Properties and analytical applications of the heteropolymolybdates of phosphorus, arsenic, silicon and germanium-II Modifications of the heteropoly acids.
Phospho-, arseno-, silico- and germanomolybdic acids may all exist in an alpha- and a beta-modification. alpha-Silicomolybdic acid is formed if the degree of acidification is below [H(+)]/[Mo] = 1.5. At higher acid concentrations the beta-modification is formed, and spontaneously transformed into the alpha-form. Germanomolybdic acid behaves similarly, but the transformation is much faster. Arsenomolybdic acid is always formed as the alpha-modification and is transformed into the beta-modification only later and in a very slow reaction if the pH of the solution is below 1. Formation of the two modifications is also dependent on acid concentration in the case of phosphomolybdic acid, but both modifications are stable and no spontaneous transformation occurs. Compounds similar to the beta-modifications but showing different molar absorptivities are formed in acetone-containing solution.
Halász
A
A
Institute for Analytical Chemistry, University of Chemical Engineering, Veszprém, Hungary.
Pungor
E
E