Ammonium iron(II) sulfate

Ammonium iron(II) sulfate
Ammonium iron(II) sulfate
CAS number 10045-89-3 YesY, (anhydrous)
7783-85-9 (hexahydrate)
PubChem 24863
ChemSpider 23246 YesY
EC number 233-151-8
Jmol-3D images Image 1
Molecular formula (NH4)2Fe(SO4)2·6H2O
Molar mass 284.05 g mol−1 (anhydrous)
392.14 g mol−1 (hexahydrous)
Appearance Blue-green solid
Melting point

100-110 °C, 373-383 K, 212-230 °F

Boiling point

not applicable

R-phrases R36/37/38
S-phrases S24/25
Main hazards Irritant (Xi)
NFPA 704
NFPA 704.svg
Related compounds
Related compounds Ammonium iron(III) sulfate
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Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Ammonium iron(II) sulfate, or Mohr's Salt, is the inorganic compound with the formula (NH4)2Fe(SO4)2·6H2O. 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. 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.[1]



In analytical chemistry, this salt is preferred over other salts of ferrous sulfate for titration purposes as it is much less prone to oxidation by air 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 slows this oxidation process.[1]

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. Several similar salts are known, which were once called Tutton's salts.

Preparation of Mohr's salt

Mohr's salt is prepared by dissolving an equimolar mixture of hydrated ferrous sulfate and ammonium sulfate in water containing a little sulfuric acid, and then subjecting the resulting solution to crystallization. Ferrous ammonium sulfate forms light green crystals.

Shape of ferrous ammonium sulfate crystals

The crystals of Mohr's salt are monoclinic in shape.


  1. ^ a b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Oxford: Butterworth-Heinemann. ISBN 0080379419. 

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