MULTICOMPONENT POLYANIONS .47. THE AQUEOUS VANADOPHOSPHATE SYSTEM

Equilibrium and metastable speciations in the aqueous vanadophosphate system in 0.6 M Na(Cl) medium at 25-degrees-C have been characterized by pH-potentiometry and high-field P-31 and V-51 NMR spectrometry. Equ ilibrations are slow at 25-degrees-C, requiring months to years to rea ch equilibrium. The simultaneous analysis of [H+]-emf data with P-31 a nd V-51 NMR shift and integral data was required to establish the spec iation in aged, equilibrated solutions. Vanadophosphate species exist at equilibrium in solutions at 1.3 < pH < 4.0 with centimolar to decim olar vanadium and have the formula [HxPV14O42](9-x)-. The predominant equilibrium species is [H4PV14O42]5- (log beta = 94.84 +/- 0.07 (3sigm a) from H+, H2VO4-, and H2PO4-). Its conjugate acid, [H5PV14O42]4- (lo g beta = 96.41 +/- 0.09), and its conjugate base, [H3PV14O42]6- (log b eta = 90.7 +/- 0.6), are minor species at equilibrium, especially the latter. Fresh solutions of dissolved Na5[H4PV14O42] salt were examined to rind metastable species. Rapid pH-titrations and P-31 and V-51 NMR shift data over 0 < pH < 8.5 established [HxPV14O42](9-x)- species ha ving x = 1-6, and their pK(a) values for x = 2-6. All observed [HxPV14 O42](9-x)- Species exhibited solution NMR spectra corresponding to the trans-bicapped Keggin structure. Transient, metastable [HxPV13O41](12 -x)- species Were observed in fresh solutions of Na5[H4PV14O42] salt d issolved at pH < approximately 1.3, during its decomposition to H3PO4 and VO2+. The pK(a) values of H3PO4, H2PO4-, and HPO42- in the 0.6 M N a(Cl) medium at 25-degrees-C were determined to be 1.772, 6.418, and 1 1.232, respectively.