Diffusion coefficients of tungsten heteropolyacids

Controlled hydrodynamics at rotating platinum and amalgamated copper electrodes were used to measure diffusion coefficients for 12-phosphotungstic and polyphosphotungstic acids in 1 M phosphoric acid, D_112-PTA=2.48 · 10^-6; D_polymer= 2.68 · 10^-7 cm²/sec, assuming only 1 tungsten atom in 12 is reduced at each diffusing entity. The phosphotungstic acids in phosphoric acid probably exist as polymers on initial formation, not greater than tetramers, rearranging to give monomers from calculations based on one reduced tungsten atom to each complex ion. Diffusion parameters of the phosphotungstic acids are compared with diffusion results of tungsten compounds from the literature obtained with dropping mercury electrodes. In all instances, low diffusion values are obtained indicating large diffusing entities.