No abstract

A new polarographic procedure has been developed for the study of metal-ligand equilibria at pH < 2 by applying an insitu monitoring of the diffusion junction potential by introducing a non-complexing witness metal ion, Tl(I). As a case study, this procedure was applied to the Cd(II)-picolinic acid (pyridine-2-carboxylic acid) system; DC TAST polarography was employed at 25°C and ionic strength of 0.25-0.5 M (H,Na)NO 3 . Log b values of 4.26±0.03, 7.86±0.11 and 10.47±0.12 were obtained for the ML, ML 2 and ML 3 complexes, respectively, and compare well to literature values. The first log b (6.27±0.07) value for the MLH species was successfully determined, for which a crystal structure has been reported recently.

The free metal ion potential, E(M), is a critical parameter in the calculation of formation constants when using voltammetry. When studying complex formation of Bi(III), however, E(Bi) cannot be directly measured. In this work a nitrate background electrolyte was employed to obtain reversible reduction waves. In order to determine E(Bi), measurements have to be made below pH 2 before the bismuth-oxy-nitrate species precipitates and thus corrections for the diffusion junction potential (monitored using Tl(I) as an internal reference ion) must be made.

The use of polarography to accurately determine stability constants of complexes formed under very acidic conditions (below pH 2) is demonstrated. The diffusion junction potentials, which must be accounted for below pH 2, were evaluated by applying protocols developed where Tl(I) is used as an internal reference. The Cu(II)-picolinic acid (2-pyridinecarboxylic acid) system studied was chosen since the CuL + species only exists in solution below pH 2 under the conditions used and literature data exists to confirm the accuracy of procedure. Additionally, the reduction of Cu(II) was quasi-reversible and procedures to determine the reversible half-wave potentials were investigated. Average log  values of 7.75  0.09 for CuL + and 14.8  0.1 for CuL 2 were obtained, which compared well to literature data.