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

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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 ⇌ HpAlq(H2L)rp+3q and the uncertainties reported are 3σ(log βp,q,r). Data were analysed with the least-squares computer program LETAGROPVRID.