Equilibrium and structural studies of silicon(IV) and aluminium(III) in aqueous solution. 14. Speciation and equilibria in the aluminium(III)-lactic acid-OH- system

Abstract

Three-component equilibria between H+, aluminium (III) and lactic acid (CH3CH(OH)COOH, HL) were studied by means of potentiometric (glass electrode) titrations at 25 °C in an ionic medium of 0.6 M NaCI. The total concentrations of aluminium, B, and lactic acid, C, were varied within the limits 0.010 ≥ B ≥ 0.001 M and 0.035 ≥ C ≥ 0.0025 M and the ratios C/B between 30 ≥ C/B ≥ 1. Data with Zc ≤ 0.95 (where Zc is the average number of OH- reacted per C)were explained with the following species (defined according to the reaction pH+ + qAl3+ + rHL ⇔ Hp Alq (HL)p+3r; βpqr) and corresponding equilibrium constants: AIL2+, Igβ-111 = -1.21 ± 0.017; AIL2+, lgβ-212 = -2.72 ± 0.028; AIL3, lg β-313 = -4.92 ± 0.14; and Al(H-1L)L, lg β-312 = -6.17 ± 0.012. The standard deviations were 3σ(lg βpqr). The carboxylic acid dissociation constant HL ⇔ H+ + L- was studied in separate titrations and was found to be lgβ-101 = -3.572 ± 0.001. Data were analyzed with the least-squares computer program LETAGROPV-RID.