Abstract
The relaxation spectra of aqueous molybdate has been determined by temperature jump at 25° and 1.0 M ionic strength. The pH ranged from 5.50 to 6.75; monomer concentration (i.e., NazMoCh concentration) from 0.01 to 0.25 M. The observed spectrum consisted of one or two relaxation effects, depending on pH and concentration. The longer, more concentration- dependent effect (1-180 msec) is most sensitive to heptamer formation; formally, 7MoC>42~ + 8H+ fi MojO»*- + 4H20. The shorter, less concentration-dependent effect (200-500 Msec) is most sensitive to octamer formation; formally, M07O246"" + M0O42- + 4H + <± MosChe4- + 2H20. Rate constants for these two equilibria were varied until a set of constants yielding the best agreement between measured and observed relaxation time was found. The interpretation of these results depends heavily on the equilibrium constants used to describe the system. The best results were found with the equilibrium data of Aveston, Anacker, and Johnson.2 The rapidity of condensation to form the heptameric isopolymolybdate species is explained by a mechanism involving reactions of protonated monomeric molybdate, which is then assumed to have octa- hedral coordination. These species, presumably of composition OMo(OH)s- and Mo(OH)i, have the coordination appro- priate to units of the polymer structure. The dissociation rate constant for the octamer (yielding heptamer and monomer) is larger than that for the heptamer (complete breakup to monomer), reflecting the enhanced stability of the heptamer.