Vanadium potentiometry

V NMR spectroscopy is a particularly useful method for solution studies of vanadium(V) complexes. Several correlations of the shielding in vanadium(V) complexes have been reported and have been used as empirical guides to the coordination number of the vanadium.57-64 Application of these correlations should, however, be used cautiously. 170 NMR spectroscopy65 and extended X-ray absorption fine structure (EXAFS) spectroscopy have been used for the characterization of solution complexes.66 Methods including potentiometry, X-ray diffraction, and UV-visible and IR spectroscopic techniques are also widely used in the characterization of the properties of vanadium complexes. Other methods for the characterization of low-level (sub-microgram) vanadium complexes have been described 67,68... 

Table 2.5 provides an overview of the concentrations of important components of human blood, important in the sense that they constitute potential ligands for vanadium. The concentrations of lactate (1.5 mM) and citrate (0.1 mM) are comparatively high. Along with phosphate (1.2 mM), they are the major potential low molecular weight llgators for vanadate and vanadyl. The system containing vanadate(V) and lactate (lac) has been extensively smdled,[ °] and the species Identified on the basis of NMR and H+ potentiometry Include mono-, di-, trl- and tetranuclear forms, for which the structures... 

We have performed a study of aeidie properties of boron (HI) and vanadium (V) oxo-compounds in molten Nal at 700"C (see Table 10.4.1). A eomparative study of strength of Lux bases OH", C03 , S04 was determined " " by potentiometrie titration using sodium pyrophosphate as acid. Two moles of the first base may be neutralized by 1 mole of P407 , while two other moles react with the acid in ratio 1 1. On the base of e.m.f. drop magnitude at the equivalence point, the bases have been arranged in sequence 0H>C03 >S04 of basicity decrease. The equilibrium constants were not estimated. "" " ... 

The results of the linearization of the polarization curves using Equation 4.5.2 for current densities corresponding to the range of concentrations in which Henry s law is fulfilled [16] are presented in Table 4.5.1. One can see that the value of n (number of electrons involved in the electrode process) is below two. Based on the conventional electrochemical analysis this indicates that vanadium is partially dissolved following a one-electron reaction. This however contradicts our data obtained previously by coulometry, oxidimetry, and potentiometry [16]. 

There is some conjecture over whether trimeric or tetrameric vanadium(V) species form and, indeed, whether both may form simultaneously. Cruywagen, Heyns and Visagie (1989) studied the hydrolysis of vanadium(V) using both potentiometry and spectrophotometry. Their results indicated that a model with trimers gave a slightly better fit than did that with tetramers however, only the model with the tetramers could be used to fit the spectrophotometric... 

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