Rhodium, a metal belonging to the platinum group, discovered by Wollaston in 1803. He found 0.4 per cent. in ore from Brazil, and in a specimen from another locality as much as 3 per cent. It usually forms about one half of one per cent. of the ore. It may be extracted from the solution from which platinum and palladium have been separated in the manner which has been described. (See Osmium, Palladium, and Platinum.) The solution is mixed with hydrochloric acid and evaporated to dryness, and the residue is treated with alcohol of sp. gr. 0.837; this dissolves everything except the double chloride of sodium and rhodium, which remains behind as a red powder. This is dissolved in water, and the rhodium precipitated by the action of metallic zinc, or the salt may be reduced by heating it in a current of hydrogen gas. Rhodium is white and very hard. When quite pure it is malleable after fusion upon lime, and is then of. sp. gr. 12.1. Wollaston's estimate was 11, but he experimented on the unfused metal. The fusing point of rhodium is higher than that of platinum, and it will only melt in the voltaic arc or in the oxyhydrogen furnace; the precise degree cannot be estimated.
Its symbol is Ro; its atomic weight 104 or 104.3. It is unalterable in the air at ordinary temperatures, but oxidizes, and also combines with chlorine, at a red heat. It resists the action of the strongest acid, singly or combined, unless alloyed with some other metal, when it will dissolve in nitro-muriatic acid. Rhodium forms four oxides : a monoxide, RoO; a sesquioxide, Ro2O3; a dioxide, RoO2; and a trioxide, RoO3. The principal oxygen salts are the acetate, nitrate, phosphate, sulphate, and sulphite. There are two sulphides, RoS and Ro2S3. According to Berzelius, there are three chlorides, RoCl2, Ro2Cl5, and RoCl3; but more recent investigations by Claus make it probable that there is only one, the trichloride, RoCl3, which forms double chlorides with the alkalies. The best known salt of rhodium is the sodic rhodic chloride, which is obtained in the extraction of the metal as described above. The rhodic salts generally form rose-colored solutions, and are decomposed by metallic iron or zinc, with precipitation of metallic rhodium. Potassic and sodic hydrates added to rhodic salt solutions slowly precipitate a yellow hydrated rhodic oxide, which is soluble in excess of alkali as well as in acids. Potassic iodide precipitates a sparingly soluble yellow tri-iodide of rhodium.
Sulphuretted hydrogen in hot solutions slowly forms a brown sulphide. Rhodium salts heated in a current of hydrogen gas are reduced to the metallic state.