A. Mitchell
Aug 1, 1984
Citations
1
Influential Citations
13
Citations
Journal
Journal of Pharmacy and Pharmacology
Abstract
Ferrous sulphate BP, FeSO4, 7H2O, dehydrates to form FeSO4, 4H2O when the relative humidity (RH) is less than 65%, or on heating at 40 °C. The tetrahydrate was also prepared by recrystallization at 60 °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 < 65% (25 °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 °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 °C under vacuum with the removal of water vapour. Crystalline FeSO4, H2O rehydrates to the heptahydrate when RH > 65% but the amorphous form recrystallizes as the stable tetrahydrate at RH > 0 and <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.