Multicomponent polyanions, 49 - A potentiometric and (P-31, V-51) NMR study of the aqueous molybdovanadophosphate system

The equilibrium speciation of the quaternary system H+/MoO42-/HVO42-/HPO42- in aqueous 0.600 M Na(Cl) at 25 degrees C and 90 degrees C was studied by pH potentiometry and P-31 (202.5 MHz) and V-51 (131.6 MHz) NMR spectrometry . The study focused on solutions containing the oxoanion components in the ratios (Mo + V)/P = 12:1 and Mo/V greater than or equal to 3.8, in the pH r ange 0-5, wherein Keggin anions of the formula [HzMo12-xVxPO40]((3+x-z)-) w ith x = 1-3 are the predominant species. Formation constants (log beta) and pK(a) values for Keggin anions with x = 1-3 were determined from P-31 and V-51 resonance chemical shift data, P-31 resonance intensity data, and pH p otentiometric data with the multidata least squares calculation program LAK E. The [Mo11VPO40](4-) anion does not protonate, [Mo10V2PO40](5-) anions ca n be monoprotonated, and [Mo9V3PO40](6-) anions can be triprotonated. The s ix most intense P-31 NMR resonances of [HzMo9V3PO40]((6-z)-) could be follo wed over the pH range studied. It was also possible to assign some P-31 res onances to a specific isomer of [Mo9V3PO40](6-).