Note 9 tri — Chapter XXIII · THE PLATINUM METALS
George Kamensky translation
We have yet to become acquainted with the two remaining associates of platinum—ruthenium and osmium—whose most important property is that they are oxidised even when heated in air, and that they are able to give volatile oxides of the form RuO4 and OsO4; these have a powerful odour (like iodine and nitrous anhydride). Both these higher oxides are solids; they volatilise with great ease at 100°; the former is yellow and the latter white. They are known as ruthenic and osmic anhydrides, although their aqueous solutions (they both slowly dissolve in water) do not show an acid reaction, and although they do not even expel carbonic anhydride from potassium carbonate, do not give crystalline salts with bases, and their alkaline solutions partially deposit them again when boiled (an excess of water decomposes the salts). The formulæ OsO4 and RuO4 correspond with the vapour density of these oxides. Thus Deville found the vapour density of osmic anhydride to be 128 (by the formula 127·5) referred to hydrogen. Tennant and Vauquelin discovered this compound, and Berzelius, Wöhler, Fritzsche, Struvé, Deville, Claus, Joly, and others helped in its investigation; nevertheless there are still many questions concerning it which remain unsolved. It should be observed that RO4 is the highest known form for an oxygen compound, and RH4 is the highest known form for a compound of hydrogen; whilst the highest forms of acid hydrates contain SiH4O4, PH3O4, SH2O4, ClHO4—all with four atoms of oxygen, and therefore in this number there is apparently the limit for the simple forms of combination of hydrogen and oxygen. In combination with several atoms of an element, or several elements, there may be more than O4 or H4, but a molecule never contains more than four atoms of either O or H to one atom of another element. Thus the simplest forms of combination of hydrogen and oxygen are exhausted by the list RH4, RH3, RH2, RH, RO, RO2, RO3, RO4. The extreme members are RH4 and RO4, and are only met with for such elements as carbon, silicon, osmium, ruthenium, which also give RCl4 with chlorine. In these extreme forms, RH4 and RO4, the compounds are the least stable (compare SiH4, PH3, SH2, ClH, or RuO4, MoO3, ZrO2, SrO), and easily give up part, or even all, their oxygen or hydrogen.
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.