Ammonium iron(II) sulfate

Ammonium iron(II) sulfate
IUPAC name Iron(II) ammonium sulfate
Other names Ferrous ammonium sulphate Ammonium iron sulphate Mohr's salt
Identifiers
CAS number	10045-89-3 Y, (anhydrous) 7783-85-9 (hexahydrate)
PubChem	24863
ChemSpider	23246 Y
EC number	233-151-8
Jmol-3D images	Image 1
SMILES [Fe+2].[O-]S(=O)(=O)[O-].[O-]S([O-])(=O)=O.[NH4+].[NH4+]
InChI InChI=1S/Fe.2H3N.2H2O4S/c;;;2*1-5(2,3)4/h;2*1H3;2*(H2,1,2,3,4)/q+2;;;;/p-2 Y Key: IMBKASBLAKCLEM-UHFFFAOYSA-L Y InChI=1/Fe.2H3N.2H2O4S/c;;;2*1-5(2,3)4/h;2*1H3;2*(H2,1,2,3,4)/q+2;;;;/p-2 Key: IMBKASBLAKCLEM-NUQVWONBAX
Properties
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
Hazards
MSDS	Fisher MSDS
R-phrases	R36/37/38
S-phrases	S24/25
Main hazards	Irritant (Xi)
NFPA 704	0 2 0
Related compounds
Related compounds	Ammonium iron(III) sulfate
Y (verify) (what is: Y/N?) 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]

Applications

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.

References

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