ON THE THERMODYNAMICS OF HIGHER SOLID NICKEL OXOCOMPOUNDS

On the basis of the qualitative reevaluation [1] of critically selecte d data from various authors on the equilibrium or rest potentials of s olid nickel oxocompounds it was concluded that the so called ''alpha-N i(OH)2/gamma-NiOOH'' systems form sufficiently stable solid solutions of Ni(II) and Ni(IV) components in the whole studied range of the oxid ation number, z(Ni) = 2.0-3.6, without any participation of Ni(III) co mpounds. A quantitative reevaluation was based on the model of an unin terrupted 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 i n the entire range Of z(Ni) was achieved using empirical Margules' rel ation of the concentration dependence of activity coefficients of indi vidual components of the solid redox systems. The correlation led to t he 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 a nd 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 ou t using a corrected value for DELTAG(f)0(Ni(OH)2) = -457.7 kJ mol-1 le ading to DELTAG(f)0(Ni2+, aq) = -45.03 kJ mol-1 and E(Ni2+/Ni)0 = -0.2 334 V at 25-degrees-C.