Ammonium iron(II) sulphate (formerly also Mohr’s salt) is the sulphate of ammonium and iron, it forms water-soluble, light green monoclinic crystals. It is usually commercially available with six molecules of water of crystallization as ammonium iron(II) sulfate hexahydrate. It is a double salt from the group of Tutton salts.
Ammonium iron(II) sulphate is mainly used as hexahydrate. The light green crystals are very soluble in water and form an acidic solution. They crystallize according to the monoclinic system. The salt with the two different cations Fe2+ and NH4+ belongs to the double salts. When dissolved in water, an iron-water complex [Fe(H2O)6]2+ is formed, which is particularly stable. The central Fe2+ ion is surrounded in the complex by six water molecules as ligands. The substance known as Mohr’s salt is named after the German chemist Karl Friedrich Mohr (1806-1879).
After mixing two hot saturated solutions of ferrous sulphate and ammonium sulphate with a little sulphuric acid, the Mohr’s salt crystallizes out after cooling.
(NH4)2SO4 + FeSO4 → (NH4)2Fe(SO4)2
Mohr’s salt is used in chemistry lessons for crystallization experiments. The synthesis of the salt with yield calculation is a classical experiment in chemistry studies. The salt is also used in analytical chemistry to adjust potassium permanganate solutions for dimensional analysis. In contrast to iron(II) sulfate, the double salt is much more stable in aqueous solution. Ordinary iron(II) salts oxidize in aqueous solution to form iron(III) salts.