Equilibrium of the protonic species in hydrates of some heteropolyacids at elevated temperatures

Abstract Hydrates of three heteropolyacids: 12-molybdophosphoric H 3 PMo 12 O 40 · n H 2 O (MoPA· n H 2 O), 12-tungstophosphoric H 3 PW 12 O 40 · n H 2 O (WPA· n H 2 O) and 12-tungstosilisilic acid H 4 SiW 12 O 40 · n H 2 O (WSiA· n H 2 O) were examined by using thermal analysis, infrared spectroscopy and impedance measurements. These compounds are known as superionic proton conductors and their behaviour was examined as a function of temperature. At the TGA and DSC curves, in the 300–500 K temperature range, transformations were observed at several temperatures. These transformations are partially due to the dehydration process and partially to the change of equilibrium of protonic species (H 3 O + , H 2 O and OH - and their possible association: H 5 O + 2 , H 7 O + 3 , ...), as shown by thermal analysis and infrared spectroscopy.