Chemical Properties of Rhodium

Chemical Properties of Rhodium are related with its very inert metallic state. Finely divided rhodium, such as that obtained by the reduction of its salts in hydrogen, is a greyish, porous powder which only absorbs small quantities of hydrogen, although it acts as a catalyser in promoting the union of hydrogen and oxygen. When heated strongly in air, oxygen is absorbed and the product, once believed to be rhodium monoxide, RhO, appears, from more recent research, to be an ill-defined mixture of metallic rhodium and its sesquioxide. The reaction is noticeable at 600° C., and increases in velocity with rise of temperature. At ordinary temperatures oxygen is without action upon rhodium. Chlorine attacks the finely divided metal, the reaction beginning at 250° C., yielding the trichloride, RhCl₃, which at high temperatures undergoes partial dissociation.

Bromine begins to react at the same temperature (250° C.), but the product has a variable composition, indicative of simultaneous dissociation. Thus:

2Rh + 3Br₂ ⇔ 2RhBr₃.

Rhodium is insoluble in acids, even in aqua regia, although when its alloys are attacked by this latter mixture a portion of the rhodium passes into solution. When fused with potassium hydrogen sulphate, rhodium dissolves, yielding the sulphate. This reaction is interesting as affording a convenient method of separating the metal from iridium and platinum.

Rhodium that has been precipitated from solution evolves considerable quantities of various gases, such as carbon di-oxide, hydrogen, and oxygen, when heated in vacuo. After this treatment the metal does not occlude appreciable quantities of hydrogen or of carbon di-oxide between 420° C. and 1020° C.