Potentiometric studies on the formation equilibria of binary and ternary complexes of some metal ions with dipicolinic acid and amino acids

The binary and ternary complexes of Mg2+, Ca2+, Sr2+, Ba2+, Co2+, Ni2+, Mn2+, and Cd2+ metal ions with dipicolinic acid as a primary ligand and some selected mono- and dicarboxylic amino acids as biologically relevant secondary ligands were studied in solution using a potentiometric technique. The acidity constants of the ligands selected were determined potentiometrically and used for determining the stability constants of the complexes formed in aqueous solution at 25 degreesC and ionic strength I = 0.10 mol dm(-3) (NaNO3). The lower stability of 1:2 binary complexes of amino acids compared to that of the corresponding 1:1 systems is in accord with statistical considerations. The formation of ternary complexes was found to take place in a stepwise manner. The order of stability of the binary and ternary complexes in terms of the nature of the amino acid is investigated and discussed. The values of Delta log K for ternary systems involving amino acids have been evaluated and discussed.