Aqueous nickel diphosphato complexes

The complexes of the highly charged pyrophosphate ion with nickel are generally stronger than the phosphate complexes, but interpretation of the experiments is beset by the same difficulties as the interpretation of the phosphate studies with respect to unambiguous identification of the species. In the literature, results of studies of association of Ni with pyrophosphate have been discussed in terms of the following equilibria  

The results of Vaid and Rama Char [58VA1/RAM] were obtained under ill-defined conditions, and are in poor agreement with results from the other studies. The equilibrium constant of Yatsimirskii and Vasilev for Reaction (V.98) is consistent with other values, but was obtained at moderately high and variable ionic strength. Values from the measurements of Hammes and Morrell [64HAM/MOR] in 0.1 M Me4NCl(aq) for Reactions (V.98) and (V.99), after correction to / = 0, are log (V.98) = (8.73 0.25) and log,o K° (V.99) = (5.14 0.25), at 298.15 K. 

There appears to be only one reported value [56YAT/VAS] for the cumulative second association constant (Reaction (V.lOO)), 10.50) x lo . As discussed 

The value for Reaction (V.98) is not inconsistent with the rough calorimetric value reported by Yatsimirskii and Vasilev (see discussion of [56YAT/VAS] in Appendix A). No value is selected for the enthalpy of Reaction (V.lOO). 

It must be emphasised that the values selected in this review for formation of NiHPjO, and NiPjO, should not be used for solutions more than 0.01 M in alkali metal ions unless explicit values are introduced for the pyrophosphate-alkali metal ion association constants (the values of De Stefano et al. [94STE/FOT] may be useful, cf. Appendix A for that reference). 

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