Note 11 — Chapter XXIII · THE PLATINUM METALS
George Kamensky translation
This acid character is explained by the influence of the platinum on the hydrogen, and by the attachment of the cyanogen groups. Thus cyanuric acid, H3(CN)3O3, is an energetic acid compared with cyanic acid, HCNO. And the formation of a compound with five molecules of water of crystallisation, (PtH2(CN)4,5H2O), confirms the opinion that platinum is able to form compounds of still higher types than that expressed in its saline compounds, and, moreover, the combination of hydroplatinocyanic acid with water does not reach the limit of the compounds which appears in PtCl4,2HCl,6H2O.
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.