THE THERMAL DEHYDRATION OF NA2WO4-CENTER-DOT-2H(2)O

The thermal dehydration in flowing dry nitrogen of commercial crystalline powders and of laboratory prepared single crystals of sodium tungstate dihydrate has been studied by power compensation DSC (both isothermal and dynamic runs) and optical microscopy. The two water molecules are lost in a single thermal event ending below 373 K in dynamic runs at scan speeds beta lower than 1 K min(-1). Kinetic analysis of both types of thermal curve indicates that a partial transformation precedes the main decomposition for which the contracting circle equation is the best fitting one. Supporting evidence is drawn from optical microscopy which, however, also shows the formation and growth of randomly distributed nuclei on the single crystal surfaces. Furthermore, optical microscopy reveals that in some instances the observed surfaces undergo reorganization at the very beginning of nucleation. This indicates that a role is played in the decomposition by the interphasal zones of the crystals and is in agreement with recent findings on dehydration of other hydrates. Thermal data and kinetic parameters are reported and the mechanism of dehydration discussed.