Pseudopolarographic Determination of Cd2+ Complexation in Freshwater

Pseudopolarography was used to detect Cd2+ complexes in samples collected at several locations along the Potomac River in June and September, 2004. Irrespective of site and sampling time, no weak inorganic Cd2+ species were present. However, up to two stable Cd2+-organic complexes were detected at each site. These unknown Cd2+ complexes were characterized by their half-wave potential (E 1/2). The E 1/2 values indicated certain Cd2+ complexes were common at different sites during each sampling but different complexes were observed in June and September. A Cd2+ chelate scale, generated from model ligands, was used to estimate the thermodynamic stability constants (K THERM) of the unknown complexes, which ranged from log K THERM = 21.5−32.0. Pseudopolarography did not recover all Cd2+ in the samples. This was partly attributed to highly stable Cd-sulfide species; owing to the presence of acid volatile sulfide at concentrations greater than total dissolved Cd2+. These electrochemically inert species may be multinuclear Cd-sulfide clusters and/or nanoparticles with K THERM values that exceed the detection window of pseudopolarography (log K THERM > 34.4).