High‐Performance adsorptive cathodic stripping voltammetry of nickel and cobalt in seawater

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Abstract
The voltammetric determination of trace metals in seawater suffers from the interference of dissolved oxygen and engine vibrations onboard ship. It is here attempted to overcome these interferences using a high‐frequency (1 to 20 kHz) staircase modulation during the voltammetric scan (high‐performance adsorptive cathodic stripping voltammetry, HPACSV). Comparative experiments using the square‐wave modulation showed that this modulation was not effective at such a high frequency. A comparison between various complexing agents (DMG and nioxime) and pH buffers (HEPES/NH4OH, borate/NaOH, and TEA/NH4OH) showed that the determination of nickel and cobalt was similarly improved by the high‐speed potential scan in all conditions. Fast potential scans cause a large increase of the peak area and peak heights using staircase modulation. The peaks are wider, and the peak potentials are shifted toward negative values due to the ohmic drop, but the analytical determination is not disturbed. Optimal sensitivity for the determination of cobalt and nickel in seawater was obtained using TEA buffer and DMG as the adsorptive ligand. The nioxime wave was found to interfere in the cobalt peak at high scan rates so that this ligand is not recommended for HPACSV. Comparative tests showed that the sensitivity for nickel and cobalt determinations was highly improved by the fast scan rates. Furthermore, the reduction current of dissolved oxygen was partially masked so that measurements could be taken without a previous purge of the sample. Finally, the scans were insensitive to the solution turbulence so measurements could be readily carried out onboard of an oceanographic vessel or without switching off the stirrer. Low levels of nickel (2 nM) could be determined in seawater by using a deposition time of 20 seconds and of 120 seconds for 0.05 nM cobalt from turbulent and unpurged solutions. A fast determination method for labile nickel and cobalt is proposed.