OXIDES, SULPHIDES, AND CHLORIDES OF PALLADIUM.

837

resembling those of suboxide of copper, and which, according to Kane, is obtained by heating the hydrated protoxide to incipient redness; a protoxide, Pde, which is the base of the ordinary salts of the metal; and a binoxide, PdO..

The protoxide(Pd+=122 ̊5, or PdO=61·2) may be procured as a black powder, by heating the nitrate to low redness; or it may be obtained upon adding carbonate of potassium or of sodium to its salts, as a dark brown hydrate, soluble both in acids and in alkalies, and from which the water may be expelled by heat. At a bright red heat it loses its oxygen.

The binoxide (Pd) is prepared by decomposing the solid double chloride of palladium and potassium by a solution of caustic potash; it forms a yellowish-brown hydrate, which obstinately retains a portion of alkali: it is soluble in the alkalies: by boiling it with water it is rendered anhydrous, and is then deposited as a black powder.

Sulphide of Palladium (PdS) may be formed either directly, by heating powdered sulphur with palladium, or by precipitating the salts of the protoxide by means of sulphuretted hydrogen; it forms a fusible, greyish-white, lustrous mass, from which patient roasting in air expels the sulphur.

If a piece of palladium foil or wire be held in the flame of a spirit-lamp, soot is speedily deposited in large quantity, the foil or wire is corroded, and the mass of soot is found to contain palladium throughout, owing to the formation of a carbide of the metal.

(979) Palladious chloride, or Chloride of palladium (PdCl2, or PdCl) is obtained by evaporating to dryness a solution of palladium in aqua regia; it forms brown hydrated crystals, which become black when the water is expelled; if heated to redness metallic palladium is left. Chloride of palladium forms double salts with the soluble chlorides; those with potassium and ammonium are dark bottle-green. With ammonia, chloride of palladium forms a series of compounds analogous to those of platinum (973) one of them, palladamine, (PdH.N2), is a crystallizable powerfully alkaline base. The perchloride of palladium (PdC1, or PdCl2) exists in solution in aqua regia, but cannot be obtained in crystals it forms double salts with the chlorides of the alkaline metals; the double salt with potassium crystallizes in ruby-red prisms.

Iodide of Palladium (PdI,=3605, or PdI=1802): Comp. in 100 parts, Pd, 300; I, 700.—This compound is obtained by adding a solution of a salt of palladium in slight excess to one of

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SALTS OF PALLADIUM.

iodide of potassium. It is a black powder, insoluble in water but soluble in ammonia, and in a solution of iodide of potassium; a solution of palladium is sometimes employed as a precipitant for iodine when it is necessary to separate iodine from chlorine and bromine (541). Iodide of palladium loses its iodine when strongly heated.

Cyanogen has a stronger attraction for palladium than for any other metal, so that a salt of palladium will decompose even cyanide of mercury. This cyanide is procured as a yellowish precipitate by adding potassium to neutral solutions of any of the salts of palladium; it is soluble in ammonia, in acids, and in cyanide of potassium; it forms a series of double cyanides.

The Sulphate of palladium (Pd§‡, or PdO,SO) may be ob tained by decomposing the nitrate by sulphuric acid, or by dissolving the oxide in sulphuric acid. It is a deliquescent salt which forms a deep brownish-red solution; when heated it loses acid, and furnishes a basic salt.

The nitrate is formed by boiling nitric acid on palladium: it may be obtained in rhombic prisms; they are freely soluble in a small quantity of water, and yield a deep reddish-brown liquid; but on being largely diluted, the normal salt is decomposed, and an insoluble basic nitrate is precipitated. If ammonia in excess be added to the solution of the nitrate, an ammoniacal nitrate of palladium may be crystallized from it in rectangular tables.

(980) CHARACTERS OF THE SALTS OF PALLADIUM.-The protosalts, or ordinary salts of palladium, form either brown or red solutions, which when neutral are distinguished by the yellowish precipitate of cyanide of palladium, formed on adding cyanide of mercury. The fixed hydrated alkalies precipitate compounds of palladium in the form of a red or orange basic salt, which is soluble in excess of the alkali by the aid of heat. Ammonia and its carbonate, when added to a solution of palladious chloride, give a flesh-coloured precipitate, soluble in excess of ammonia. Nitrate of palladium gives a brown precipitate with ammonia. Car. bonates of potassium and sodium yield a brown precipitate of the hydrated oxide, with salts of palladium. Iodide of potassium precipitates a black iodide of palladium. Sulphuretted hydrogen and sulphide of ammonium throw down a black sulphide of palladium, insoluble in the sulphides of the alkaline metals. Solutions of the salts of palladium are reduced by a solution of green sulphate of iron, and by many of the metals, the reduction being facilitated

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by heat. Stannous chloride produces a dark brown precipitate, which is soluble in hydrochloric acid, forming an intensely green solution, which becomes reddish brown on dilution.

Palladium may be separated from all other metals, except copper and lead, by the addition of the cyanide of mercury to the solution previously neutralized by means of carbonate of sodium. The cyanide of palladium when ignited in the air leaves metallic palladium.

VI. RHODIUM: Ro"" 1043, or Ro=52:16. Sp. Gr. 121.

(981) RHODIUM was discovered by Wollaston in 1803. It usually forms about one-half per cent. of the ore of platinum; it may be extracted from the solution of this ore in aqua regia after the platinum and palladium have been separated by the addition of sal ammoniac and cyanide of mercury: the excess of cyanide of mercury is then decomposed by acidulating the solution with hydrochloric acid, adding common salt, and evaporating to dryness; the chloride of sodium thus forms double chlorides with all the metals in solution; the residue is treated with alcohol (of sp. gr. 0837), which dissolves all these double salts, except that of sodium and rhodium, which remains behind as a red powder: this is dissolved in water, and the rhodium thrown down in a pulverulent form by means of bars of metallic zinc. The chloride of sodium and rhodium may also be decomposed by heating it in a current of hydrogen gas, when, on washing the mass with water, the rhodium is left in a pulverulent form.

Rhodium is a white, very hard metal; when quite pure, it is malleable after fusion upon lime, and it then has a sp. gr. of 12'1. It requires a stronger heat to fuse it than platinum, and when melted has a similar tendency to absorb oxygen, and to spit as the globule sets.

Deville says that rhodium furnishes an alloy with platinum, which is easily worked; when the proportion of rhodium forms 30 per cent. or upwards of the alloy it is not attacked by aqua regia. When pure, rhodium is insoluble in the acids, though if alloyed in small quantity with platinum, copper, bismuth, or lead, it is dissolved with them in nitrohydrochloric acid. Rhodium has a considerable attraction for oxygen, and may be oxidized by fusion with a mixture of nitre and carbonate of potassium; acid sulphate of potassium also oxidizes the metal and forms a soluble double sulphate of rhodium and potassium, whilst sulphurous anhydride escapes. If heated in contact with chloride of sodium, in a

840

OXIDES, SULPHIDES, AND CHLORIDES OF RHODIUM. current of chlorine, a soluble double chloride of sodium and rhodium is produced.

(982) OXIDES OF RHODIUM.-Rhodium has a considerable attraction for oxygen: it appears to form two definite oxides, Roe, and Ro,,, besides some compounds intermediate between them. The protoxide, however, has not been obtained in a state of purity.

Rhodic oxide, or Sesquioxide of rhodium (Ro ̧Ð ̧=252·6, or Ro2O=1283).-This is the only salifiable oxide of rhodium; it may be procured by heating rhodium with a mixture of nitre and carbonate of potassium; the oxide forms an insoluble compound with potash, which is to be well washed, and decomposed by digestion with hydrochloric acid: the sesquioxide is thus left as a greenishgrey hydrate which is insoluble in all acids.

SULPHIDES.-Rhodium forms two sulphides, RoS and Ro2S. If the metal be heated in the vapour of sulphur, the two bodies unite with incandescence, and form the protosulphide, which has a bluish-grey colour, and fuses at a very high temperature; the sulphur burns off in the open air, and leaves a forgeable mass of metallic rhodium. The sesquisulphide may be obtained in the form of a brown hydrate by decomposing a hot solution of the double chloride of sodium and rhodium by means of sulphide of potassium or of sodium.

CHLORIDES OF RHODIUM.-Three of these, viz., RoCl; Ro.Cl,; and RoCl,, are stated by Berzelius to exist, but the last is the only one of importance.

Terchloride of rhodium (RoCl,), or sesquichloride (RoCl), is formed by decomposing the chloride of potassium and rhodium by silicofluoric acid, which separates the potassium as a gelatinous silicofluoride; the filtered liquid when evaporated to dryness leaves the terchloride of rhodium. This chloride unites with many of the soluble chlorides to form crystallizable double salts, which are of a ruby or rose colour (whence the metal receives its name, from pódov, a rose): the sodium salt crystallizes in cubes or in octohedra, which are efflorescent in the air (3 NaCl,RoCl, 9 H,, or 3 NaCl, Ro,Cl ̧,18 Aq); they are insoluble in alcohol. When terchloride of rhodium is supersaturated with ammonia, the precipitate formed at first is redissolved, and a characteristic yellow compound, consisting of (RoCl,, 5 H,N) is formed by boil ing, and may be purified by evaporation and re-crystallization. This compound when ignited, leaves pure rhodium in the form of a powder.

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(983) CHARACTERS OF THE SALTS OF RHODIUM.-The double

CHARACTERS OF THE SALTS OF RHODIUM-RUTHENIUM.

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chloride of sodium and rhodium is the best known of these compounds. The salts corresponding to the sesquioxide of the metal generally form rose-coloured solutions; they are decomposed by iron or zinc, which causes a deposit of metallic rhodium. Hydrates of potash and soda slowly occasion a precipitate of yellow hydrated oxide, which obstinately retains a portion of the alkali; it is soluble in the excess of the alkali as well as in acids; if alcohol be added to the alkaline solution, a black precipitate gradually occurs without applying heat. Iodide of potassium throws down a sparingly soluble yellow iodide of rhodium. Sulphuretted hydrogen, when the solution is heated, slowly forms a brown precipitate insoluble in the alkaline sulphides. The soluble sulphites give a characteristic pale yellow precipitate. If the salts of rhodium be heated in a current of hydrogen, the metal is readily reduced in this form it is insoluble in aqua regia, but if it be fused with acid sulphate of potassium, a double salt is formed which is soluble in water, with a pink colour.

:

§ VII. RUTHENIUM: Ru=104'2, or Ru=52'11.
Sp. Gr., from 11 to 114.

(984) Treatment of the Platinum residue.-After the platinum. ore has been exhausted with aqua regia, a residue is obtained which frequently contains both titaniferous iron and chrome iron; but its most important constituent is an alloy in flat plates or scales of a white colour and metallic lustre. This was formerly considered to be an alloy of osmium and iridium. It has, however, been found to consist of four metals-viz., osmium, iridium, ruthenium, and a small quantity of rhodium.

Fremy in separating the different metals contained in this residue avails himself of the oxidability of osmium and the volatility of its tetroxide. His process is the following:-About 3000 grains of the platinum residue placed in a porcelain or platinum tube, and heated to redness, is roasted in a current of dry air; in the portion of the tube which projects from the furnace some fragments of porcelain are placed, and the tube is connected with a series of glass flasks for the purpose of condensing the tetroxide of osmium as it distils; in the last flask a solution of caustic potash is placed in order to retain such portions of the tetroxide as may have escaped condensation; and this flask is connected with an aspirator, by means of which a current of atmospheric air is maintained through the apparatus. The air is dried, and freed from organic particles before it enters the heated tube, by