Aluminum(III)-Citrate Complexes: a Potentiometric and 13C N.M.R.-Study

The equilibrium reactions of citrate ion with protons and aluminium(III) have been studied by potentiometric titration in aqueous solution, I 0.10 M KCl, 25°C. The protonation constants for citrate, log Ki, corrected for K+-citrate ion pairing, were determined as 5.90±0.02, 4.35±0.01 and 2.91±0.02, i = 1-3 respectively. The model which gave the best fit to experimental data for the aluminium-citrate equilibrium system (TL/TM > 5.3) included AlHL + ( logβ 11.02±0.02), AlL (8.35±0.23), AlL23- (13.40±0.35), Al(HL)L2- (17.36±0.11), AlL2H-14-[ pK (AlL23-)6.07±0.24] and Al(LH-1)25- [ pK (AlL2H-14-) 7.09±0.08]. 13C n.m.r.data established that ligand exchange in the aluminium(III) complexes is slow on the n.m.r . time scale. The spectra were consistent with deprotonation of the coordinated C(2)-OH group commencing between pH 5 and 6, corresponding to formation of the species designated AlL2H-14- and Al(LH-1)25-. The broad structure of the- CH2-, -COOH (central) and -COOH (terminal) resonances for coordinated citrate was indicative of coordination isomers and/or the presence of both mono and bis complexes.