Properties and analytical applications of the heteropolymolybdates of phosphorus, arsenic, silicon and germanium-I Spectrophotometric examination of the heteropoly acids.
Spectrophotometric studies have been made of the heteropoly acids of phosphorus, arsenic, silicon and germanium, formed at molybdate concentrations of 10(-1)-10(-3)M. It was found that the heteropoly anion is quantitatively produced at a Z-value of 1.60 in the case of a quinquevalent central atom and 1.50 for a quadrivalent central atom, if the amount of the latter is not higher than 50% of the stoichiometric (Z is the number of moles of protons taken up by one mole of molybdenum.) On addition of further acid the anion takes up protons, but its absorbance remains unchanged if measured at a wavelength at which the isopolymolybdates show no absorption. At Z-values higher than 2.2-2.3, i.e., in highly acidic media, a decomposition reaction starts; its products are octamolybdate and molybdenyl ions. The different molar absorptivities and other "irregularities" found in the various analytical procedures in the literature are due not to the heteropoly acids but to the isopolymolybdate formed together with the former.
Halász
A
A
Institute for Analytical Chemistry, University for Chemical Engineering, Veszprém, Hungary.
Pungor
E
E