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ammonium ferric sulphate
Ferric Ammonium Sulfate Doceahydrate, NH4Fe(SO4)2 . 12 H2O (FAS), is a double salt in the class of aluns, which consists of compunds with the general formula AB(SO4)2 . 12 H2O(1). It has the appearance of weakly violet, octahedrical crystals. There has been some discussion regarding the origin of the crystals colour, with some ascribing it to impurities in the compound(2), and others claiming it to be a property of the crystal itself