Note 18 — Chapter XXII · IRON, COBALT, AND NICKEL
George Kamensky translation
Ferrous sulphate, like magnesium sulphate, easily forms double salts—for instance, (NH4)2SO4,FeSO4,6H2O. This salt does not oxidise in air so readily as green vitriol, and is therefore used for standardising KMnO4.