Sulphochromic Acid, Cr₂O₃.4SO₃

Sulphochromic Acid, Cr₂O₃.4SO₃.3H₂O, and certain of its salts, have been prepared; chromosulphochromic acid, its salts, and chromodisulphochromic acid are also known.

On allowing a mixture of chromic acid and concentrated sulphuric acid to stand for several days, the compound H₂SO₄.CrO₃ or H₂CrSO₇ is formed; on heating a mixture of chromic and sulphuric anhydrides in a sealed tube to 75° C. or a higher temperature, the resulting yellow mass consists essentially of the mixed anhydride, CrO₃.SO₃.

Chromium Chlorsulphates may be prepared either by the action of sulphuric acid on chromic chloride, or by the interaction of hydrochloric acid with chromium sulphate. Isomeric modifications of these salts exist, for it has been shown that in certain cases the - SO₄, and in others the -Cl, is ionised.

By dissolving hydrated chromic chloride in its own weight of water and adding fairly concentrated sulphuric acid, there are precipitated pale green crystalline plates, having a formula [CrCl.(H₂O)₅]SO₄.3H₂O. From a solution of this salt the sulphate ion, but not the chlorion, is immediately precipitable. On the other hand, when violet chromic sulphate, Cr₂(SO₄)₃.17H₂O, is heated at 80° C. until 3H₂O is lost, yielding a green powder, and then is dissolved in water, kept for three hours, cooled and saturated with hydrogen chloride, a green crystalline powder separates which has the formula [CrSO₄.(H₂O)₅]Cl, and in a solution of which the chloride, and not the sulphate, is ionised.

By dissolving chromic sulphate in hot concentrated hydrochloric acid, the green salt, CrSO₄Cl.6H₂O, is obtained from which the sulphate ion but not the chlorion is immediately precipitable. It is considered to be chlorpentaquochromium sulphate, [Cr(OH₂)₅Cl]SO₄.H₂O, and possibly has the constitution [CrCl₂(H₂O)₄][Cr(H₂O)₆](SO₄)₂.2H₂O; it can also be obtained by the action of concentrated sulphuric acid on a mixture of 1 molecule of violet chromium sulphate and 2 to 6 molecules of green chromic chloride; another method of preparation is by the action of sulphuric acid upon chlorchromium dichloride, CrCl₃.6H₂O (two-thirds of the chlorine precipitable), alone. At 85° C. the pentahydrate is obtained.