Abstract
The equilibrium speciation of the quaternary system H+/MoO42−/HVO42−/HPO42− in aqueous 0.600 M Na(Cl) at 25 °C and 90 °C was studied by pH potentiometry and 31P (202.5 MHz) and 51V (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 ⩾ 3.8, in the pH range 0−5, wherein Keggin anions of the formula [HzMo12−xVxPO40](3+x−z)− with x = 1−3 are the predominant species. Formation constants (log β) and pKa values for Keggin anions with x = 1−3 were determined from 31P and 51V resonance chemical shift data, 31P resonance intensity data, and pH potentiometric data with the multidata least squares calculation program LAKE. The [Mo11VPO40]4− anion does not protonate, [Mo10V2PO40]5− anions can be monoprotonated, and [Mo9V3PO40]6− anions can be triprotonated. The six most intense 31P NMR resonances of [HzMo9V3PO40](6 − z)− could be followed over the pH range studied. It was also possible to assign some 31P resonances to a specific isomer of [Mo9V3PO40]6−.
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