This subject was subsequently investigated by A. A. Cracow of St. Petersburg (1894), who showed that at first the absorption of hydrogen by the palladium proceeds like solution, according to the law of Dalton and Henry, but that towards the end it proceeds like a dissociation phenomenon in definite compounds; this forms another link between the phenomenon of solution and of the formation of definite atomic compounds. Cracow's observations for a temperature 18°, showed that the electro-conductivity and tension vary until a compound Pd2H is reached, and namely, that the tension p rises with the volume v of hydrogen absorbed, according to the law of Dalton and Henry—for instance, for
The maximum tension at 18° is 9 mm. At a temperature of about 140° (in the vapour of xylene) the maximum tension is about 760 mm., and when v = 10–50 vols. the tension (according to Cracow's experiments) stands at 90–450 mm.—that is, increases in proportion to the volume of hydrogen absorbed. But from the point of view of chemical mechanics it is especially important to remark that Moutier clearly showed, through palladium hydride, the similarity of the phenomena which proceed in evaporation and dissociation, which fact Henri Sainte-Claire Deville placed as a fundamental proposition in the theory of dissociation. It is possible upon the basis of the second law of the theory of heat, according to the law of the variation of the tension p of evaporation with the temperature T (counted from -273°), to calculate the latent heat of evaporation L (see works on physics) because 424L = T(?d - ?/D)dp/dt, where d and D are the weights of cubic measures of the gas (vapour) and liquid. (Thus, for instance, for water, when t = 100°, T = 373, d = 0·605, D = 960, dp/dt = 0·027 m., 13,596 = 367, L = 536, whence 424L = 227,264, and the second portion of the equation 226,144, which is sufficiently near, within the limits of experimental error, see Chapter I., Note 11.) The same equation is applicable to the dissociation of Na2H and K2H—(Chapter XII., Note 42)—but it has only been verified in this respect for Pd2H, since Moutier, by calculating the amount of heat L evolved, for t = 20, according to the variation of the tension (dp/dt) obtained 4·1 thousand calories, which is very near the figure obtained experimentally by Favre (see Chapter XII., Note 44). The absorbed hydrogen is easily disengaged by ignition or decreased pressure. The resultant compound does not decompose at the ordinary temperature, but when exposed to air the metal sometimes glows spontaneously, owing to the hydrogen burning at the expense of the atmospheric oxygen. The hydrogen absorbed by palladium acts towards many solutions as a reducing agent; in a word, everything here points to the formation of a definite compound and at the same time of a physically-compressed gas, and forms one of the best examples of the bond existing between chemical and physical processes, to which we have many times drawn attention. It must be again remembered that the other metals of the eighth group, even copper, are, like palladium and platinum, able to combine with hydrogen. The permeability of iron and platinum tubes to hydrogen is naturally due to the formation of similar compounds, but palladium is the most permeable. From what has been said respecting the separation of platinum and rhodium it will be understood how the compounds of iridium, which is the main associate of platinum, are obtained. In describing the treatment of osmiridium we shall again have an opportunity of learning the method of extraction of the compounds of this metal, which has in recent times found a technical application in the form of its oxide, Ir2O3; this is obtained from many of the compounds of iridium by ignition with water, is easily reduced by hydrogen, and is insoluble in acids. It is used in painting on china, for giving a black colour. Iridium itself is more difficultly fusible than platinum, and when fused it does not decompose acids or even aqua regia; it is extremely hard, and is not malleable; its sp. gr. is 22·4. In the form of powder it dissolves in aqua regia, and is even partially oxidised when heated in air, sets fire to hydrogen, and, in a word, closely resembles platinum. Heated in an excess of chlorine it gives iridic chloride, IrCl4, but this loses chlorine at 50°; it is, however, more stable in the form of double salts, which have a characteristic black colour—for instance, Ir(NH4)2Cl6—but they give iridious chloride, IrCl3, when treated with sulphuric acid. The primary source from which the compounds of ruthenium and osmium are obtained is either osmiridium (the osmium predominates, from IrOs to IrOs4, sp. gr. from 16 to 21), which occurs in platinum ores (it is distinguished from the grains of platinum by its crystalline structure, hardness, and insolubility in aqua regia), or else those insoluble residues which are obtained, as we saw above, after treating platinum with aqua regia. Osmium predominates in these materials, which sometimes contain from 30 p.c. to 40 p.c. of it, and rarely more than 4 p.c. to 5 p.c. of ruthenium. The process for their treatment is as follows: they are first fused with 6 parts of zinc, and the zinc is then extracted with dilute hydrochloric acid. The osmiridium thus treated is, according to Fritzsche and StruvÉ's method, then added to a fused mixture of potassium hydroxide and chlorate in an iron crucible; the mass as it begins to evolve oxygen acts on the metal, and the reaction afterwards proceeds spontaneously. The dark product is treated with water, and gives a solution of osmium and ruthenium in the form of soluble salts, R2OsO4 and R2RuO4, whilst the insoluble residue contains a mixture of oxides of iridium (and some osmium, rhodium, and ruthenium), and grains of metallic iridium still unacted on. According to FrÉmy's method the lumps of osmiridium are straightway heated to whiteness in a porcelain tube in a stream of air or oxygen, when the very volatile osmic anhydride is obtained directly, and is collected in a well-cooled receiver, whilst the ruthenium gives a crystalline sublimate of the dioxide, RuO2, which is, however, very difficultly volatile (it volatilises together with osmic anhydride), and therefore remains in the cooler portions of the tube; this method does not give volatile ruthenic anhydride, and the iridium and other metals are not oxidised or give non-volatile products. This method is simple, and at once gives dry, pure osmic anhydride in the receiver, and ruthenium dioxide in the sublimate. The air which passes through the tube should be previously passed through sulphuric acid, not only in order to dry it, but also to remove the organic and reducing dust. The vapour of osmic anhydride must be powerfully cooled, and ultimately passed over caustic potash. A third mode of treatment, which is most frequently employed, was proposed by WÖhler, and consists in slightly heating (in order that the sodium chloride should not melt) an intimate mixture of osmiridium and common salt in a stream of moist chlorine. The metals then form compounds with chlorine and sodium chloride, whilst the osmium forms the chloride, OsCl4, which reacts with the moisture, and gives osmic anhydride, which is condensed. The ruthenium in this, as in the other processes, does not directly give ruthenic anhydride, but is always extracted as the soluble ruthenium salt, K2RuO4, obtained by fusion with potassium hydroxide and chlorate or nitrate. When the orange-coloured ruthenate, K2RuO4, is mixed with acids, the liberated ruthenic acid immediately decomposes into the volatile ruthenic anhydride and the insoluble ruthenic oxide: 2K2RuO4 + 4HNO3 = RuO4 + RuO2,2H2O + 4KNO3. When once one of the above compounds of ruthenium or osmium is procured it is easy to obtain all the remaining compounds, and by reduction (by metals, hydrogen, formic acid, &c.) the metals themselves. Osmic anhydride, OsO4, is very easily deoxidised by many methods. It blackens organic substances, owing to reduction, and is therefore used in investigating vegetable and animal, and especially nerve, preparations under the microscope. Although osmic anhydride may be distilled in hydrogen, still complete reduction is accomplished when a mixture of hydrogen and osmic anhydride is slightly ignited (just before it inflames). If osmium be placed in the flame it is oxidised, and gives vapours of osmic anhydride, which become reduced, and the flame gives a brilliant light. Osmic anhydride deflagrates like nitre on red-hot charcoal; zinc, and even mercury and silver, reduce osmic anhydride from its aqueous solutions into the lower oxides or metal; such reducing agents as hydrogen sulphide, ferrous sulphate, or sulphurous anhydride, alcohol, &c., act in the same manner with great ease. The lower oxides of osmium, ruthenium, and of the other elements of the platinum series are not volatile, and it is noteworthy that the other elements behave differently. On comparing SO2, SO3; As2O3, As2O5; P2O3, P2O5; CO, CO2, &c., we observe a converse phenomenon; the higher oxides are less volatile than the lower. In the case of osmium all the oxides, with the exception of the highest, are non-volatile, and it may therefore be thought that this higher form is more simply constituted than the lower. It is possible that osmic oxide, OsO2, stands in the same relation to the anhydride as C2H4 to CH4—i.e. the lower oxide is perhaps Os2O4, or is still more polymerised, which would explain why the lower oxides, having a greater molecular weight, are less volatile than the higher oxides, just as we saw in the case of the nitrogen oxides, N2O and NO. Ruthenium and osmium, obtained by the ignition or reduction of their compounds in the form of powder, have a density considerably less than in the fused form, and differ in this condition in their capacity for reaction; they are much more difficultly fused than platinum and iridium, although ruthenium is more fusible than osmium. Ruthenium in powder has a specific gravity of 8·5, the fused metal of 12·2; osmium in powder has a specific gravity of 20·0, and when semi-fused—or, more strictly speaking, agglomerated—in the oxyhydrogen flame, of 21·4, and fused 22·5. The powder of slightly-heated osmium oxidises very easily in the air, and when ignited burns like tinder, directly forming the odoriferous osmic anhydride (hence its name, from the Greek word signifying odour); ruthenium also oxidises when heated in air, but with more difficulty, forming the oxide RuO2. The oxides of the types RO, R2O3, and RO2 (and their hydrates) obtained by reduction from the higher oxides, and also from the chlorides, are analogous to those given by the other platinum metals, in which respect osmium and ruthenium closely resemble them. We may also remark that ruthenium has been found in the platinum deposits of Borneo in the form of laurite, Ru2S3, in grey octahedra of sp. gr. 7·0. For osmium, Moraht and Wischin (1893) obtained free osmic acid, H2OsO4, by decomposing K2OsO4 with water, and precipitating with alcohol in a current of hydrogen (because in air volatile OsO4 is formed); with H2S, osmic acid gives OsO3(HS)2 at the ordinary temperature. Debray and Joly showed that ruthenic anhydride, RuO4, fuses at 25°, boils at 100°, and evolves oxygen when dissolved in potash, forming the salt KRuO4 (not isomorphous with potassium permanganate). Joly (1891), who studied the ruthenium compounds in greater detail, showed that the easily-formed KRuO4 gives RuKO4RuO3 when ignited, but it resembles KMnO4 in many respects. In general, Ru has much in common with Mn. Joly (1889) also showed that if KNO3 be added to a solution of RuCl3 containing HCl, the solution becomes hot, and a salt, RuCl3NO2KCl, is formed, which enters into double decomposition and is very stable. Moreover, if RuCl3 be treated with an excess of nitric acid, it forms a salt, RuCl3NOH2O, after being heated (to boiling) and the addition of HCl. The vapour density of RuO4, determined by Debray and Joly, corresponds to that formula. A whole series of platinocyanides of the common type PtR2(CN)4nH2O is obtained by means of double decomposition with the potassium or hydrogen or silver salts. For example, the salts of sodium and lithium contain, like the potassium salt, three molecules of water. The sodium salt is soluble in water and alcohol. The ammonium salt has the composition Pt(NH4)2(CN)4,2H2O and gives crystals which reflect blue and rose-coloured light. This ammonium salt decomposes at 300°, with evolution of water and ammonium cyanide, leaving a greenish platinum dicyanide, Pt(CN)2, which is insoluble in water and acid but dissolves in potassium cyanide, hydrocyanic acid, and other cyanides. The same platinous cyanide is obtained by the action of sulphuric acid on the potassium salts in the form of a reddish-brown amorphous precipitate. The most characteristic of the platinocyanides are those of the alkaline earths. The magnesium salt PtMg(CN)4,7H2O crystallises in regular prisms, whose side faces are of a metallic green colour and terminal planes dark blue. It shows a carmine-red colour along the main axis, and dark red along the lateral axes; it easily loses water, (2H2O), at 40°, and then turns blue (it then contains 5H2O, which is frequently the case with the platinocyanides). Its aqueous solution is colourless, and an alcoholic solution deposits yellow crystals. The remainder of the water is given off at 230°. It is obtained by saturating platinocyanic acid with magnesia, or else by double decomposition between the barium salt and magnesium sulphate. The strontium salt, SrPt(CN)4,4H2O crystallises in milk-white plates having a violet and green iridescence. When it effloresces in a desiccator, its surfaces have a violet and metallic green iridescence. A colourless solution of the barium salt PtBa(CN)4,4H2O is obtained by saturating a solution of hydroplatinocyanic acid with baryta, or by boiling the insoluble copper platinocyanide in baryta water. It crystallises in monoclinic prisms of a yellow colour, with blue and green iridescence; it loses half its water at 100°, and the whole at 150°. The ethyl salt, Pt(C2H5)2(CN)4,2H20, is also very characteristic; its crystals are isomorphous with those of the potassium salt, and are obtained by passing hydrochloric acid into an alcoholic solution of hydroplatinocyanic acid. The facility with which they crystallise, the regularity of their forms, and their remarkable play of colours, renders the preparation of the platinocyanides one of the most attractive lessons of the laboratory. By the action of chlorine or dilute nitric acid, the platinocyanides are converted into salts of the composition PtM2(CN)5, which corresponds with Pt(CN)3,2KCN—that is, they express the type of a non-existent form of oxidation of platinum, PtX3 (i.e. oxide Pt2O3), just as potassium ferricyanide (FeCy3,3KCy) corresponds with ferric oxide, and the ferrocyanide corresponds with the ferrous oxide. The potassium salt of this series contains PtK2(CN)5,3H2O, and forms brown regular prisms with a metallic lustre, and is soluble in water but insoluble in alcohol. Alkalis re-convert this compound into the ordinary platinocyanide K2Pt(CN)4, taking up the excess of cyanogen. It is remarkable that the salts of the type PtM2Cy5 contain the same amount of water of crystallisation as those of the type PtM2Cy4. Thus the salts of potassium and lithium contain three, and the salt of magnesium seven, molecules of water, like the corresponding salts of the type of platinous oxide. Moreover, neither platinum nor any of its associates gives any cyanogen compound corresponding with the oxide, i.e. having the composition PtK2Cy6, just as there are no compounds higher than those which correspond to RCy3nMCy3 for cobalt or iron. This would appear to indicate the absence of any such cyanides, and indeed, for no element are there yet known any poly-cyanides containing more than three equivalents of cyanogen for one equivalent of the element. The phenomenon is perhaps connected with the faculty of cyanogen of giving tricyanogen polymerides, such as cyanuric acid, solid cyanogen chloride, &c. Under the action of an excess of chlorine, a solution of PtK2(CN)4 gives (besides PtK2Cy5) a product PtK2Cy4Cl2, which evidently contains the form PtX4, but at first the action of the chlorine (or the electrolysis of, or addition of dilute peroxide of hydrogen to, a solution of PtK2Cy4, acidulated with hydrochloric acid) produces an easily soluble intermediate salt which crystallises in thin copper-red needles (Wilm, Hadow, 1889). It only contains a small amount of chlorine, and apparently corresponds to a compound 5PtK2Cy4 + PtK2Cy4Cl2 + 24H2O. Under the action of an excess of ammonia both these chlorine products are converted either completely or in part (according to Wilm ammonia does not act upon PtK2Cy4) into PtCy2,2NH3, i.e. a platino-ammonia compound (see further on). It is also necessary to pay attention to the fact that ruthenium and osmium—which, as we know, give higher forms of oxidation than platinum—are also able to combine with a larger proportion of potassium cyanide (but not of cyanogen) than platinum. Thus ruthenium forms a crystalline hydroruthenocyanic acid, RuH4(CN)6, which is soluble in water and alcohol, and corresponds with the salts M4Ru(CN)6. There are exactly similar osmic compounds—for example, K4Os(CN)6,3H2O. The latter is obtained in the form of colourless, sparingly-soluble regular tablets on evaporating the solution obtained from a fused mixture of potassium osmiochloride, K2OsCl6, and potassium cyanide. These osmic and ruthenic compounds fully correspond with potassium ferrocyanide, K4Fe(CN)6,3H2O, not only in their composition but also in their crystalline form and reactions, which again demonstrates the close analogy between iron, ruthenium, and osmium, which we have shown by giving these three elements a similar position (in the eighth group) in the periodic system. For rhodium and iridium only salts of the same type as the ferricyanides, M3RCy6, are known, and for palladium only of the type M2PdCy4, which are analogous to the platinum salts. In all these examples a constancy of the types of the double cyanides is apparent. In the eighth group we have iron, cobalt, nickel, copper, and their analogues ruthenium, rhodium, palladium, silver, and also osmium, iridium, platinum, gold. The double cyanides of iron, ruthenium, osmium have the type K4R(CN)6; of cobalt, rhodium, iridium, the type K3R(CN)6; of nickel, palladium, platinum the type K2R(CN)4 and K2R(CN)5; and for copper, silver, gold there are known KR(CN)2, so that the presence of 4, 3, 2, and 1 atoms of potassium corresponds with the order of the elements in the periodic system. Those types which we have seen in the ferrocyanides and ferricyanides of iron repeat themselves in all the platinoid metals, and this naturally leads to the conclusion that the formation of similar so-called double salts is of exactly the same nature as that of the ordinary salts. If, in expressing the union of the elements in the oxygen salts, the existence of an aqueous residue (hydroxyl group) be admitted, in which the hydrogen is replaced by a metal, we have then only to apply this mode of expression to the double salts and the analogy will be obvious, if only we remember that Cl2, (CN)2, SO4, &c., are equivalent to O, as we see in RO, RCl2, RSO4, &c. They all = X2, and, therefore, in point of fact, wherever X (= Cl or OH, &c.) can be placed, there (Cl2H), (SO4H), &c., can also stand. And as Cl2H = Cl + HCl and SO4H = OH + SO3, &c., it follows that molecules HCl or SO3, or, in general, whole molecules—for instance, NH3, H2O, salts, &c., can annex themselves to a compound containing X. (This is an indirect consequence of the law of substitution which explains the origin of double salts, ammonia compounds, compounds with water of crystallisation, &c., by one general method.) Thus the double salt MgSO4,K2SO4, according to this reasoning, may be considered as a substance of the same type as MgCl2, namely, as = Mg(SO4K)2, and the alums as derived from Al(OH)(SO4), namely, as Al(SO4K)(SO4). Without stopping to pursue this digression further, we will apply these considerations to the type of the ferrocyanides and ferricyanides and their platinum analogues. Such a salt as K2PtCy4 may accordingly be regarded as Pt(Cy2K)2, like Pt(OH)2; and such a salt as PtK2Cy5 as PtCy(Cy2K)2, the analogue of PtX(OH)2, or AlX(OH)2, and other compounds of the type RX3. Potassium ferricyanide and the analogous compounds of cobalt, iridium, and rhodium, belong to the same type, with the same difference as there is between RX(OH)2 and R(OH)3, since FeK3Cy6 = Fe(Cy2K)3. Limiting myself to these considerations, which may partially elucidate the nature of double salts, I will now pass again to the complex saline compounds known for platinum. (A) On mixing a solution of potassium thiocyanate with a solution of potassium platinosochloride, K2PtCl4, they form a double thiocyanate, PtK2(CNS)4, which is easily soluble in water and alcohol, crystallises in red prisms, and gives an orange-coloured solution, which precipitates salts of the heavy metals. The action of sulphuric acid on the lead salt of the same type gives the acid itself, PtH2(SCN)4, which corresponds with these salts. The type of these compounds is evidently the same as that of the cyanides. (B) Platinous chloride, PtCl2, which is insoluble in water, forms double salts with the metallic chlorides. These double chlorides are soluble in water, and capable of crystallising. Hence when a hydrochloric acid solution of platinous chloride is mixed with solutions of metallic salts and evaporated it forms crystalline salts of a red or yellow colour. Thus, for example, the potassium salt, PtK2Cl4, is red, and easily soluble in water; the sodium salt is also soluble in alcohol; the barium salt, PtBaCl4,3H2O, is soluble in water, but the silver salt, PtAg2Cl4, is insoluble in water, and may be used for obtaining the remaining salts by means of double decomposition with their chlorides. (C) A remarkable example of the complex compounds of platinum was observed by SchÜtzenberger (1868). He showed that finely-divided platinum in the presence of chlorine and carbonic oxide at 250°-300° gives phosgene and a volatile compound containing platinum. The same substance is formed by the action of carbonic oxide on platinous chloride. It decomposes with an explosion in contact with water. Carbon tetrachloride dissolves a portion of this substance, and on evaporation gives crystals of 2PtCl2,3CO, whilst the compound PtCl2,2CO remains undissolved. When fused and sublimed it gives yellow needles of PtCl2,CO, and in the presence of an excess of carbonic oxide PtCl2,2CO is formed. These compounds are fusible (the first at 250°, the second at 142°, and the third at 195°). In this case (as in the double cyanides) combination takes place, because both carbonic oxide and platinous chloride are unsaturated compounds capable of further combination. The carbon tetrachloride solution absorbs NH3 and gives PtCl2,CO,2NH3, and PtCl2,2CO,2NH3, and these substances are analogous (Foerster, Zeisel, JÖrgensen) to similar compounds containing complex amines (for instance, pyridine, C5H5N), instead of NH3, and ethylene, &c., instead of CO, so that here we have a whole series of complex platino-compounds. The compound PtCl2CO dissolves in hydrochloric acid without change, and the solution disengages all the carbonic oxide when KCN is added to it, which shows that those forces which bind 2 molecules of KCN to PtCl2 can also bind the molecule CO, or 2 molecules of CO. When the hydrochloric acid solution of PtCl2CO is mixed with a solution of sodium acetate or acetic acid, it gives a precipitate of PtOCO, i.e. the Cl2 is replaced by oxygen (probably because the acetate is decomposed by water). This oxide, PtOCO, splits up into Pt + CO2 at 350°. PtSCO is obtained by the action of sulphuretted hydrogen upon PtCl2CO. All this leads to the conclusion that the group PtCO is able to assimilate X2 = Cl2, S, O, &c. (Mylius, Foerster, 1891). Pullinger (1891), by igniting spongy platinum at 250°, first in a stream of chlorine, and then in a stream of carbonic oxide, obtained (besides volatile products) a non-volatile yellow substance which remained unchanged in air and disengaged chlorine and phosgene gas when ignited; its composition was PtCl6(CO)2, which apparently proves it to be a compound of PtCl2 and 2COCl2, as PtCl2 is able to combine with oxychlorides, and forms somewhat stable compounds. (D) The faculty of platinous chloride for forming stable compounds with divers substances shows itself in the formation of the compound PtCl2,PCl3 by the action of phosphorus pentachloride at 250° on platinum powder (Pd reacts in a similar manner, according to Fink, 1892). The product contains both phosphorus pentachloride and platinum, whilst the presence of PtCl2 is shown in the fact that the action of water produces chlorplatino-phosphorous acid, PtCl2P(OH)3. (E) After the cyanides, the double salts of platinum formed by sulphurous acid are most distinguished for their stability and characteristic properties. This is all the more instructive, as sulphurous acid is only feebly energetic, and, moreover, in these, as in all its compounds, it exhibits a dual reaction. The salts of sulphurous acid, R2SO3, either react as salts of a feeble bibasic acid, where the group SO3 presents itself as bivalent, and consequently equal to X2, or else they react after the manner of salts of a monobasic acid containing the same residue, RSO3, as occurs in the salts of sulphuric acid. In sulphurous acid this residue is combined with hydrogen, H(SO3H), whilst in sulphuric acid it is united with the aqueous residue (hydroxyl), OH(SO3H). These two forms of action of the sulphites appear in their reactions with the platinum salts—that is to say, salts of both kinds are formed, and they both correspond with the type PtH2X4. The one series of salts contain PtH2(SO3)2, and their reactions are due to the bivalent residue of sulphurous acid, which replaces X2. The others, which have the composition PtR2(SO3H)4, contain sulphoxyl. The latter salts will evidently react like acids; they are formed simultaneously with the salts of the first kind, and pass into them. These salts are obtained either by directly dissolving platinous oxide in water containing sulphurous acid, or by passing sulphurous anhydride into a solution of platinous chloride in hydrochloric acid. If a solution of platinous chloride or platinous oxide in sulphurous acid be saturated with sodium carbonate, it forms a white, sparingly soluble precipitate containing PtNa2(SO3Na)4,7H2O. If this precipitate be dissolved in a small quantity of hydrochloric acid and left to evaporate at the ordinary temperature, it deposits a salt of the other type, PtNa2(SO3)2,H2O, in the form of a yellow powder, which is sparingly soluble in water. The potassium salt analogous to the first salt, PtK2(SO3K)4,2H2O, is precipitated by passing sulphurous anhydride into a solution of potassium sulphite in which platinous oxide is suspended. A similar salt is known for ammonium, and with hydrochloric acid it gives a salt of the second kind, Pt(NH4)2(SO3)2,H2O. If ammonio-chloride of platinum be added to an aqueous solution of sulphurous anhydride, it is first deoxidised, and chlorine is evolved, forming a salt of the type PtX2; a double decomposition then takes place with the ammonium sulphite, and a salt of the composition Pt(NH4)2Cl3(SO3H) is formed (in a desiccator). The acid character of this substance is explained by the fact that it contains the elements SO3H—sulphoxyl, with the hydrogen not yet displaced by a metal. On saturating a solution of this acid with potassium carbonate it gives orange-coloured crystals of a potassium salt of the composition Pt(NH4)2Cl3(SO3K). Here it is evident that an equivalent of chlorine in Pt(NH4)2Cl4 is replaced by the univalent residue of sulphurous acid. Among these salts, that of the composition Pt(NH4)Cl2(SO3H)2,H2O is very readily formed, and crystallises in well-formed colourless crystals; it is obtained by dissolving ammonium platinosochloride, Pt(NH4)2Cl4, in an aqueous solution of sulphurous acid. The difficulty with which sulphurous anhydride and platinum are separated from these salts indicates the same basic character in these compounds as is seen in the double cyanides of platinum. In their passage into a complex salt, the metal platinum and the group SO2 modify their relations (compared with those of PtX2 or SO2X2), just as the chlorine in the salts KClO, KClO3, and KClO4 is modified in its relations as compared with hydrochloric acid or potassium chloride. (F) No less characteristic are the platinonitrites formed by platinous oxide. They correspond with nitrous acid, whose salts, RNO2, contain the univalent radicle, NO2, which is capable of replacing chlorine, and therefore the salts of this kind should form a common type PtR2(NO2)4, and such a salt of potassium has actually been obtained by mixing a solution of potassium platinosochloride with a solution of potassium nitrite, when the liquid becomes colourless, especially if it be heated, which indicates the change in the chemical distribution of the elements. As the liquid decolorises it gradually deposits sparingly soluble, colourless prisms of the potassium salt K2Pt(NO2)4, which does not contain any water. With silver nitrate a solution of this salt gives a precipitate of silver platinonitrite, PtAg2(NO2)4. The silver of this salt may be replaced by other metals by means of double decomposition with metallic chlorides. The sparingly soluble barium salt, when treated with an equivalent quantity of sulphuric acid, gives a soluble acid, which separates, under the receiver of an air-pump, in red crystals; this acid has the composition PtH2(NO2)4. To the potassium salt, K2Pt(NO2)4, there correspond (VÈzes, 1892) K2Pt(NO2)4Br2 and K2Pt(NO2)4Cl2 and other compounds of the same type K2PtX6, where X is partly replaced by Cl or Br and partly by (NO2), showing a transition towards the type of the double salts like the platino-ammoniacal salts. (The corresponding double sodium nitrite salt of cobalt is soluble in water, while the K,NH4 and many other salts are insoluble in water, as I was informed by Prof. K. Winkler in 1894). In all the preceding complex compounds of Pt we see a common type PtX2,2MX (i.e. of double salts corresponding to PtO) or PtM2X4 = Pt(MX2)2, corresponding to Pt(HO)2 with the replacement of O by its equivalent X2. Two other facts must also be noted. In the first place these X's generally correspond to elements (like chlorine) or groups (like CN, NO2, SO3, &c.), which are capable of further combination. In the second place all the compounds of the type PtM2X4 are capable of combining with chlorine or similar elements, and thus passing into compounds of the types PtX3 or PtX4. Judging from the most complex platino-ammonium compounds PtCl4,4NH3, we should admit the possibility of the formation of compounds of the type PtX4Y4, where Y4 = 4X2 = 4NH3, and this shows that those forces which form such a characteristic series of double platinocyanides PtK2(CN)4,3H2O, probably also determine the formation of the higher ammonia derivatives, as is seen on comparing—
Moreover, it is obviously much more natural to ascribe the faculty for combination with nY to the whole of the acting elements—that is, to PtX2 or PtX4, and not to platinum alone. Naturally such compounds are not produced with any Y. With certain X's there only combine certain Y's. The best known and most frequently-formed compounds of this kind are those with water—that is, compounds with water of crystallisation. Compounds with salts are double salts; also we know that similar compounds are also frequently formed by means of ammonia. Salts of zinc, ZnX2, copper, CuX2, silver, AgX, and many others give similar compounds, but these and many other ammonio-metallic saline compounds are unstable, and readily part with their combined ammonia, and it is only in the elements of the platinum group and in the group of the analogues of iron, that we observe the faculty to form stable ammonio-metallic compounds. It must be remembered that the metals of the platinum and iron groups are able to form several high grades of oxidation which have an acid character, and consequently in the lower degrees of combination there yet remain affinities capable of retaining other elements, and they probably retain ammonia, and hold it the more stably, because all the properties of the platinum compounds are rather acid than basic—that is, PtXn recalls rather HX or SnXn or CXn than KX, CaX2, BaX2, &c., and ammonia naturally will rather combine with an acid than with a basic substance. Further, a dependence, or certain connection of the forms of oxidation with the ammonia compounds, is seen on comparing the following compounds:
We know that platinum and palladium give compounds of lower types than iridium and rhodium, whilst ruthenium and osmium give the highest forms of oxidation; this shows itself in this case also. We have purposely cited the same compounds with 4NH3 for osmium and ruthenium as we have for platinum and palladium, and it is then seen that Ru and Os are capable of retaining 2H2O and 3H2O, besides Cl2 and NH3, which the compounds of platinum and palladium are unable to do. The same ideas which were developed in Note 35, Chapter XXII. respecting the cobaltia compounds are perfectly applicable to the present case, i.e. to the platinia compounds or ammonia compounds of the platinum metals, among which Rh and Ir give compounds which are perfectly analogous to the cobaltia compounds. Iridium and rhodium, which easily give compounds of the type RX3, give compounds (Claus) of the type IrX3,5NH3, of a rose colour, and RhX3,5NH3, of a yellow colour. JÖrgensen, in his researches on these compounds, showed their entire analogy with the cobalt compounds, as was to be expected from the periodic system. The quality of the X's, retainable in the platino-ammonium salts, may be considerably modified, and they may frequently be wholly or partially replaced by hydroxyl. For example, the action of ammonia on the nitrate of Gerhardt's base, Pt(NO3)4,2NH3, in a boiling solution, gradually produces a yellow crystalline precipitate which is nothing else than a basic hydrate or alkali, Pt(OH)4,2NH3. It is sparingly soluble in water, but gives directly soluble salts PtX4,2NH3 with acids. The stability of this hydroxide is such that potash does not expel ammonia from it, even on boiling, and it does not change below 130°. Similar properties are shown by the hydroxide Pt(OH)2,2NH3 and the oxide PtO,2NH3 of Reiset's second base. But the hydroxides of the compounds containing 4NH3 are particularly remarkable. The presence of ammonia renders them soluble and energetic. The brevity of this work does not permit us, however, to mention many interesting particulars in connection with this subject. To the common properties of the platino-ammonium salts, we must add not only their stability (feeble acids and alkalis do not decompose them, the ammonia is not evolved by heating, &c.), but also the fact that the ordinary reactions of platinum are concealed in them to as great an extent as those of iron in the ferricyanides. Thus neither alkalis nor hydrogen sulphide will separate the platinum from them. For example, sulphuretted hydrogen in acting on Gros's salts gives sulphur, removes half the chlorine by means of its hydrogen, and forms salts of Reiset's first base. This may be understood or explained by considering the platinum in the molecule as covered, walled up by the ammonia, or situated in the centre of the molecule, and therefore inaccessible to reagents. On this assumption, however, we should expect to find clearly-expressed ammoniacal properties, and this is not the case. Thus ammonia is easily decomposed by chlorine, whilst in acting on the platino-ammonium salts containing PtX2 and 2NH3 or 4NH3, chlorine combines and does not destroy the ammonia; it converts Reiset's salts into those of Gros and Gerhardt. Thus from PtX2,2NH3 there is formed PtX2Cl2,2NH3, and from PtX2,4NH3 the salt of Gros's base PtX2Cl2,4NH3. This shows that the amount of chlorine which combines is not dependent on the amount of ammonia present, but is due to the basic properties of platinum. Owing to this some chemists suppose the ammonia to be inactive or passive in certain compounds. It appears to me that these relations, these modifications, in the usual properties of ammonia and platinum are explained directly by their mutual combination. Sulphur, in sulphurous anhydride, SO2, and hydrogen sulphide, SH2, is naturally one and the same, but if we only knew of it in the form of hydrogen sulphide, then, having obtained it in the form of sulphurous anhydride, we should consider its properties as hidden. The oxygen in magnesia, MgO, and in nitric peroxide, NO2, is so different that there is no resemblance. Arsenic no longer reacts in its compounds with hydrogen as it reacts in its compounds with chlorine, and in their compounds with nitrogen all metals modify both their reactions and their physical properties. We are accustomed to judge the metals by their saline compounds with haloid groups, and ammonia by its compounds with acid substances, and here, in the platino-compounds, if we assume the platinum to be bound to the entire mass of the ammonia—to its hydrogen and nitrogen—we shall understand that both the platinum and ammonia modify their characters. Far more complicated is the question why a portion of the chlorine (and other haloid simple and complex groups) in Gros's salts acts in a different manner from the other portion, and why only half of it acts in the usual way. But this also is not an exclusive case. The chlorine in potassium chlorate or in carbon tetrachloride does not react with the same ease with metals as the chlorine in the salts corresponding with hydrochloric acid. In this case it is united to oxygen and carbon, whilst in the platino-ammonium compounds it is united partly to platinum and partly to the platino-ammonium group. Many chemists, moreover, suppose that a part of the chlorine is united directly to the platinum and the other part to the nitrogen of the ammonia, and thus explain the difference of the reactions; but chlorine united to platinum reacts as well with a silver salt as the chlorine of ammonium chloride, NH4Cl, or nitrosyl chloride, NOCl, although there is no doubt that in this case there is a union between the chlorine and nitrogen. Hence it is necessary to explain the absence of a facile reactive capacity in a portion of the chlorine by the conjoint influence of the platinum and ammonia on it, whilst the other portion may be admitted as being under the influence of the platinum only, and therefore as reacting as in other salts. By admitting a certain kind of stable union in the platino-ammonium grouping, it is possible to imagine that the chlorine does not react with its customary facility, because access to a portion of the atoms of chlorine in this complex grouping is difficult, and the chlorine union is not the same as we usually meet in the saline compounds of chlorine. These are the grounds on which we, in refuting the now accepted explanations of the reactions and formation of the platino-compounds, pronounce the following opinion as to their structure. In characterising the platino-ammonium compounds, it is necessary to bear in mind that compounds which already contain PtX4 do not combine directly with NH3, and that such compounds as PtX4,4NH3 only proceed from PtX2, and therefore it is natural to conclude that those affinities and forces which cause PtX2 to combine with X2 also cause it to combine with 2NH3. And having the compound PtX2,2NH3, and supposing that in subsequently combining with Cl2 it reacts with those affinities which produce the compounds of platinic chloride, PtCl4, with water, potassium chloride, potassium cyanide, hydrochloric acid, and the like, we explain not only the fact of combination, but also many of the reactions occurring in the transition of one kind of platino-ammonium salts into another. Thus by this means we explain the fact that (1) PtX2,2NH3 combines with 2NH3, forming salts of Reiset's first base; (2) and the fact that this compound (represented as follows for distinctness), PtX2,2NH3,2NH3, when heated, or even when boiled in solution, again passes into PtX2,2NH3 (which resembles the easy disengagement of water of crystallisation, &c.); (3) the fact that PtX2,2NH3 is capable of absorbing, under the action of the same forces, a molecule of chlorine, PtX2,2NH3,Cl2, which it then retains with energy, because it is attracted, not only by the platinum, but also by the hydrogen of the ammonia; (4) the fact that this chlorine held in this compound (of Gerhardt) will have a position unusual in salts, which will explain a certain (although very feebly-marked) difficulty of reaction; (5) the fact that this does not exhaust the faculty of platinum for further combination (we need only recall the compound PtCl4,2HCl,16H2O), and that therefore both PtX2,2NH3,Cl2 and PtX2,2NH3,2NH3 are still capable of combination, whence the latter, with chlorine, gives PtX2,2NH3,2NH3,Cl2, after the type of PtX4Y4 (and perhaps higher); (6) the fact that Gros's compounds thus formed are readily reconverted into the salts of Reiset's first base when acted on by reducing agents; (7) the fact that in Gros's salts, PtX2,2NH3(NH3X)2, the newly-attached chlorine or haloid will react with difficulty with salts of silver, &c., because it is attached both to the platinum and to the ammonia, for both of which it has an attraction; (8) the fact that the faculty for further combination is not even yet exhausted in the type of Gros's salts, and that we actually have a compound of Gros's chlorine salt with platinous chloride and with platinic chloride; the salt PtSO4,2NH3,2NH3,SO4 combines further also with H2O; (9) the fact that such a faculty of combination with new molecules is naturally more developed in the lower forms of combination than in the higher. Hence the salts of Reiset's first base—for example, PtCl2,2NH3,2NH3—both combine with water and give precipitates (soluble in water but not in hydrochloric acid) of double salts with many salts of the heavy metals—for example, with lead chloride, cupric chloride, and also with platinic and platinous chlorides (Buckton's salts). The latter compounds will have the composition PtCl2,2NH3,2NH3,PtCl2—that is, the same composition as the salts of Reiset's second base, but it cannot be identical with it. Such an interesting case does actually exist. The first salt, PtCl2,4NH3,PtCl2, is green, insoluble in water and in hydrochloric acid, and is known as Magnus's salt, and the second, PtCl2,2NH3, is Reiset's yellow, sparingly soluble (in water). They are polymeric, namely, the first contains twice the number of elements held in the second, and at the same time they easily pass into each other. If ammonia be added to a hot hydrochloric acid solution of platinous chloride, it forms the salt PtCl2,4NH3, but in the presence of an excess of platinous chloride it gives Magnus's salt. On boiling the latter in ammonia it gives a colourless soluble salt of Reiset's first base, PtCl2,4NH3, and if this be boiled with water, ammonia is disengaged, and a salt of Reiset's second base, PtCl2,2NH3, is obtained. A class of platino-ammonium isomerides (obtained by Millon and Thomsen) are also known. Buckton's salts—for example, the copper salt—were obtained by them from the salts of Reiset's first base, PtCl2,4NH3, by treatment with a solution of cupric chloride, &c., and therefore, according to our method of expression, Buckton's copper salt will be PtCl2,4NH3,CuCl2. This salt is soluble in water, but not in hydrochloric acid. In it the ammonia must be considered as united to the platinum. But if cupric chloride be dissolved in ammonia, and a solution of platinous chloride in ammonium chloride is added to it, a violet precipitate is obtained of the same composition as Buckton's salt, which, however, is insoluble in water, but soluble in hydrochloric acid. In this a portion, if not all, of the ammonia must be regarded as united to the copper, and it must therefore be represented as CuCl2,4NH3,PtCl2. This form is identical in composition but different in properties (is isomeric) with the preceding salt (Buckton's). The salt of Magnus is intermediate between them, PtCl2,4NH3,PtCl2; it is insoluble in water and hydrochloric acid. These and certain other instances of isomeric compounds in the series of the platino-ammonium salts throw a light on the nature of the compounds in question, just as the study of the isomerides of the carbon compounds has served and still serves as the chief cause of the rapid progress of organic chemistry. In conclusion, we may add that (according to the law of substitution) we must necessarily expect all kinds of intermediate compounds between the platino and analogous ammonia derivatives on the one hand, and the complex compounds of nitrous acid on the other. Perhaps the instance of the reaction of ammonia upon osmic anhydride, OsO4, observed by Fritsche, FrÉmy, and others, and more fully studied by Joly (1891), belongs to this class. The latter showed that when ammonia acts upon an alkaline solution of OsO4 the reaction proceeds according to the equation: OsO4 + KHO + NH3 = OsNKO3 + 2H2O. It might be imagined that in this case the ammonia is oxidised, probably forming the residue of nitrous acid (NO), while the type OsO4 is deoxidised into OsO2, and a salt, OsO(NO)(KO), of the type OsX4 is formed. This salt crystallises well in light yellow octahedra. It corresponds to osmiamic acid, OsO(ON)(HO), whose anhydride [OsO(NO)]2, has the composition Os2N2O5, which equals 2Os + N2O5 to the same extent as the above-mentioned compound PtCO2 equals Pt + CO2 (see Note 11). |
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