The thermodynamics of solid solutions of Ni(II)/Ni(IV) oxo compounds

The measured data of equilibrium and rest potentials of higher nickel oxo compounds, E' (converted for 25 degrees C and a(H2O) = a(KOH) = a(H2) = 1.0) selected in the range of the overall oxidation number of 2.0 < z(Ni) less than or equal to 3.6, were newly evaluated with the aid of a simplified model of the quasi-binary solid solutions Ni(OH)2/Ni(OH), assuming unlimited miscibility. It was found that the dependence of E' on the composition of the redox system can be described by a Nernst equation in the entire range of z(Ni) investigated, where the activity coefficients of individual components are expressed by relations for regular solutions. The so calculated activities of considered components showed positive deviations from the ideal behaviour of the solid solutions. All results obtained were discussed in more detail assuming the non-existence of Ni(III) oxo compounds in such solid solutions.