Rhodium Black

Rhodium Black is the name given to the black precipitate of indefinite composition obtained by reduction of solutions of rhodium salts, as, for example, by treatment with alcohol and potassium hydroxide or with a mixture of ammonium hydroxide, formate, and acetate. The precipitate consists of metallic rhodium associated with more or less hydride or oxide, and in an exceedingly fine state of subdivision. Inactive rhodium black becomes active after absorbing oxygen for a time.

For the preparation of rhodium black of permanent catalytic activity the presence of sulphur compounds appears to be necessary. Convenient methods of obtaining such consist in reducing rhodium sesquisulphide with formic acid; and by electrolytic deposition of rhodium black from a solution of rhodium in 60 per cent, sulphuric acid by a current of 0.02 ampere at 180° C.

Rhodium black dissolves not only in aqua regia, but also in concentrated sulphuric acid and in hydrochloric acid in the presence of air. It possesses powerful catalytic properties, decomposing formic acid into carbon dioxide and hydrogen at ordinary temperatures; alcoholic potash into potassium acetate and hydrogen; and liberating oxygen from solutions of alkali hypochlorites. The molecule of ozone is disrupted by rhodium black, yielding ordinary oxygen.

Precipitated rhodium, when heated in a vacuum, evolves a considerable volume of gas consisting of a mixture of carbon dioxide, oxygen, hydrogen, and water.