THE TEMPERATURE-DEPENDENCE OF STABILITY-CONSTANTS FOR THE FORMATION OF POLYNUCLEAR CATIONIC COMPLEXES

The temperature dependence of the formation of polynuclear cationic co mplexes is analyzed on the basis of a simple electrostatic model of el ectrolytic dissociation proposed by RYZHENKO and BRYZGALIN. it is poss ible to expand this theory to the case of polynuclear cationic complex es, considering the formation of such complexes as a result of two com peting processes: a nonelectrostatic attraction and an electrostatic r epulsion of the spheres consisting of mononuclear ''building blocks.'' The relative stability of polynuclear complexes decreases with temper ature because of the large increase of electrostatic repulsion due to the decrease of the dielectric constant of water. The predictive prope rties of the model is exemplified by the system Al+3-OH-, where it is possible to describe the temperature dependence of stability constants for both mononuclear and polynuclear complexes up to 200-250 degrees C at saturation water vapor pressure using the values of log K degrees (298, 1) and only one fitting parameter alpha, which has the same valu e for all complexes in the system Al+3-OH- (alpha is close to the meta l-hydroxide/oxide bond distance). Additional experimental data for the formation of polynuclear hydroxide complexes of divalent Mg, Co, Ni, Pb, and UO22+ confirmed the predictive properties of the model. The pr oposed model may be used to estimate entropies of hydrolysis reactions . These turn out to follow the empirical correlations previously estab lished by BAES and MESMER (1976) very well.