RHODIUM, a metal always found in platinum ores but never in large quantities, was first discovered by Wollaston and an nounced by him in the memoir disclosing the discovery of palla dium (Phil. Trans., 1804). (Symbol Rh, atomic number 45, atomic weight 102.9.) Its name is due to the rose-red colour of its salts (566ov, a rose).
If a platinum ore is treated with aqua regia, most of the os mium, iridium and ruthenium is left undissolved (some rhodium is also left if much is present). The platinum can be nearly all removed from the solution by precipitation with ammonium chloride and, if the acid solution is treated with scrap iron ("footed"), the remaining metals are deposited. If this pre cipitate is fused with potassium bisulphate the rhodium is all converted into potassium rhodium sulphate which on extraction with water and recrystallization gives the pure salt. Another method is to heat the above metallic precipitate with common salt in a stream of chlorine, when the double sodium rhodium chloride, is formed; this is extracted with water and crystallizes with 12 molecules of water in dark red crystals ap pearing almost black by transmitted light. For other and more complicated methods of extraction, see Claus, T. Prakt. Chem., (1843-45); Gibbs, ibid. (1861) p. 65, (1865) p. Io.
Rhodium in the massive state is a slightly bluish-white metal having a specific gravity of about 12.1. It fuses at about 194o° C and volatilizes at a somewhat higher temperature. It oxidizes su perficially when heated in the air, becoming coated with a film of a blue oxide. It is practically insoluble in either hydrochloric or nitric acid or in aqua regia, but it is attacked by hot concentrated sulphuric acid or on fusion with potassium bisulphate—an unique property amongst the platinum metals ; when alloyed with other metals and especially with lead, it is soluble in other strong acids. Fused with nitre and caustic potash it is oxidized to From the commonest commercial salt, sodium rhodium chloride, other rhodium salts may be readily prepared by precipitating from its solution the yellow hydroxide with an equivalent amount of caustic potash (the hydroxide is soluble in excess), filtering and washing the precipitate, and dis solving it in the desired acid. The hydroxide being a stronger
base than that of most of the other metals of this group, rhodium salts, even of organic acids, may easily be prepared by this process.
Four oxides of rhodium are known, R11,0, RhO, and of these is of some importance as being the oxide from which ordinary rhodium salts are derived. It is a black, sparingly soluble powder produced by heating the corresponding hydroxide.
Rhodium chloride, can be produced either by heating finely divided rhodium in a stream of chlorine or by dissolving the corresponding hydroxide in hydrochloric acid. The latter process first gives a yellow solution, which on heating assumes a rose-red colour, and on careful evaporation gives a residue of a gummy, deliquescent hydrated chloride. This on further heating loses its water and becomes insoluble. Two new chlorides of rhodium, RhC1 and have been prepared by Wohler and Muller (1926). The simple chloride RhC1, readily produces double chlo rides with the chlorides of the alkali metals having the general formula Rhodium sulphate, is formed from the yel low hydroxide by solution in dilute sulphuric acid. The double sulphate produced by fusing rhodium with potas sium bisulphate, is soluble in water and gives rose-coloured crys tals on evaporation to dryness. Rhodium sulphate also combines with alkaline sulphates to form true alums isomorphous with ordinary alum. The best known example is potassium rhodium alum, (See ALUM.) Rhodium f orms several series of compounds with ammonia : the best known are the roseo, purpureo and luteo compounds which are similar to the corresponding cobalt compounds (see COBALT). Technically, little use has been found for rhodium compounds, although it is essential in small amounts in the manu facture of "liquid gold" (see GOLD). A rhodium–platinum alloy is in general use as one of the elements in a thermocouple for measuring high temperatures. (F. E. M.)