Mohr's salt

Chembox new
ImageFile = Mohr's salt.png ImageSize = 200px
IUPACName = Iron(II) ammonium sulfate
OtherNames = Ferrous ammonium sulphate Ammonium iron sulphate
Section1 = Chembox Identifiers
CASOther = [10045-89-3] (Anhydrous) [7783-85-9] (Hexahydrate)
PubChem = 24863
SMILES = [NH4+] . [NH4+] . [O-] S(=O)(=O) [O-] . [O-] S(=O)(=O) [O-] . [Fe+2]
Section2 = Chembox Properties
Formula = FeH₂₀N₂O₁₄S₂
MolarMass = 392.14 g/mol [hexahydrate]
Appearance = Blue-green solid
Density =
MeltingPt =
BoilingPt =
Section3 = Chembox Hazards
Solubility = 269 g/L (hexahydrate)
MainHazards = Irritant (Xi)
RPhrases = R36/37/38
SPhrases = S24/25
FlashPt =
Autoignition =
ExternalMSDS = [http://fscimage.fishersci.com/msds/09715.htm Fisher MSDS]

Mohr's Salt, or 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 from occurring. The relevant equation for this is::4 Fe²⁺ + O₂ + (4+2x) H₂O unicode| 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.