Surface ionization of polynuclear species in Al(III) hydrolysis—II. A conditional equilibrium model

Abstract

Base and acid titrations of aluminum salt solutions were modeled by including in the reaction scheme the titration of protons from ionizable surface (or “edge”) sites on polynuclear species and the charged precipitate. It was assumed that the OH/Al ratio of the polynuclear species consists of a structural part, the hydroxide ions that link the Al ions and a more labile surface site titration part. The hydrolysis products included in the best-fit reaction scheme are two monomeric species, Al(OH) 2+ and Al(OH) − 4, and two polynuclear species, Al 14(OH) 10+ 32 and Al p (OH)( 3 p− q)+ q where ( q/ p) ≈ 2.7. The precipitate is assumed to consist of aggregates of the larger polynuclear species destabilized by surface site deprotonation. The conditional, infinite dilution equilibrium constants determined by fitting experimental base titration data are: pK 1.1 = 4.4, pK 14.32 = 103, pK sp = − 10.9 and pK 1.4 = 24.