The dehydration of Na2S2O3· 5H2O single crystals as studied by thermal analysis and optical microscopy

The dehydration of cleaved single crystal of sodium thiosulphate pentahydrate in vacuum or in flowing dry N2 has been studied employing optical microscopy and thermal methods. Evidence of the formation of a dihydrate as intermediate has been obtained in both dehydration and rehydration experiments. Comparative studies were made of reactions on single crystals cleaved parallel to three different crystallographic planes, but the dependence of the dehydration on face was ascertained only for the initial portions of vacuum or high-N2 flow experiments. Microscopic evidence and the initial splitting of the constant-temperature dehydration thermal curves indicated that, at the onset of dehydration, the formation of a fully dehydrated layer on the crystal surface takes place. The subsequent evolution of the transformation is determined by the properties of this layer. These in turn appear to depend on the perfection of the cleaved surface as well as on the efficiency of the dehydrating agents. Dehydration mechanisms for strong and mild dehydrating conditions are suggested, based on the presence of the dehydrated surface layer and on the formation of the intermediate dihydrate. Kinetic analysis of the constant-temperature thermal dehydration responses gave a good fit to the data of the pentahydrate-to-dihydrate transformation by a contracting circle equation, while for the dihydrate-to-anhydrous reaction an n= 2 Avrami–Erofeev law was obeyed. The sensitivity of the transformation to the defect content of the crystals and to the changes in N2 flow and temperature prevented the evaluation of reliable Arrhenius parameters, particularly for the first dehydration stage. However, this provides further support for the surface-layer mechanism proposed here, which is reputed to apply more generally to other dehydration reactions in place of the preceding nucleation-and-growth model.