The Stability Constants of Ethylenediaminetetraacetato, Trimethylenediaminetetraacetato and Propylenediaminetetraacetato Complexes of Some Divalent Metal Ions

Two methods are presented for the determination of the stability constants of ethylenediaminetetraacetato, trimethylenediaminetetraacetato and propylenediaminetetraacetato complexes of some divalent metal ions. One is a new method which is based on the polarographic measurement of the concentration of an uncomplexed ligand in a solution containing two kinds of ligands and an appropriate metal ion. The other is a modification of the method described in the previous paper. From the results obtained by these methods and those obtained previously, the individual stability constants have been calculated. The values of the logarithmic stability constants of complexes at 25°C and ionic strength 0.2 are: Mn 13.64, Co 15.71, Ni 18.12, Cu 18.46, Zn 15.94, Cd 15.98 and Pb 17.76 for ethylenediaminetetraacetato complexes; Mn <10.8, Co 14.48, Ni 17.29, Cu 18.04, Zn 14.26, Cd 12.69 and Pb 13.04 for trimethylenediaminetetraacetato complexes, and Mn 14.85, Co 17.07, Ni 19.42, Cu 19.64, Zn 17.14, Cd 17.43 and Pb 18.69 for propylenediaminetetraacetato complexes.