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Mohr's salt

Mohr's salt
IUPAC name Iron(II) ammonium sulphate
Other names Ferrous ammonium sulphate
Ammonium iron sulphate
CAS number
PubChem 24863
SMILES [NH4+].[NH4+].[O-]S(=O)(=O)[O-].[O-]S(=O)(=O)[O-].[Fe+2]
Molecular formula FeH8N2O8S2
Molar mass 392.14 g/mol
Appearance Blue-green solid
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

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+

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.

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Mohr's_salt". A list of authors is available in Wikipedia.
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