Comparison of Alternate Reactants for pM Level Cobalt Analysis in Seawater by the Use of Catalytic Voltammetry

The analysis of Cobalt (Co) at low pM concentrations in seawater with Adsorptive Cathodic Stripping Voltammetry involves high concentrations of sodium nitrite (NaNO₂) to enhance the signal in an electrocatalytic reaction. In this study we found three substitutes for NaNO₂ that critically affected the sensitivity. Optimisation of a method with potassium bromate (KBrO₃) resulted in an excellent detection limit (0.9 pM) after a 90 s adsorption period. Reactant concentration and consumption were 10× reduced compared to protocols with NaNO₂ and reagent blanks were lower. Accuracy and precision were verified with SAFe intercalibration standards and the method was applied using open ocean seawater samples. The reaction mechanism is discussed and differences to NaNO₂ are shown.