Ammonium iron(II) sulfate

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Ammonium iron(II) sulfate
Mohr's salt.png
MohriteD.jpg
Síran železnato-amonný.JPG
Names
IUPAC name
Ammonium iron(II) sulfate
Other names
Ferrous ammonium sulfate
Ammonium iron sulfate
Mohr's salt
Identifiers
  • 10045-89-3 YesY
  • (anhydrous)
    7783-85-9 (hexahydrate)
3D model (Jmol)
ChEBI
ChemSpider
ECHA InfoCard 100.030.125
EC Number 233-151-8
UNII
Properties
(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
269 g/L (hexahydrate)
Hazards
Main hazards Irritant (Xi)
Safety data sheet Fisher MSDS
R-phrases (outdated) R36/37/38
S-phrases (outdated) S24/25
NFPA 704
Flammability code 0: Will not burn. E.g., water Health code 2: Intense or continued but not chronic exposure could cause temporary incapacitation or possible residual injury. E.g., chloroform Reactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g., liquid nitrogen Special hazards (white): no codeNFPA 704 four-colored diamond
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.[1]

Structure[edit]

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(OH2)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 = large 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[edit]

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 dosemeter to measure high doses of gamma rays.[4]

Preparation[edit]

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[edit]

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.[5]

References[edit]

  1. ^ a b Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9. 
  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)". Radiatiation Protection Dosimetry. Oxford Journals. 17 (1-4): 255–257. 
  5. ^ Vogel, Arthur I. (1961). A Text-book of Quantitative Inorganic Analysis Including Elementary Instrumental Analysis (3 ed.). Longmans. pp. 281–282.