Ferrous potassium sulphate, FeSO₄.K₂SO₄

Ferrous potassium sulphate, FeSO₄.K₂SO₄.6H₂O, may be prepared in a similar manner to the ammonium salt. It yields monoclinic crystals of density 2.189. Its solubility in water is given as follows: -

Temperature, °C.	0	10	16	25	35	40	55	70
Grams FeSO4.K2SO4 in 100 grams water	19.6	24.5	30.9	36.5	41	45	56	64.2

From the solution of FeSO4 in water three definite hydrates can be obtained according to the temperature of crystallisation

These data, however, are only approximately correct. From the solution of this salt in water three definite hydrates can be obtained according to the temperature of crystallisation. This is well illustrated by fig., where the vertical column gives the solubility of the salt (expressed in the original memoir as c.c. of KMnO₄.

The dihydrate is colourless, the tetrahydrate faintly green, and the hexahydrate distinctly green. The transition temperatures are as follow: -

FeSO₄.K₂SO₄.6H₂O ⇔ FeSO₄.K₂SO₄.4H₂O + 2H₂O at 30° C.
FeSO₄.K₂SO₄.4H₂O ⇔ FeSO₄.K₂SO₄.2H₂O + 2H₂O at 87° C.
FeSO₄.K₂SO₄.6H₂O ⇔ FeSO₄.K₂SO₄.2H₂O + 4H₂O at 54° C.

The tetra- and di-hydrates may also be obtained by crystallisation from solutions containing varying quantities of sulphuric acid.