ON THE THERMODYNAMICS OF HIGHER SOLID NICKEL OXOCOMPOUNDS

On the basis of the qualitative reevaluation [1] of critically selected data from various authors on the equilibrium or rest potentials of solid nickel oxocompounds it was concluded that the so called ''alpha-Ni(OH)2/gamma-NiOOH'' systems form sufficiently stable solid solutions of Ni(II) and Ni(IV) components in the whole studied range of the oxidation number, z(Ni) = 2.0-3.6, without any participation of Ni(III) compounds. A quantitative reevaluation was based on the model of an uninterrupted series of solid solutions of completely undissociated Ni(OH)2, for NiO2.xH2O a complete dissociation into H+ and NiO2.(x-1)H2O.OH- was considered. A satisfactory correlation of all experimental data in the entire range Of z(Ni) was achieved using empirical Margules' relation of the concentration dependence of activity coefficients of individual components of the solid redox systems. The correlation led to the following standard data at 25-degrees-C: E(NiO2.xH2O/Ni(OH)2)0 = 1.3250 +/- 0.0167 V, DELTAG(f)0(NiO2 .xH2O) = -202.02-x.237.141 +/- 3.21 kJ.mol-1. With the aid of the empirical relation between DELTAH(f)0 and DELTAG(f)0 after Karapet'jants the following values were estimated: DELTAH(f)0(NiO2) = -257.4 +/- 5 kJ mol-1 and DELTAH(f)0(NiO2 .2H2O) = -844 +/- 5 kJ mol-1, S-degrees(NiO2) = 49.2 +/- 17 J mol-1 K-1 and S0(NiO2.2H2O) = 139 +/- 17 J mol-1 K-1. The calculations were carried out using a corrected value for DELTAG(f)0(Ni(OH)2) = -457.7 kJ mol-1 leading to DELTAG(f)0(Ni2+, aq) = -45.03 kJ mol-1 and E(Ni2+/Ni)0 = -0.2334 V at 25-degrees-C.