Abstract
1. The mechanism of dehydration and the structure of hydrates with a differing degree of dehydration of 12-silicotungstic and 12-phosphotungstic acids are the same. The mechanism of dehydration of 12-phosphomoiybdic acid differs from that of 12-phospho- and 12-silicotungstic acids. The valence state of molybdenum in 12-phosphomolybdic acid does not affect the structure of the 12-phosphomolybdat anion in crystals of the composition H3PMo12O40 · 29H2O or the structure of hydrates with a differing degree of dehydration. 2. The formation of a plateau on the dehydration curves of 12-phosphotungstic and 12-silicotungstic abids may be due to the selective layer removal of water in each cluster of 29 water molecules. 3. When 12-phosphotungstic and 12-silicotungstic acids are heated at temperatures of 700–900° and 400–850° isomorphic compounds are formed with a tetragonal lattice.
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