Potassium Chromium Alum, K₂SO₄.Cr₂(SO₄)₃

Potassium Chromium Alum, "Chrome Alum", K₂SO₄.Cr₂(SO₄)₃.24H₂O, first isolated by Mussin-Puschin, is obtained by the reduction of a mixture of potassium dichromate and sulphuric acid by sulphurous acid,4 alcohol, oxalic acid, etc.

The aqueous solution (blue, with a reddish tinge, but becoming green at 50° to 70° C.) deposits violet octahedra; of the several forms in which chrome alum crystallises, including the rhombic dodecahedron, cube, and trapezohedron, the octahedron is the most stable. The crystals have a density of about 1.84, a specific heat of 0.324, and a coefficient of expansion of 0.0000246. The transformation of the violet to the green variety takes place at 78° C., the substance melting at 89° C. to a green mass. Other physical constants have been determined, and the absorption spectra of its aqueous solution studied.

At ordinary temperatures, over concentrated sulphuric acid, chrome alum loses 12H₂O; on heating to 300° to 350° C. it becomes anhydrous, but decomposes in the air. If heated much above 350° C. the residue is then insoluble in water.

Chromium alum is soluble in water, 100 c.c. of water at 25° C. dissolving 24.39 grams of the hydrated salt. The violet solution, on heating at 50° to 70° C.. becomes green, and is then crystallisable with difficulty. The violet solution, even at ordinary temperatures, slowly changes to green, the change being attended by increase of volume and vice versa, irrespectively of concentration. The change appears to be due to hydrolysis, and the resulting basic chromic salts polymerise when the solution is heated, and, more slowly, even on standing. The addition of alkali carbonate causes liberation of carbon dioxide due to the sulphuric acid formed, and precipitation occurs owing to coagulation of colloidal chromic hydroxide or of the polymerised basic salts. The amount of carbonate necessary to produce a permanent precipitate varies considerably with the age of the chrome-alum solution and with its method of preparation.

Crystalline rubidium chromium alum has been described.

With ammonium sulphate, chromic sulphate forms the double salts, 3(NH₄)₂SO₄.Cr₂(SO₄)₃ and (NH₄)₂SO₄.Cr₂(SO₄)₃, besides the alum, (NH₄)₂SO₄.Cr₂(SO₄)₃.24H₂O. The former are obtained on fusing oxides or certain other compounds of chromium with ammonium sulphate. The compound 3(NH₄)₂SO₄.Cr₂(SO₄)₃ forms green needles slightly attacked by water; the salt, (NH₄)₂SO₄.Cr₂(SO₄)₃, identical with anhydrous ammonium chrome alum, forms hexagonal crystals.