Mohr's salt

Mohr's Salt, ammonium iron sulfate, is a double salt of iron sulfate and ammonium sulfate, with the formula [NH₄]₂[Fe][SO₄]₂·6H₂O. Mohr's salt is preferred over iron(II) sulfate for titration purposes as it is much less affected by oxygen in the air than iron(II) sulfate, solutions of which tend to oxidise to iron(III). The oxidation of solutions of iron(II) is very pH dependent, occurring much more readily at high pH. The ammonium ions make solutions of Mohr's salt slightly acidic, which prevents this oxidation occurring. The relevant equation for this is:

4 Fe²⁺ + O₂ + (4+2x) H₂O ⇌ 2 Fe₂O₃.xH₂O + 8 H⁺

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The presence of protons keeps this equilibrium to the left, the Fe(II) side.

Mohr's salt is named after the German chemist Karl Friedrich Mohr, who made many important advances in the methodology of titration in the 19th century.