842

ANALYSIS OF PLATINUM RESIDUES-RUTHENIUM.

causing it to pass through tubes filled with pumice moistened with sulphuric acid. During the operation the osmium and ruthenium become oxidized; the tetroxide of osmium condenses in beautiful needles in the flasks, and mechanically carries forward the binoxide of ruthenium, which is deposited upon the fragments of porcelain in regular square prisms.*

The fixed residue consists of an alloy of iridium and rhodium, mixed with a little osmium and ruthenium. This is to be fused with caustic potash, by which the oxide of ruthenium is removed and is dissolved out on washing the fused mass with water. The undissolved portion is ignited with four times its weight of nitrate of potassium, and the product is treated with boiling water, which dissolves the osmium, and on cooling, often deposits it in octohedral crystals as osmite of potassium. The residue now contains only sesquioxides of iridium and rhodium in combination with potash. Aqua regia, when boiled upon it, converts most of the iridium into the soluble perchloride; a solution of chloride of potassium is added to the liquid, after which crystals of the double chloride of iridium and potassium are deposited as it cools. The sesquioxide of rhodium, which is left undissolved, since it is insoluble in aqua regia, is converted into a soluble double salt by mixing it intimately with an equal weight of chloride of sodium, and heating the mass to dull redness in a current of dry chlorine.

(985) RUTHENIUM is a metal which, in 1845, was shown by Claus to exist in the ore of platinum. It is very hard, and brittle, and is scarcely fusible even before the oxyhydrogen blowpipe. The melted metal, according to Deville and Debray, has a sp. gr. of from II to 11'4. It absorbs oxygen at a red heat, and the oxide so obtained is not decomposed by simple elevation of temperature. The metal is readily oxidized by fusion with nitre, or with caustic potash. Ruthenium accompanies the alloy of osmium and iridium in a proportion varying from 3 to 6 per cent. ; but it is not found in the portion of platinum ore which is soluble in aqua regia. It is most easily obtained by Fremy's process (984). The binoxide of ruthenium is not volatile when heated alone, but is carried forward mechanically by the tetroxide of osmium, and becomes condensed in crystals near to the source of heat. By heating this oxide in a current of hydrogen, the metal is obtained in the form

* Sometimes the osmide of iridium does not readily undergo oxidation. In such a case Deville fuses it with 8 or 10 times its weight of zinc, and heats it for some hours to full redness; he then dissolves out the zinc by hydrochloric acid, which leaves the platinum metals, in the form of a fine black powder, which is very easily oxidized in a current of air.

COMPOUNDS OF RUTHENIUM.

843

of a dark grey powder. The metal forms an alloy with tin, RuSn,, which crystallizes in cubes of perfect regularity.

3

Ruthenium forms four compounds with oxygen, Rue; Ru¿Ð ̧; Ru,; and Rug.

Ruthenic anhydride, or ruthenic acid (Ru), is insoluble in water it may be obtained by heating any of the preceding oxides with nitre; the rutheniate of potassium forms an orange-yellow solution in water. The sesquioxide is the most stable of the oxides of the metal; it is obtained in the anhydrous form by igniting the metal in a current of air. It is insoluble in the alkalies: but with acids it forms soluble salts which have a yellow colour. The alkalies precipitate the hydrated oxide (Ru‚Ð ̧,зH¿Ð) from these solutions as a bulky blackish-brown powder.

There are three chlorides of ruthenium, RuCl,; RuCl ̧; and RuCl. The terchloride is obtained by dissolving the sesquioxide in hydrochloric acid: on evaporation it yields a greenish blue deliquescent mass, which is soluble in alcohol. Sulphuretted hydrogen causes a brown precipitate of sulphide of ruthenium in solutions of the terchloride, leaving a supernatant liquid of a fine blue colour, probably owing to the formation of a lower chloride of the metal; this reaction is very delicate, and characteristic of ruthenium. Metallic zinc also reduces the yellow terchloride to the blue bichloride, and ultimately precipitates the metal as a black powder. Formiate or oxalate of sodium, if boiled with salts of ruthenium, renders the solution colourless, but does not, occasion any precipitate of reduced metal. With acetate of lead a purplish-red characteristic precipitate is formed. Cyanide of mercury renders the solution blue, whilst a blue precipitate is formed. The caustic and carbonated alkalies yield a black precipitate of the sesquioxide, insoluble in excess of the precipitant.

§ VIII. OSMIUM: Os=199, or Os=99'41. Sp. Gr. 21'4.

(986) OSMIUM Occurs associated with platinum in the form of an alloy of osmium, iridium, and ruthenium. It was discovered in the ore of platinum by Tennant, in 1803. Osmium may be obtained in the metallic condition by several processes. One of the simplest consists in treating volatile oxide of osmium (se) obtained by Fremy's method (984) with hydrochloric acid and metallic mercury. Calomel is thus produced by the decomposition of the mercurous oxide, which is formed at the expense of the oxygen contained in the oxide of osmium; Ose,+8 Hg+8 HCl= Os+8 HgCl +4 H2O. The water and the superfluous acid are

expelled by evaporation to dryness, and on heating the residue in a small porcelain retort, the excess of mercury and calomel are driven off, leaving pure osmium in a pulverulent form. In this finely divided state, it emits the odour of tetroxide of osmium, when exposed to a moist atmosphere; it takes fire when heated in the open air, and is dissolved by strong nitric acid, or by aqua regia, being converted into tetroxide of osmium. After ignition, however, it is no longer soluble in the acids. The specific gravity of osmium in the pulverulent form is about 10, but after it has been heated to the fusing-point of rhodium in the oxyhydrogen jet, it acquires a sp. gr. of 214. In order to obtain compact osmium, Deville and Debray oxidize the alloy of osmium and iridium by mixing it intimately with 5 times its weight of peroxide of barium, heating it to a bright red for 2 hours, after which they distil with a mixture of eight parts of hydrochloric and one of nitric acid. The tetroxide of osmium which passes over is received into a solution of ammonia, supersaturated with sulphuretted hydrogen, and boiled. The sulphide of osmium is separated by filtration, dried at a low temperature, placed in a crucible of gas-coke, which is enclosed in a clay crucible and luted down, then exposed for 4 or 5 hours to a heat sufficient to melt nickel. The osmium is reduced, and furnishes a brittle mass, the colour of which has more of a bluish cast than that of zinc. At a still higher temperature in the oxyhydrogen jet, at the fusing-point of rhodium, it becomes still denser. It may be heated in this condition to the fusing-point of zinc without emitting vapour, but it takes fire at a higher temperature. If heated with 7 or 8 times its weight of tin in a charcoal crucible to a full red heat, the osmium is dissolved by the tin, and crystallizes out on slow cooling. On treat. ing the mass with hydrochloric acid, the osmium is left as very hard crystalline powder. It may also be combined with zinc, and on dissolving the zinc in hydrochloric acid the osmium is left as an amorphous combustible powder. Osmium appears to be the least fusible of the metals. In the oxyhydrogen jet platinum is volatilized, and iridium and ruthenium undergo fusion, but osmium does not melt, though it is volatilized by the intense heat.

Osmium differs remarkably from the other metals of this group, and presents more analogy with arsenic and antimony than with the noble metals.

(987) FIVE OXIDES OF OSMIUM are known:-Os; Ч¿Ð ̧; ᎾsᎾ, ; ᎾsᎾ ; ᎾsᎾ, The anhydrous prolocide of osmium is of a grey-black, and insoluble in acids: its bluish-black hydrate is soluble in hydrochloric acid, forming a deep indigo-blue solution

OXIDES OF OSMIUM.

2

845

of chloride, OsCl2, which absorbs oxygen rapidly, and becomes converted into the chloride OsCl. The sesquioxide has not been isolated; it forms rose-red uncrystallizable salts. The binoxide is black. The teroxide possesses a feebly acid character; it cannot be isolated, but it forms a crystalline compound with potassium (osmite of potassium, K ̧‡s✪4,2 H ̧‡, or KO,OsO ̧ . 2 Aq), which is sparingly soluble. This compound furnishes a good source of pure osmium. It is easily obtained by the addition of a little alcohol to a solution of the tetroxide of osmium in potash; the osmite separates in large rose-coloured octohedra, which are permanent in a dry air, but absorb oxygen if moist. If this salt be digested in a solution of chloride of ammonium, a yellow sparingly soluble salt is formed (2 HNCI,→s→2HN2), which, when ignited in a current of hydrogen, leaves pure osmium.

2

Tetroxide of osmium, or osmic acid (Os; Mol. vol. ; sp. gr. of vapour, 8.88) is the volatile compound which is produced when the metal is heated with nitre, or when roasted in air: it forms colourless, acicular, transparent, flexible crystals which are readily fusible, and are freely soluble in water; it boils at about 2120, emitting an extremely irritating and deleterious vapour, with a pungent characteristic odour somewhat resembling that of chlorine : hence the name of the metal osmium (from ooμn, odour): it does not combine with acids; but though it unites with the alkalies, its solution does not redden litmus, and its solutions in the alkalies give off tetroxide when boiled. It produces a permanent black stain upon the skin when touched, owing to the partial reduction of the metal, and gives a characteristic blue precipitate when its solutions are mixed with tincture of galls. According to Fremy, another oxide of osmium (s?) exists, but it is very unstable; it forms compounds with potash and soda which have a dark-brown colour; they sometimes crystallize from concentrated alkaline solutions.

If the aqueous solution of tetroxide of osmium be treated with sulphuretted hydrogen, an immediate precipitate of the black hydrated tetrasulphide occurs, which is slightly soluble in solutions of the sulphides of the alkaline metals. Four inferior degrees of sulphuration of osmium also exist; they correspond in composition with the oxides. These sulphides are decomposed by prolonged ignition, and pure osmium is left.

(988) There are four chlorides of osmium, viz.-OsCl2; OsCls; OsCl; OsCl: the osmious subchloride, (OsCl) or protochloride, is green, and sublimes in green needles; it is produced by heating powdered osmium in a current of chlorine; the double salts which it forms are of a green colour. The osmic subchloride (OsCl ̧), or bi

846

SALTS OF OSMIUM-IRIDIUM.

chloride, is formed in the same way as osmious subchloride, by employing an excess of chlorine; it is more volatile, and condenses as a red, crystalline, fusible, deliquescent powder: both this and the preceding chloride are dissolved by water, which soon decomposes them, forming tetroxide of osmium and hydrochloric acid, and depositing metallic osmium. Osmic subchloride forms with chloride of potassium a beautiful sparingly soluble red salt, which furnishes octohedral crystals (2 KCl,OsCl, or KCl,OsCl); this salt is obtained by heating a mixture of osmium with chloride of potassium in a current of chlorine: it is isomorphous with the corresponding platinum salt, and yields a characteristic dark olive-green precipitate with nitrate of silver (2 AgCl,→sCl; Claus). Mercurous nitrate gives with it a reddish-brown precipitate: tannic acid gives with it, when heated, a dark-blue solution, and ferrocyanide of potassium, a chrome-green liquid, passing into dark blue.

Double salts may also be formed which contain both osmious chloride (OsCl) and osmic chloride (OsCl).

A compound of nitrogen, oxygen, and osmium (Os,N ̧Ð ̧), was formed by Fritsche and Struve. It may be obtained by acting upon a mixture of caustic potash and ammonia by means of tetroxide of osmium: these chemists termed it osman-osmic acid. With the alkalies it forms yellow crystalline compounds, which detonate readily when they are struck or suddenly heated. The potassium salt may be represented by the formula, K,Os,N,5, or KO, Os2NO4.

2

The properties of the salts of osmium have been but incompletely ascertained. When boiled with nitric acid they all evolve vapours of tetroxide of osmium.

§ IX. IRIDIUM: Fr=1971, or Ir=98.56.

Sp. Gr. 21'15.

(989) IRIDIUM was discovered at the same time as osmium, by Smithson Tennant. It is occasionally found native and nearly pure in considerable masses among the Uralian ores of platinum, but it usually occurs combined with osmium as an alloy in flat scales. Iridium appears to be dimorphous, for it is found crystallized both in cubes and in double six-sided pyramids (G. Rose). In order to obtain the metal in the separate state, Wöhler recommends the powdered alloy to be intimately mixed with an equal weight of finely powdered fused chloride of sodium, and the mixture to be heated to dull redness in a glass tube through which a current of dry chlorine is transmitted so long as it is absorbed.