A new polarographic procedure has been developed for the study of metal-ligand equilibria at pH<2 by applying
an in situ monitoring of the diffusion junction potential by introducing a noncomplexing witness metal ion, Tl(I).
As a case study, this procedure was applied to the Cd(II)-picolinic acid (pyridine-2-carboxylic acid) system; DCTAST
polarography was employed at 25 8C and ionic strength of 0.25–0.5 M (H,Na)NO3. Log b values of 4.26 0.03,
7.86 0.11 and 10.47 0.12 were obtained for the ML, ML2 and ML3 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.