Ferrous ammonium sulphate, FeSO₄.(NH₄)₂SO₄

Ferrous ammonium sulphate or Mohr's salt, FeSO₄.(NH₄)₂SO₄.6H₂O, may be prepared by adding a saturated solution of ammonium sulphate at 40° C. to the requisite quantity of ferrous sulphate dissolved also at 40° C. in the smallest amount of water. The solution, on cooling, deposits an abundant crop of pale green crystals, monoclinic, and of density 1.864 at 20° C. The salt is kept dry by pressure between filter papers.

Ferrous ammonium sulphate is a stable salt at ordinary temperatures, and its solutions in the cold do not readily oxidise. Hence it is largely used in the laboratory for standardising solutions of potassium permanganate for volumetric analysis. Its solubility in water is as follows: -

Temperature, °C	0	15	40	50	70
Grams FeSO4.(NH4)2SO4 in 100 grams H2O	12.5	20.0	33.0	40	52

According to Locke, 351 grams of the anhydrous salt dissolve in one litre of water at 25° C.

These solutions readily absorb nitric oxide in a precisely similar manner to those of pure ferrous sulphate.