Solution Studies of Cd(II) and Gd(III) Involving Malonic Acid and Some Catecholamines

The solution equilibria of mixed ligand complexes of Cd (II) and Gd (III) with primary ligand A [A = malonic acid] and secondary ligand B [B = Tyrosine, Dopa (3, 4 dihyroxyphenylalanine), Dopm (Dopamine)] having (111) mol ratio in aqueous solution is investigated potentiometrically at different temperatures (20C1, 30C1 and 40C1) and at three ionic strengths (=0.05M, 0.10M and 0.15M). Equilibrium analysis shows that the ternary complexes are formed in stepwise manner. Stability constants of the complexes have been determined through the method suggested by Chaberek and Martell as modified by Dey et al. and refined by SCOGS (stability constant of generalized species) computer program. Speciations of various species formed in different equilibria are obtained. The thermodynamic parameters, such as free energy change G, enthalpy change H and entropy change S are obtained by Van-t Hoff equation. The value of G and H are negative whereas S is positive. The relative stability of ternary complexes as compared to binary species is expressed in term of logK. Values of logK are positive in most of the systems which suggest that the formation of ternary complexes is favorable. However in few cases logK is found to be negative. The stability of Gd (III) complexes are found to be greater than that of Cd (II) complexes.