Spectroscopic and scattering investigation of isopoly-molybdate and tungstate solutions

Abstract

Detailed spectroscopic and scattering investigations of isopoly-molybdate and -tungstate solutions as a function of concentration and pH were made. From scattering results the apparent molecular weights were determined as a function of time and concentration. We find that the result agree with the aggregation scheme of simple --> hepta --> octa --> [Mo36O112]-8 --> protonated polymeric species, for molybdate solutions. Tungstate solutions aggregate according to simple --> Y-polytungstate --> paratungstate-A --> paratungstate-B --> [Psi]-metatungstate. The molybdate solutions exhibited very rapid equilibration, but the tungstate solutions required several days to reach equilibrium. From the discrete changes in the Raman spectra of both systems we find that the formation of isopoly anions is not a continuous process and that only certain species are present in solution. Our results do not rule out the formation of significant quantities of the octamolybdate anion as suggested by previous investigators.