Ammonium iron(II) sulfate

Ammonium iron(II) sulfate, or Mohr's salt, is the inorganic compound with the formula (NH₄)₂Fe(SO₄)₂(H₂O)₆. Containing two different cations, Fe²⁺ and NH₄⁺, it is classified as a double salt of ferrous sulfate and ammonium sulfate. It is a common laboratory reagent because it is readily crystallized, and crystals resist oxidation by air. Like the other ferrous sulfate salts, ferrous ammonium sulfate dissolves in water to give the aquo complex [Fe(H₂O)₆]²⁺, which has octahedral molecular geometry. Its mineral form is mohrite.

In analytical chemistry, this salt is the preferred source of ferrous ions as the solid has a long shelf life, being resistant to oxidation. This stability extends somewhat to solutions reflecting the effect of pH on the ferrous/ferric redox couple. This oxidation occurs more readily at high pH. The ammonium ions make solutions of Mohr's salt slightly acidic, which slows this oxidation process. Sulfuric acid is commonly added to solutions to reduce oxidation to ferric iron.

It is used in the Fricke's dosemeter to measure high doses of gamma rays