Rhodium [symbol Rh; atomic weight 102.9 (O=16)], in chemistry, a metallic chemical element found, associated with the other elements of the platinum group, in crude platinum ore, wherein it was discovered in 1803 by W. H. Wollaston (Phil. Trans., 1804, p. 419). It may be obtained from the residues of platinum ore after treatment with aqua regia and removal of the platinum as chlorplatinate. The mother liquors are decomposed by treatment with metallic iron, the precipitate obtained being warmed with concentrated nitric acid and heated in an iron crucible with concentrated caustic potash. The residue thus obtained is mixed with salt and heated in a current of chlorine, any iridium present being converted into its chloride by treatment with nitric acid and precipitated by ammonium chloride, whilst rhodium ammonium chloride goes into solution with its characteristic rose-red colour (C. E. Claus, Jour. prakt. Chem., 1843–1845). For other methods of extraction see Gibbs, ib., 1861, 84, p. 65; 1865, 94, p. 10; T. Wilm, Bull. soc. chim., 1880 (2), 34, p. 679; E. Frémy, Comptes rendus, 1854, 38, p. 1008, &c.). The metal itself is best obtained by the reduction of chlorpurpureo rhodium chloride, (Cl2Rh2·10NH3)·Cl4, in a current of hydrogen, the metal after reduction being cooled in a stream of carbon dioxide (S. M. Jorgensen, Zeit. anorg. Chem., 1903, 34, p. 82). It somewhat resembles aluminium in colour; its specific gravity varies from 11 to 12·1; and its specific heat is 0·05527 (V. H. Regnault, Ann. chim. phys., 1861, 63, p. 15). It is less fusible than platinum. It oxidizes superficially when heated, and may be distilled in the electric furnace. It is insoluble in acids, but forms a soluble sulphate when fused with potassium bisulphate (a reaction which distinguishes it from the other metals of the platinum group). It oxidizes when fused with potassium hydroxide and potassium nitrate, to the dioxide, RhO2. It absorbs hydrogen readily. Rhodium black is obtained by reducing rhodium salts with formic acid; by alcohol in the presence of alkali; or by precipitation with zinc and iron. A colloidal rhodium may be prepared by reducing the sesquichloride with hydrazine hydrate. Rhodium salts may be recognized by their characteristic reaction with freshly prepared sodium hypochlorite solution. A yellow precipitate is obtained, which on shaking for some time with acetic acid gradually dissolves to an orange-coloured solution. This solution after a short time deposits a grey precipitate, and the supernatant liquid becomes azure blue in colour (E. Demarçay, Comptes rendus, 1885, 101, p. 951).
Several oxides of rhodium are known. The monoxide, RhO, formed when the hydrated sesquioxide is heated (Claus) or when finely divided rhodium is heated in a current of air (Wilm), is a grey powder which is insoluble in acids. The sesquioxide, Rh2O3, is a black insoluble powder, formed when the corresponding hydrate is heated. This hydrate, Rh2(OH)3, is obtained as a yellow powder, by decomposing rhodium salts(not the sulphate)with dilute solutions of the caustic alkalis. It is soluble in acids, and in the moist condition is also soluble in concentrated alkalis. A hydrated rhodium dioxide, RhO2·2H2O, is formed when chlorine is passed into a solution of the sesquioxide in concentrated caustic potash, or by adding an alkaline hypochlorite to a concentrated alkaline solution of rhodium and sodium chlorides. It is a greenish-black powder which. is soluble in hydrochloric acid. Rhodium chloride, Rh2Cl6, is obtained impure by heating the metal to dull redness in a current of chlorine, or, purer, by heating an alloy of rhodium and tin in chlorine or by heating the double ammonium rhodium chloride in chlorine at 440° C. (E. Leidié, Ann. chim. phys., 1889, (6), 17, p. 265; Comptes rendus, 1899, 129, p. 1249). It is a red powder, which decomposes at a red heat, leaving a residue of the metal. It is insoluble in water and acids, but dissolves in concentrated solutions of potassium cyanide. The hydrated form Rh2Cl6·8H2O is obtained impure by dissolving the hydrated sesquioxide in hydrochloric acid, by the action of hydrofluosilicic acid on potassium rhodium chloride, and by the action of chlorine on rhodium in the presence of sodium chloride. In the last method the product is dissolved in a dilute hydrochloric acid (1:1), and the solution saturated with hydrochloric acid gas at 0° C., allowed to stand for some time, decanted, and finally evaporated in vacuo (Leidié, loc. cit.). It forms a very deliquescent, red, amorphous mass, which decomposes on exposure. It is very soluble in water, forming a yellow solution. It forms double salts with the alkaline chlorides. Rhodium mono sulphide, RhS, is formed when rhodium or rhodium ammonium chloride are heated with sulphur, and also by precipitating rhodium salts with sulphuretted hydrogen, the precipitate being dissolved in ammonium sulphide and thrown down again by dilute sulphuric acid (Lecoq de Boisbaudran, Ber., 1883, 16, p. 579). It is a dark-coloured powder which is insoluble in acids and other solvents. It loses all its sulphur when heated in air. The sesquisulphide, Rh2S3, is prepared by heating anhydrous rhodium chloride, Rh2Cl6, in a current of sulphuretted hydrogen at 360° C., or by passing the gas into a boiling solution of the chloride. It is a black powder which is insoluble in acids and in alkaline sulphides. It decomposes when strongly heated. Rhodium sulphate, Rh2(SO4)3, is prepared by oxidizing the sulphide, by fusing the metal with acid potassium sulphate, or by the action of concentrated sulphuric acid on an alloy of rhodium and lead, or on the hydrated sesquioxide. It is a red powder which decomposes when heated or when boiled with much water. It forms alums (Leidié, Comptes rendus, 1888, 107, p. 234). Rhodium potassium alum, Rh2(SO4)3·K2SO4·24H2O, is obtained by dissolving the sesquioxide in sulphuric acid and adding two-thirds of the calculated amount of potassium sulphate to the solution (A. Piccini and L. Marino, Zeit. anorg. Chem., 1901, 27, p. 62). It crystallizes in cubes. Rhodium cyanide, Rh2(CN)6, is a carmine-red powder formed when rhodium potassium cyanide, is boiled with acetic acid. Rhodium potassium cyanide, K6Rh2(CN)12, is formed when the sesquioxide is dissolved in caustic potash and an excess of hydrocyanic acid added gradually, the solution being then evaporated in vacuo. It is a colourless crystalline solid soluble in water, and isomorphous with the corresponding iron, cobalt, chromium and manganese compounds.
The rhodium ammonia salts correspond almost with the similar cobalt compounds and may be divided into three series—namely, hexammine salts (luteo-salts), [Rh(NH3)6]X3; aquopentammine salts (roseo-salts), [Rh(NH3)5·H2O]X3; and pentammine salts (purpureo-salts), [Rh(NH3)5X]X2. (See S. F. Jorgensen, C. W. Blomstrand, Jour. prak. Chem., 1882, et seq.)
The atomic weight of rhodium has been determined by S. F. Jorgensen (Jour. prakt. Chem., 1883, 27, p. 486), by the analysis of chlorpurpureo rhodium chloride, the mean value obtained being 103; whilst K. Seubert and K. Kobbé (Ann., 1890, 260, p. 314), by analysis of the double chloride and sulphate, obtained as a mean value 102·86.