Ammonium iron(II) sulfate

Ammonium iron(II) sulfate, or Mohr's salt, is the inorganic compound with the formula (NH₄)₂SO₄·Fe(SO₄)·6H₂O. Containing two different cations, Fe²⁺ and NH+4, it is classified as a double salt of ferrous sulfate and ammonium sulfate. It is a common laboratory reagent because it is readily crystallized, and crystals resist oxidation by air. 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] Its mineral form is mohrite.

Ammonium iron(II) sulfate

Names
IUPAC name Ammonium iron(II) sulfate
Other names Ferrous ammonium sulfate Ammonium iron sulfate Mohr's salt
Identifiers
CAS Number	anhydrous: 10045-89-3 Y hexahydrate: 7783-85-9
3D model (JSmol)	Interactive image
ChEBI	anhydrous: CHEBI:76243 hexahydrate: CHEBI:76181 Y
ChemSpider	anhydrous: 23246 Y hexahydrate: 13084980 Y
ECHA InfoCard	100.030.125
EC Number	233-151-8
PubChem CID	anhydrous: 24863 hexahydrate: 15942308
UNII	anhydrous: 9Q764AYJ9G Y hexahydrate: 99A04ICQ3J
CompTox Dashboard (EPA)	DTXSID4041028
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	Fe(SO4)(NH4)2(SO4) (anhydrous) Fe(SO4)(NH4)2(SO4)·6H2O (hexahydrate)
Molar mass	284.05 g mol−1 (anhydrous) 392.14 g mol−1 (hexahydrate)
Appearance	Blue-green solid
Density	1.86 g/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)
Hazards
GHS labelling:
Pictograms
Signal word	Warning
Hazard statements	H315, H319, H335
Precautionary statements	P261, P264, P271, P280, P302+P352, P304+P340, P305+P351+P338, P312, P321, P332+P313, P337+P313, P362, P403+P233, P405, P501
NFPA 704 (fire diamond)	2 0 0
Safety data sheet (SDS)	Fisher MSDS
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). N verify (what is YN ?) Infobox references

Structure

This compound is a member of a group of double sulfates called Schönites or Tutton's salts. Tutton's salts form monoclinic crystals and have formula M₂N(SO₄)₂·6H₂O (M = various monocations). With regards to the bonding, crystals consist of octahedra [Fe(H₂O)₆]²⁺ centers, which are hydrogen bonded to sulfate and ammonium.^[2]

Structure of ferrous ammonium sulfate with hydrogen bonding network highlighted (N is violet, O is red; S is orange, Fe is light red).

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.

Applications

In analytical chemistry, this salt is the preferred source of ferrous ions as the solid has a long shelf life, being resistant to oxidation. This stability extends somewhat to solutions reflecting the effect of pH on the ferrous–ferric redox couple. This oxidation occurs more readily at high pH. The ammonium ions make solutions of Mohr's salt slightly acidic, which slows this oxidation process.^[1][3] Sulfuric acid is commonly added to solutions to reduce oxidation to ferric iron.

It is used in the Fricke's dosimeter to measure high doses of gamma rays.^[4]

Preparation

Mohr's salt forms upon evaporation of an equimolar mixture of aqueous ferrous sulfate and ammonium sulfate.^[5]

Contaminants

Common impurities include magnesium, nickel, manganese, lead, and zinc, many of which form isomorphous salts.^[6]

References

1. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
2. Ephraim, Fritz (1926). Inorganic Chemistry. tr P. C. L. Thorne. London: Gurney and Jackson. pp. 484–485.
3. "Ammonium Ferrous Sulphate 100 g (Mohr's Salt)". 2012. Retrieved 13 June 2013.
4. Hickman, C.; Lorrain, S.; Barthe, J.R.; Portal, G. (1986). "Use of Mohr's Salt for High Level Gamma Dosimetry (Up to 10⁸ Gy)". Radiation Protection Dosimetry. 17 (1–4). Oxford Journals: 255–257. doi:10.1093/oxfordjournals.rpd.a079818.
5. Wildermuth, Egon; Stark, Hans; Friedrich, Gabriele; Ebenhöch, Franz Ludwig; Kühborth, Brigitte; Silver, Jack; Rituper, Rafael (2000). "Iron Compounds". Ullmann's Encyclopedia of Industrial Chemistry. doi:10.1002/14356007.a14_591. ISBN 978-3-527-30385-4.
6. Vogel, Arthur I. (1961). A Text-book of Quantitative Inorganic Analysis Including Elementary Instrumental Analysis (3 ed.). Longmans. pp. 281–282.