Ferrous copper sulphate, (Fe, Cu)SO₄

Ferrous copper sulphate, (Fe, Cu)SO₄.H₂O, or CuSO₄.2FeSO₄.3H₂O, occurs in yellowish brown crystals which are not hygroscopic in ordinary air, but take up water in a warm atmosphere saturated with moisture.

The brown colour increases in depth from both ends of the series, the maximum being reached with 18 per cent, of iron and 16 per cent, of copper. The crystals are perfectly homogeneous under the microscope. When gently heated they become chocolate-brown and finally mauve, but the original colour is restored upon exposure to air.

The origin of the brown colour has been a matter of discussion. The monohydrates of ferrous sulphate and copper sulphate are white and pale blue respectively, so cannot be the direct cause of the colour. Neither is the brown due to ferric salts or to cuprous oxide. The suggestion has been made that it is the result of an oscillation of electrons due to the presence of metals in different stages of oxidation, the copper being saturated whilst the iron is not.