Equilibria of polyoxometalates in aqueous solution |
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Authors: | Lage Pettersson |
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Affiliation: | (1) Department of Inorganic Chemistry, University of Umeå, S - 901 87 Umeå, Sweden |
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Abstract: | In most aqueous polyoxometalate systems, numerous, often highly negatively charged species, are formed. To establish the speciation in such complex polyanion systems, many experimental methods and techniques must be used. Moreover, it is of vital importance that the experimental data are of the highest accuracy and that the data collected from different methods are treated simultaneously with an appropriate computer program. In systems containing one or more sensitive NMR nuclei, a combined EMF-NMR method has been shown to be extremely powerful.This article firstly gives some general comments on equilibria in aqueous polyoxometalate systems including ionic medium effects, and then describes an equilibrium EMF-NMR (31P and51V) study of the five component system H+-Mo(VI)-V(V)-P(V)-e–. The study has been focused on so-called Keggin ratio solutions, Mo+V):P=12:1, since these are commonly used in selective oxidation processes. Special attention has been given to Mo10V2P and Mo9V3P solutions, where positional isomers occur. We have been able to identify and characterize all the five possible isomers of -Mo10V2PO405– at 25 and 90 °C. Besides the results from the five component system, some interesting findings from the binary, ternary and quarternary sybsystems are also reported. |
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Keywords: | Polyoxometalate equilibria potentiometry NMR(31P,51V) redox ESR molybdate vanadate |
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