Equilibrium studies of metal-ion complexes of interest to natural waters—III: Simple and mixed dl- and meso-tartrate complexes of Al(III), In(III) and Pb(II)

A pH-titration method has been used to determine stability constants for simple and mixed dl- and meso-tartrate complexes of Al3+, In3+ and Pb2+, at 25°C and μ = 0·1M(NaClO4). In all systems the mixed complex is more stable than expected on statistical grounds. Values of log KDXY, where KDXY is the equilibrium constant for the disproportionation reaction MX2 + MY2 ⇄ 2MXY, for Al3+ and In3+, are 4·16 and 4·12 respectively, compared to the statistical value of 0·6. A comparable value for Pb system could not be computed as Pb forms only 1:1 complexes in simple systems. However, log K for the reaction MX + MY ⇄ MXY + M is 2·41. The factors contributing to the enhanced stabilities of the mixed complexes are discussed.