Detection of Rhodium

Metallic Rhodium, when pure and compact, is insoluble in mineral acids, not excepting aqua regia, although it dissolves in the latter when alloyed with certain other elements such as lead, bismuth, copper and platinum, although not when alloyed with silver or gold. When fused with a mixture of potassium hydroxide and nitrate it is readily oxidised, whilst fusion with potassium hydrogen sulphate converts it into the soluble salt, potassium rhodium sulphate, Rh₂(SO₄)₃.3K₂SO₄, or K₆Rh₂(SO₄)₆ - a point of distinction between rhodium and ruthenium, since the latter metal is not affected by potassium hydrogen sulphate.

When heated with sodium chloride in a current of chlorine, the double chloride, sodium chlor-rhodite, Na₃RhCl₆.Aq., is formed, which dissolves in water, yielding a rose-red solution.

Ammonium sulphide effects the precipitation of rhodium sesquisulphide from hot solutions of rhodium salts. In contradistinction to platinum and iridium sulphides, which are soluble in excess of ammonium sulphide, rhodium sulphide is insoluble in excess of this reagent.

Alkalies yield a yellowish brown precipitate of rhodium hydroxide, Rh₂O₃.3H₂O, on boiling. This precipitate is soluble in excess of the reagent (contrast ruthenium).

Potassium hydroxide causes no precipitation from rhodium trichloride solution in the cold, but on adding alcohol a brown precipitate of hydroxide is obtained. This reaction is characteristic for rhodium.

Another noteworthy reaction consists in adding potassium nitrite to a solution of the trichloride, whereby an orange-yellow precipitate is obtained which is only slightly soluble in water and is but slightly decomposed by concentrated hydrochloric acid.

If the gases evolved by the action of hydrochloric acid upon potassium chlorate are passed into a solution of a rhodium salt rendered alkaline by addition of sodium hydroxide, the liquid becomes yellowish red, then red, and finally a slight green precipitate forms. This latter redissolves, yielding a blue solution containing sodium rhodate, Na₂RhO₄. This colour is destroyed by sodium peroxide or persulphate, and by sulphur dioxide.

Metallic rhodium is precipitated from solutions of its salts by addition of metallic zinc.