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

Abstract

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 K_i, 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 (T_L/T_M > 5.3) included AlHL ⁺ ( logβ 11.02±0.02), AlL (8.35±0.23), AlL₂^3- (13.40±0.35), Al(HL)L^2- (17.36±0.11), AlL₂H_-1^4-[ pK (AlL₂^3-)6.07±0.24] and Al(LH_-1)₂^5- [ pK (AlL₂H_-1^4-) 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_-1^4- and Al(LH_-1)₂^5-. The broad structure of the- CH_2-, -COOH (central) and -COOH (terminal) resonances for coordinated citrate was indicative of coordination isomers and/or the presence of both mono and bis complexes.