Polarographic determination of the stability constants of metal complexes based on the shifts in half-wave or peak potentials of the anodic oxidation of mercury: methylthioacetato- and 2,2′-thiobisacetato complexes

Abstract

The stability constants of metal-ion complexes, other than those of mercury ions, can be determined from the shifts, caused by an excess of that metal ion, in the anodic waves of mercury oxidation in the presence of the ligand. Let a solution contain L, a ligand that can react with mercury ion from the anodic oxidation of mercury to form the complexes HgL p . The half-wave potential of the anodic wave will be shifted by the presence of an excess of a metal ion M, which can also react with L forming the complexes M j L (where j = 1, 1 2 ,…1/ N). A general expression is derived relating the half-wave potential shift Δ E 1 2 to the overall stability constants. If the excess of metal ion M is great enough for the 1:1 complex to predominate in solution (and if HgL 2 is the predominating mercury complex), for reversible, diffusion-controlled processes the general equation can be simplified to: ▪ where c M is the total concentration of metal ion M. This equation provides an easy and fast method for β 1 determination. The simplified equation is best suited for experimental data obtained from techniques such as DP, AC 1 and AC 2 polarography, whose increased precision in Δ E measurement enables poorly-developed dc-polarographic anodic waves to be used for β 1 determination. Since the metal-to-ligand concentration ratio must be large in order to apply the simplified equation, the determination can be carried out at very small ligand concentrations. This fact renders the new method especially useful when the ligand solubility does not allow the high ligand concentrations needed in the DeFord—Hume method to be reached. The adequacy of the deduced equations when applied to polarographic processes which are irreversible or not controlled by diffusion is discussed. The simplified equation is tested using several metal complexes of methylthioacetate and 2,2′-thiobisacetate ligands and comparing some of the values obtained from experimentally measured Δ E's with known β 1 values taken from the literature. Good agreement is found. For the U(VI)—methylthioacetate complex, which has not been previously reported in the literature, log β 1 = 1.75 ± 0.1.