Polymorphic transitions in anhydrous sodium sulphate

Abstract

The heat capacity of anhydrous sodium sulphate has been measured recently in a water calorimeter by Kreidl and Simon, who observed only one high-temperature transition, at about 238°C. However, there are two known polymorphic transitions between room temperature and 300°C.: (1) the transformation of the stable low-temperature form V to the intermediate form III and (2) transformation of III to the stable high-temperature form I. The heat of transition observed by Kreidl and Simon was nearly equal to the sum of the heats of the two known transitions. They concluded that in the presence of water, the transition V→I occurs directly and reversibly, and supported this conclusion with X-ray diffraction data. More recently, Bird has reported an irreversible transformation, V→I, upon heating sodium sulphate in the absence of water. This occurred at a considerably lower temperature of about 176° C., and is based on comparison of high-temperature X-ray diffraction patterns.