Ammonium iron(II) sulfate

Ammonium iron(II) sulfate

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Names
IUPAC name Ammonium iron(II) sulfate
Other names Ferrous ammonium sulfate Ammonium iron sulfate Mohr's salt
Identifiers
CAS Number	10045-89-3 Y (anhydrous) 7783-85-9 (hexahydrate)
ChEBI	CHEBI:76181 N
ChemSpider	23246 Y
EC Number	233-151-8
Jmol 3D model	Interactive image
PubChem	24863
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
SMILES [Fe+2].[O-]S(=O)(=O)[O-].[O-]S([O-])(=O)=O.[NH4+].[NH4+]
Properties
Chemical formula	(NH4)2Fe(SO4)2·6H2O
Molar mass	284.05 g mol−1 (anhydrous) 392.13 g mol−1 (hexahydrous)
Appearance	Blue-green solid
Density	1.86g/cm3
Melting point	100 to 110 °C (212 to 230 °F; 373 to 383 K)
Boiling point	not applicable
Solubility in water	269 g/L (hexahydrate)
Vapor pressure	{{{value}}}
Related compounds
Related compounds	Ammonium iron(III) sulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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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.

This compound is a member of a group of double sulfates called Schönites or Tutton's salts. which form monoclinic crystals and have formula M₂N(SO₄)₂.6H₂O. This group can mix metals and crystallise on other schönite crystals.

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. Sulfuric acid is commonly added to solutions to reduce oxidation to ferric iron.

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.

It is used in the Fricke's dosemeter to measure high doses of gamma rays.

Preparation

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.

Contaminants

The Analar Standards for Laboratory Chemicals only specify ≥99% purity for the standard Mohr's salt. Before use in titration the salt should be recrystalised, filtered, washed and dried. Common contaminant metal impurities include Mg, Mn, Ni, Pb, and Zn.