Electrochemical and Spectroscopic Investigation of the Reduction of Dimethylglyoxime at Mercury Electrodes in the Presence of Cobalt and Nickel

Voltammograms (polarograms) obtained from solutions of cobalt and nickel containing dimethylglyoxime (dmgH2) are widely used for the trace determination of these metals. Detailed electrochemical and spectroscopic studies on the reduction process observed in the analytically important ammonia buffer media at mercury dropping, hanging, and pool electrodes are all consistent with an overall 10-electron reduction process, in which both the dmgH2 ligand and cobalt ions are reduced in the adsorbed state: Co(II) + 2dmgH2 ⇌ (solution) [CoII(dmgH)2] + 2H+; [CoII(dmgH)2] + Hg ⇌ (electrode) [CoII(dmgH)2]adsHg; and [CoII(dmgH)2]adsHg + 10e- + 10H+ → Co(Hg) + 2[2,3-bis(hydroxylamino)butane]. The limited solubility of the nickel complex in aqueous media restricts the range of studies that can be undertaken with this system, but an analogous mechanism is believed to occur. Low-temperature voltammetric studies in dichloromethane at a frozen hanging mercury drop electrode and in situ electron spin resonance electrochemical measurements on more soluble analogues of the dimethylglyoxime complexes are consistent with an initial one-electron reduction step being available in the absence of water. Deliberate addition of water to acetone solutions enables the influence of the aqueous environment on voltammograms and polarograms to be examined. The results of the present study are compared with the wide range of mechanisms proposed in other studies.