Equilibrium and structural studies of silicon(IV) and aluminium(III) in aqueous solution. Part 22. A potentiometric study of mono- and poly-nuclear aluminium(III) tartrates

Abstract

Equilibria between H⁺, aluminium(III), and L-tartaric acid (2,3-dihydroxybutanedioic acid, H₂L) were studied in a 0.6 mol dm⁻³ Na(Cl) medium at 25 °C. The measurements were performed as e.m.f. titrations (glass electrode) within the limits 1.8 ⩽ –log[H⁺] ⩽ 6.7; 0.000 18 ⩽ B ⩽ 0.012 mol dm⁻³, 0.000 18 ⩽ C ⩽ 0.042 mol dm⁻³, and 0.5 ⩽ C/B ⩽ 25 [B and C stand for the total concentrations of aluminium(III) and tartaric acid, respectively]. All data can be explained with a mononuclear species [AlL]⁺ (log β_−2,1,1 = −3.44 ± 0.047) together with a series of binuclear complexes [Al₂L(H₋₁L)]⁺ (log β_−5,2,2 = −6.30 ± 0.040), [Al₂(H₋₁L)₂] (log β_−6,2,2 = −8.91 ± 0.016), [Al₂(H₋₁L)(H₋₂L)]⁻ (log β_−7,2,2 = −13.12 ± 0.028), and [Al₂(H₋₂L)₂]²⁻ (log β_−8,2,2 = −18.95 ± 0.19). The formulae given are tentative. The acidity constants of tartaric acid were studied in separate titrations and were found to be log β_−1,0,1 = −2.702 ± 0.002 and log β_−2,0,1 = −6.401 + 0.002. All equilibrium constants are defined according to the general reaction pH⁺ + qAl³⁺ + rH₂L ⇌ H_pAl_q(H₂L)_r^p+3q and the uncertainties reported are 3σ(log β_p,q,r). Data were analysed with the least-squares computer program LETAGROPVRID.