What is Mohr’s salt or Ferrous Ammonium Sulphate?
Mohr’s salt is a light green colour crystalline inorganic compound. It is a double salt of FeSO4 .7H2O and (NH4)2SO4 which belongs to a group of double sulfate named Schonites or tutton’s salts, { formula: M2M’ (SO4)2.6H2O where M is univalent and M’ is divalent ions.
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The chemical Mohr’s salt formula of the double salt is FeSO4.(NH4)2SO4.6H2O (Ferrous Ammonium Sulfate Hexa Hydrate) or (NH4)2Fe(SO4)2.6H2O (Ammonium Iron(II) Sulphate).Its name derived from the name of German Chemist Karl Friedrich Mohr, who proposed various important approaches of titration in the 19th century.
Ferrous Ammonium Sulfate is soluble in water and give Octahedral geometry Fe(H2O)6]+2aqua complex with hydrogen bonding with Sulfate and Ammonium ions
It prepared by adding little sulphuric acid in a equimolar solution of FeSO4 .7H2O and (NH4)2SO4, the resulting solution give the light green crystal of Ferrous Ammonium Sulphate.
Why Mohr’s Ferrous Ammonium Sulfate salt used as a primary standard in Titration
It may be noted that this salt contains only Fe+2 NH4+ ions without any trace of Fe+3 ions. In contrast, FeSO4 .7H2O always contains some Fe+3 ions due to aerial oxidation of F+2ions. It is, therefore, used as a primary standard in the volumetric analysis since a standard solution of F+2 ions can be obtained directly by weighing a known amount of the Mohr’s salt
It acts as a reducing agent and as such reduces acidified KMnO4 and K2Cr2O7solutions
MnO4– + 5Fe+2+ 8H+ —–> 5Fe+3 +Mn+2 + 4H2O
Cr2O7-2+ 6Fe+2 + 14H+ —–> 6Fe+3 + 2Cr+3 + 7H2O