Mohr's Salt, ammonium iron sulfate, is a double salt of iron sulfate and ammonium sulfate, with the formula [NH₄]₄[Fe][SO₄]₄·6H₂O. 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⁺
The presence of protons keeps this equilibrium to the left, the Fe(II) side.

Inhalation: No information found, but compound should be handled as a potential health hazard. Inhalation may cause irritation to the upper respiratory tract. Symptoms may include coughing, shortness of breath and chest pain. Other symptoms may parallel those from ingestion.