Vanadium monomeric hydrolysis specie

Vanadium(II) is unstable in water (Baes and Mesmer, 1976) being oxidised to vanadium(III), the reaction being accompanied by the production of hydrogen gas. However, hydrogen can be utilised to maintain the vanadium(II) oxidation state in solution. Data are only available for the first monomeric hydrolysis species ofvana-dium(II), VOH", which forms according to reaction (2.5) (M = p = l, q=l). 

The formation of vanadium(III) hydrolysis species is described by reaction (2.5) (M = Evidence has been given in the literature that vanadium(III) forms the species, VOH +, V(OH)2 and V2(OH)2 . Although it is probable that higher monomeric hydrolysis species would form, their detection is likely hindered by oxidation of vanadium(III) to higher oxidation states in the pH region where these species would exist. As is the case with some other trivalent transition metal cations, the formation of V3(OH)4 may also occur but no data for this species have been reported. 

There are less data available for the stability constant of V2(OH)2 than were available for those of the monomeric vanadium(III) hydrolysis species, but nevertheless, the majority of the data still come from the work of Pajdowski and co-workers (see Table 11.4). As was the case with V(OH)2, the data have been acquired in chloride media across the temperature range of 20-25 0. It is believed that utilisation of these data without correction will not significantly impact calculations for obtaining the stability constant at zero ionic strength since the change in each constant is likely to be within the uncertainty assigned to each constant. 

The determination of the maj ority of the stability constants for the vanadium( V) hydrolysis species has not been related to reaction (2.5) (M = V02 ). This is because other reactions better facilitate the calculation of the relevant stability constants at zero ionic strength, as is illustrated in the following. For consistency with data given for other cations. Table 11.9 contains the stability constants for all vanadium(V) species that relate to reaction (2.5) (M=V02" ). To undertake these calculations, the derived stability constants at zero ionic strength have been combined with the relevant stability constant of a monomeric vanadium) V) species (also given in the following) and that for water, as given in Chapter 5. 

The stability constants for the formation of the monomeric hydrolysis species of vanadium(V) are given in Table 11.11. The table contains data for the reaction of V02" with water to form the species V02(0H)(aq) (or HVOjCaq)), V02(OH)2-(or VO3-), V02(0H)32- (or HVO -) and V02(0H)/- (or VO/-). The data from Borgen, Mahmoud and Skauvik (1977) or Schiller and Thilo (1961) are not included in Table 11.11 as the complete experimental conditions used are not clear in either study. Moreover, the stability constants derived in these studies appear to be inconsistent with those of other studies. 

Data for the stability constants of monomeric vanadium(V) hydrolysis species (reactions defined earlier). 

Hydrolysis species that form for chromium(III) are similar to those that have been postulated for titanium(III) and vanadium(III), with hydrolysis being dominated by the formation of CrOH and Cr2(OH)2 - In addition, data are also available for the second monomeric hydrolysis species Cr(OH)2 and a higher polymeric species, CrjfOH), has ako been postulated. Moreover, data for the higher monomeric species, Cr(OH)3(aq) and Cr(OH)4, are ako available. Data that have been given in the hterature for CrOH are hsted in Table 11.15. 

In acidic solution, vanadium(V) exists as the univalent cation, V02. The hydrolytic behaviour of this ion is quite complex, forming a large number of monomeric and polymeric hydrolysis species. In this review, these species are related to reaction (2.5) (M = V02 ). Species with 1, 2, 4, 5 and 10 V02" molecules are believed to form, although other species have been reported. Once the univalent cation is hydrolysed as V020H(aq), or equivalently HjVOjfaq), the chemistry it displays is quite similar to that of the phosphate anion, in terms of its ability to form polymeric species and the strength of the various steps of dissociation of phosphoric acid. 

The hydrolysis of pentavalent vanadium, V(V), is very complex. Besides monomeric species, a variety of polyoxovanadate species with the nuclearities 2-6 and 10 are known to form in equilibrated solutions. The charges vary from +1 to -6. As a consequence the speciation is strongly dependent both on the total concentration of V, Vtm, and on the ionic... 

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