Jerzy Zarębski

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 (dmgH(2)) 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 dmgH(2) ligand and cobalt ions are reduced in the adsorbed state:  Co(II) + 2dmgH(2) ⇌ (solution) [Co(II)(dmgH)(2)] + 2H(+); [Co(II)(dmgH)(2)] + Hg ⇌ (electrode) [Co(II)(dmgH)(2)](ads)Hg; and [Co(II)(dmgH)(2)](ads)Hg + 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.

Catalytic Adsorptive Stripping Voltammetric Determination of Chromium(VI) in Overlying and Interstitial Waters Isolated from Sediments Contaminated by Tannery Waste

In the present paper, catalytic adsorptive stripping voltammetry (CAdSV) has been applied for the first time in a chromium speciation study conducted in pore sediments and overlying waters containing Cr(VI) in the presence of a great excess of Cr(III) and organic matter. The method is based on the adsorptive accumulation of the active Cr(III)-DTPA (diethylenetriammine-N,N,N′,N″,N″-pentaacetic acid) complex, which is formed instantaneously on the electrode surface as a result of Cr(VI) reduction to Cr(III), its complexation with DTPA, reduction to Cr(II)-DTPA, and its catalytic oxidation to Cr(III)-DTPA with nitrate. The main efforts were focused on the elimination of the influence of dissolved organic matter including organic surface-active substances (SAS) that adsorb at the electrode surface, and disturb the Cr(VI) determination or, in extreme cases, make it impossible. Of the three procedures applied for the elimination of this negative effect caused, namely, the matrix-exchange method, the removal of SAS by means of fumed silica, and their adsorption on the XAD-7 Amberlite resin present directly in voltammetric vessel, the last one proved to be the most useful. The matrix-exchange procedure also provides accurate and precise results but requires more reagents and time. The CAdSV method combined with the adsorption of surface-active substances on the XAD-7 Amberlite resin was successfully applied for the determination of the Cr(VI) in vertical profile in sediments from Dunajec River, which are severely polluted with chromium from tannery wastes.