Equilibrium and structural studies of silicon(IV) and aluminium(III) in aqueous solution—XXIII. A potentiometric study of the composition and stability of aluminium complexes with nitrilotriacetic acid

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Equilibria between H +, aluminium(III) and nitrilotriacetic acid [N(CH 2COOH) 3, H 3L] were studied in 0.6 M Na(Cl) medium at 25°C. The measurements were performed as EMF titrations (glass electrode) within the limits 1.4 < −log [H +] < 9.4; 0.0006 < B < 0.04 M; 0.0007 < C < 0.04 M and 0.25 < C/B < 5 ( B and C stand for the total concentrations of aluminium(III) and nitrilotriacetic acid, respectively). All data can be explained with a series of mononuclear species AlL(H 2O) 0 2 (log β −3,1,1 = −1.853±0.007), AlL(OH)(H 2O) − (log βin−4,1,1) = −7.28±0.026) and AlL(OH) 2− 2 (log β −5,1,1 = −15.47±0.032) together with a dimer of AlL(OH)(H 2O) −, i.e. Al 2(OH) 2L 2− 2 (log β −8,2,2 = −12.74±0.038).(The formulae given are based on an assumed resemblance with species found in the solid state.) The acidity constants of nitrilotriacetic acid were studied in separate titra tions (0.003 < C < 0.02 M) and were found to be log β −1,0,1 = −1.74±0.023, log β −2,0,1 = −4.04±0.015 and log β −3,0,1 = −12.95±0.016. All equilibrium constants given are defined according to the general reaction pH + + a Al 3+ + rH 3L ⇌ H p Al q (H 3L) p+3 q r and the uncertainties reported are 3σ(log β p,q,r ). Data were analysed with the least-squares computer program LETAGROPVRID. In a series of calculations with the modelling program SOLGASWATER, the potential ability of the nitrilotriacetate ion to increase the solubility of gibbsite, is illustrated.