The preparation and characterization of ferrous sulphate hydrates

Ferrous sulphate BP, FeSO4,7H2O, dehydrates to form FeSO4,4H2O when the relative humidity (RH) is less than 65%, or on heating at 40 degrees C. The tetrahydrate was also prepared by recrystallization at 60 degrees C from aqeuous solutions of the sulphate. Commercial dried ferrous sulphate BP is FeSO4,H2O but the method of preparation given in the BP produces FeSO4,4H2O which is the stable form at RH less than 65% (25 degrees C). A crystalline monohydrate was prepared either by heating the tetrahydrate on a boiling water bath to constant weight or heating the heptahydrate under vacuum at 60 degrees C in the presence of water vapour evolved from the water of crystallization. An X-ray amorphous form of FeSO4,H2O was made by heating the heptahydrate at 100 degrees C under vacuum with the removal of water vapour. Crystalline FeSO4,H2O rehydrates to the heptahydrate when RH greater than 65% but the amorphous form recrystallizes as the stable tetrahydrate at RH greater than 0 and less than 65%. The various hydrates were characterized by DSC and X-ray diffraction. A simple procedure is described for obtaining data to construct a RH-composition phase diagram. The method combines measurement of any weight changes of each hydrate after storage at various controlled humidities together with a direct measurement of RH over the hydrate pairs. The thermal dehydration pathways are reported together with the RH-composition phase diagrams.