Vanadium aqua ions

The occurrence of vanadium in the lower oxidation states, which as the simple aqua ions undergo acid dissociation above pH 3 [if present as V(III)) ] and pH 6 [in the case of oxo-V(IV) ], along with the high sulfur content of ascidian blood and the low pH that results when ascidian blood cells are ruptured in distilled water has led to the belief that intact vanadium-containing tunicate blood cells are acidic (145). Other lines of evidence, including vital staining and NMR (144,170), and... 

It is not always realized that only five M(IV) aqua ions are known to occur in acidic solution, colorless thorium(IV) being the most reluctant to deprotonate, with a pK close to 4 like Be(OH2)i and Al(OH2)6 whereas U(IV) and Pu(IV) aqua ions both have pK close to l.S (Kraus and Nelson 1950). Similar pK values are likely to occur for Np(lV) and Pa(IV) aqua ions. Vanadium(IV) forms the blue vanadyl ion without perceptible proton affinity in aqueous solution, which seems to... 

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