Colloidal Rhodium

Colloidal Rhodium may be prepared by Bredig's method, which consists in sparking between rhodium electrodes submerged in ice- cooled water, a current of 2 amperes at 110 volts proving useful for the purpose.4 The solution has a reddish brown colour, and is very unstable.

Colloidal rhodium may also be prepared by reduction of pure rhodium salts with hydrazine hydrate. Traces of impurity prevent the formation of the hydrosol, which is very unstable. Addition of a protective colloid, such as a 1 per cent, solution of gum acacia, renders the hydrosol stable, so much so that it admits of concentration over sulphuric acid in vacuo, yielding a dark brown solid mass of colloidal metal, containing 99.4 per cent, of rhodium, and almost completely soluble in water.5 Shaking the colloidal solution with barium sulphate or animal charcoal serves to coagulate the metal.

Sodium protalbate or lysalbate may be used instead of gum acacia.

Colloidal rhodium catalytically assists the decomposition of hydrogen peroxide in aqueous solution in a similar manner to colloidal platinum.

Colloidal rhodium prepared by Bredig's method in the form of an unstable solution containing 0.002 gram of metal per litre is toxic towards pathogenic organisms, but has no poisonous action on fish, frogs, and dogs unless injected in large doses.