Effect of Ca(II), Sr(II), and Ba(II) on the Pulsed Amperometric Detection of Alditols and Carbohydrates at a Gold Electrode in Alkaline Solutions

Abstract

The effects of some divalent nonelectroactive cations (DNCs), such as Sr(II), Ba(II), and Ca(II) on the electrochemical oxidation of alditols and carbohydrates at gold electrodes in pulsed amperometric detection have been investigated. It seems that in the presence of DNCs in alkaline solutions, two competitive processes are involved: polyhydroxy compound complexation in the following order of metal ion binding affinity, Ca(II) > Sr(II) > Ba(II), and inhibition on the onset of gold oxide formation in the same order. This last effect leads to an increased activity of the electrode surface; at the optimized value of detection potential of d-sorbitol, E(DET) = +50 mV vs Ag|AgCl, which is ∼100 mV lower in comparison to the maximum value observed with blank carrier electrolytes (i.e., 0.58 M NaOH), there is an increase in sensitivity of ∼50%, and 30% in the presence of 1.0 mM Sr(II), and 1.0 mM Ba(II), respectively. However, the current response of sample molecules results increased only when Ba(II) or Sr(II) ions were already contained in the alkaline media, that is the experimental condition normally occurring in flow injection and liquid chromatography systems. The voltammetric response, observed upon additions of Ba(II) or Sr(II) to an alkaline electrolyte containing d-sorbitol, showed progressive decrease of the anodic current. Irrespective of the experimental condition adopted, i.e., cation addition to the solution before or after the carbohydrate, the presence of Ca(II) has an adverse effect on the anodic currents. These findings have been explained by a rapid formation of adducts between polyhydroxy compounds and DNCs in sodium hydroxide solutions.