Thermal behaviour of hydrated dodecatungstosilicic acid

The thermal behaviour of H₄SiW₁₂O₄₀·24.8H₂O (SiW₁₂) was investigated by using DTA, TG and FTIR. Endothermic effects were observed at 40, 98 and 217°C, corresponding to the fusion of SiW₁₂ in its own crystallization water, boiling of the solution and decomposition of the remaining tetrahydrate into anhydrous SiW₁₂, respectively. The mass of the sample remained constant on heating from about 250 to 400°C. Subsequently, it slowly decreased and reached a constant value at about 500°C. At 526°C a DTA peak appeared. There was an abrupt change in the FTIR spectrum of the sample heated to 550°C. The typical spectrum of the Keggin unit vanished and new bands at 807.5 and 1030 cm^?1 indicated the presence of free WO₃ and SiO₂, respectively.