Direct determination of cobalt in unpurged oceanic seawater by high speed adsorptive cathodic stripping voltammetry

Abstract

A fast, sensitive method to determine Co (II) in unpurged ocean seawater is presented. The method combines high scan speed staircase voltammetry (HSACSV) and nitrite catalytic effect to enhance Co(II)-dimethylglyoxime reduction currents. The catalytic effect was magnified using high speed adsorptive cathodic stripping voltammetry and peak currents of ca. 0.1 μA were obtained for pM cobalt samples using 30-s deposition times. The effect of buffer, pH, nitrite concentration and scan speed on cobalt sensitivity was studied. Capacitance currents developed at high scan speeds were filtered by background subtraction. The same procedure was successfully applied to remove oxygen interference in unpurged solutions. Blank voltammograms were obtained in different ways yielding similar results without variation in peak reproducibility or accuracy. Nickel and zinc interferences were removed using a cleaning potential before the scan at −1.03 V. Ratios over 10 000 fold were eliminated using more negative cleaning potential steps. Nickel can also be measured by this method. Total analysis time of unpurged seawater (pM levels of cobalt) was decreased to reagents addition time, 30-s adsorption period and 10-s quiescence time. A detection limit of 5.9 pM Co was achieved in seawater. Short analysis time, sensitivity and portable hardware recommend this method for aboard determinations of cobalt. An oceanic profile of cobalt was measured at the Central East Atlantic Ocean.