Relaxation spectra of molybdate polymers in aqueous solution. Temperature-jump studies

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 concentrationdependent 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 octahedral coordination. These species, presumably of composition OMo(OH)s-and Mo(OH)i, have the coordination appropriate 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.Upon addition of acid to alkaline molybdate solutions, isopolyanions are formed. Equilibrium is established between monomeric molybdate and, predominantly, heptameric molybdate polymers. Despite the relative complexity of these polymolybdate ions, the steps leading to their formation are complete within a few milliseconds at moderate concentrations.8Unlike rapid mixing studies, which are generally independent of any associated equilibrium constants, temperature-jump investigations usually depend on a sound knowledge of these constants for successful interpretation of the results. Thus, the first measurement on these systems, carried out in connection with a temperature-jump investigation of molybdenum (VI)tartaric acid complexatjon using optical rotation detection, yielded relaxation spectra indicative of polymerization, which were not, however, fully interpreted, due to a lack of precise knowledge of the species present.4 A temperature-jump study following further equilibrium acidity measurements on these systems confirmed the validity of the earlier kinetic measurements, and were in essential agreement with the stabil-

Electrometric investigations on the system acid-molybdate and the formation of heavy metal molybdates

“…The existence of Mo7O26 6-and Mo8O26 4-is in conformity with the results of the temperature-jump studies by Honing and Kustin 15 and the Raman spectra studies by Ozeki et al 16 Murata et al 17 , however, claimed that the octamolybdate underwent further reaction with acid (Z ≥ 1.7) to form Mo12O37 2-and Mo6O19 6-, but this observation could not be confirmed by our results.…”

Multiphasic Uptake of Sulfate by Barley Roots I. Effects of Analogues, Phosphate, and pH

“…In experiments at pH 3.0, monovalent molybdate caused few or no transitions and did not inhibit at low concentrations. Definite conclusions cannot be drawn, however, due to the formation of molybdate polymers (Honig and Kustin 1972). Perchlorate and periodate (10~* M) had no effect on uptake of sulfate from 10"^ M solutions.…”

Electrospray mass spectral study of isopolyoxomolybdates

“…There are significant differences between the component correlation coefficients and the overall correlation, which suggests that the addition polymerization begins from different seeds. For example, starting with an odd-seed the addition polymerization sequence is (10) whereas…”

“…However, there is uncertainty concerning the speciation and formulation of some isopolyoxomolybdate species in solution, since they are difficult to characterize under such conditions. [6][7][8][9][10][11][12][13][14][15][16] Several studies of alkali metal molybdates have confirmed that the monomolybdate [MoO 4 ] 2Ϫ has a tetrahedral structure in both the solid state and in solution. 17, 18 On addition of acid, [MoO 4 ] 2Ϫ is protonated to yield [HMoO 4 ] Ϫ and subsequently H 2 MoO 4 .…”