Equilibrium and Structural Studies of Silicon(IV) and Aluminium(III) in Aqueous Solution. 3. Potentiometric Study of Aluminium(III) Hydrolysis and Aluminium(III) Hydroxo Carbonates in 0.6 M Na(Cl).

Abstract

Equilibria between aluminium(III), pyrocatechol (1,2-dihydroxybenzene, H2L) and OH-were studied in 0.6 M Na(Cl) medium at 25°C. The measurements were performed as emf titrations (glass electrode) within the limits 1.5 ≤ - log[H+] ≤ 9; 0.0005 ≤ B ≤ 0.015 M; 0.006 ≤ C ≤ 0.03 M and 2 ≤ C/B ≤ 30 (B and C stand for the total concentrations of aluminium(III) and pyrocatechol respectively). All data can be explained with a main series of complexes: A1L+, log β-2,1,1= - 6.337 ± 0.005; A1L2-, log β-4,1,2= -15.44 ± 0.017 and A1L33-, log β-6,1,3= - 28.62 ± 0.024 together with two minor species: Al(OH)L22-, log β-5,1,2= - 23.45 ± 0.079 and Al3(OH)3L3, log β-9,3,3= - 29.91 ± 0.066. Of the two, the latter probably is a type of average composition complex principally occurring at low C/B quotients. The first acidity constant for pyrocatechol as determined in separate experiments is log β-1,0,1= - 9.198 ± 0.001. The standard deviations given are 3σ(log β p,q,r). Data were analyzed with the least squares computer program LETAGROPVRID. In a model calculation using kaolinite as solid phase, we compared the complexation ability of this system with that of the system Al3+-OH--salicylic acid, reported earlier in this series. © 1983.