Ferrous Ammonium Sulfate

Ferrous Ammonium Sulfate
Chemical Formula Fe(NH4)2(SO4)2·6H2O
Formula Weight 392.14
Equivalent Weight 392.14 (Molar = Normal)
CAS No. 7783-85-9

Ferrous Ammonium Sulfate solutions are used as reductometric standards in titrations involving Ceric, Dichromate and Permanganate ions, as well as other oxidizers. Since Ferrous ion is easily oxidized by air to Ferric ion, Ferrous Ammonium Sulfate solutions are not as stable as other reductometric standards such as Arsenic Trioxide and Sodium Oxalate solutions, but they do not have the toxicity or environmental problems of the others. Because of this inherent instability, it is recommended that Ferrous Ammonium Sulfate solutions be restandardized periodically when extreme accuracy is required.

Freshly standardized Ferrous Ammonium Sulfate solutions are used in the determination of Chlorine in drinking water, among other applications. Typical reactions involving Ferrous ions are:

2 Fe2+ + Cl2 → 2 Fe3+ + 2 Cl-

5 Fe2+ + MnO4- + 8 H+ → 5 Fe3+ + Mn2+ + 4 H2O

6 Fe2+ + Cr2O72- + 14 H+ → 6 Fe3+ + 2 Cr3+ + 7 H2O

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