This member of the platinum group of metals is prepared from the solution remaining after throwing down ammonio-platinum chloride, by precipitating by metallic iron, and fusing the deposit with 1 part lead and 2 parts litharge to form a regulus; the copper, lead, and palladium are dissolved out by dilute nitric acid, and the insoluble residue is mixed with 5 parts barium binoxide, and heated to redness for 2 hours; the mass is lixiviated with' water, and the residue is boiled with aqua-regia to volatilize the osmium tetroxide, which is condensed (see p. 400), the barium in excess is precipitated by adding sulphuric acid, and the filtrate is evaporated at 212° F. (100° C.) with excess of sal-ammoniac; the residue is washed with sal-ammoniac solution so long as a rose-red colour is communicated to the wash-water, and the filtrate is evaporated with nitric acid in excess to decompose the sal-ammoniac; the solid residue is heated to redness with 3 or 4 parts of sulphur, and rapidly boiled out with aqua-regia and sulphuric acid.
The resulting nearly pure metallic rhodium may be refined by fusing with 3 or 4 parts of zinc, and treating the alloy with strong hydrochloric acid; it is then dissolved in aqua-regia, and evaporated with ammonia in excess; the rhodium-ammonium chloride which separates out is purified by re-crystallization, ignited with sulphur in a graphite crucible, and finally fused in an oxyhydrogen furnace to remove traces of osmium and silicon. The pure metal is less fusible than platinum, has a sp. gr. of 12.1, and resembles aluminium in lustre and colour; it is almost insoluble in all acids, though its alloys dissolve in aqua-regia; it is attacked by chlorine more readily than any other member of the platinum group, and can be dissolved by repeated ignition with phosphoric acid, acid phosphates, or fused potash bisulphate.