RHODIUM (Ito), a metal discovered by the late Dr. Wollaston in 1803, and named from fluxion (0443or, a rose), on account of the colour of one of Re solutions. This metal exists in combination with plati num. According to the analysis of Berzelitia, the ore of Colombia e egoism nearly three and a half per cent., and that of Siberia .nly 1Kr cent of ahadiuuj When the putter part of the platinum and palladium have been separated from the solution of the native grains, by the addition of chloride of ammonium and cyanide of mercury, a plate of iron is to be immersed in the residual solution, and by this the rhodium, with small quantities of platinum, copper, and lead, is thrown down in the metallic state. In order to render the rhodium pure, it is first digested in dilute nitric acid, which dissolves the copper and lead, and the rhodium and platinum are then to be dissolved in aqua regia mixed with some common salt, and the solution is to be evaporated to dryness. By this operation there are obtained the double chloride of platinum and sodium, and rhodium and sodium. The former is to be dissolved in alcohol, and the latter afterwards in water, when a plate of zinc immersed in the solution precipitates the rhodium in the metallic state. The metal thus procured is in the state of a black powder, and requires the strongest heat of a wiud furnace, or better the oxyhydrogen blowpipe, for fusion. It is less fusible than platinum.
Rhodium is white; has a metallic lustre; is extremely hard ; specific gravity 12.1 ; and is ductile and malleable. It is not dissolved by any acid or by aqua regia, except when it is alloyed by other metals ; and this circumstance accounts for its being dissolved, when alloyed with pia. tinum, in the native grains of this metal. It suffers no change by exposure to air, either dry or moist. The equivalent of rhodium is 52.1e.
Oxyyen and Rhodium form two compounds. They cannot be made to combine by direct action. The protoxide has not yet been obtained in a state of purity. When fiuely-divided rhodium, mixed with potash and a little nitre, is heated to redness in a silver crucible, the metal is oxidized, and becomes of a brown colour, and is mixed with potash ; the mass is to he washed with water and then treated with hydrochloric acid, by which hydrated peroxide of rhodium is left, of a greenish gray colour. It contains When this peroxide is heated, it becomes black, and is then probably converted into pro toxide.
Chlorine and Rhodium probably unite in three proportions, but the perehloride only has bceu hitherto obtained in a separate state. It was procured by Berzelius by adding silico-hydrofluoric acid to a solution of the chloride of potassium and rhodium, as long as the double fluoride of potassium and silicon was generated, after which the filtered liquor was evaporated to dryness, and the residue redis solved in water. The remaining perchloride thus obtained has a dark brown colour, and when heated to redness, chlorine is evolved and metallic rhodium obtained. The aqueous solution of this salt is a fine rose-red colour, whence the name of the metal which it contains.
This salt forms iouble compounds, called rhodio-chlorides, with the chloride of potassium and of sodium.
Sulphur and Rhodium may be made to combine by heating them the metal being in a state of minute division. The protu sulphide(Ro S) formed fuses at a white heat without decomposition, has a bluish-gray colour, a metallic lustre, and by the action of nitric acid is converted into sulphate of rhodium. Sesqui-sulphide of rhodium may be formed by heating the ammonio-chloride of the metal with or by heatiug its solution with sulphide of potassium.
Salts of dodium.—The salts of the peroxide only have beeu formed. Their general properties are but little known.
Nitrate of Rhodium is obtained by dissolving the peroxide in the acid : it is of a deep red colour, and uueryetallizable.
Sulphate of Rhodium is procured, as already mentioned, by acting upon the sulphide with nitric acid. The solution is of a deep red colour, and does not yield crystals. When the caustic alkalies aro added to the solution of this or the other soluble salts of rhodium, a precipitate of the hydrated sesqui-oxidc of a greenish-yellow colour is obtained after some time ; the alkaline carbonates produce no effect, nor does sulphurous acid, nor the ferro-cyanide of potassium. Hydro-sulphuric acid throws down sulphide of rhodium • but the hydrosulphates of ammonia and potash produce no immediate 'pre cipitate.
Alloys of Rhodium—When combined with steel to the amount of only two per cent., it gives the steel great hardness without causing it to crack under the hammer. Dr. Wollaston examined several of its alloys, and, on account of its hardness, he suggested its employment for the nibs of metallic pens; to which purpose it has been applied successfully. It has not been combined with mercury.