The Source of the Anomalous Cathodic Peak During ASV with In Situ Mercury Film Formation in Chloride Solutions

Abstract

In situ mercury ®lm depositions at a glassy carbon electrode at various concentrations of sodium chloride or hydrochloric acid were performed. In the chloride concentration range of 0.001 to 0.5 M, an anomalous cathodic peak is observed during the anodic scan when performing square wave voltammetry. This phenomenon has previously been attributed to the formation of mercury(I) chloride (calomel) on the electrode surface. Cyclic voltammetry indicated that electrochemically generated calomel, which forms due to the oxidation of the mercury ®lm in solutions with greater than 0.00 1 M Cl À , is reversible and is reduced to elemental mercury during cathodic polarization of the electrode. It is proposed that the cathodic peak is a result of the calomel which forms after Hg(O) is oxidized to Hg(I) by Hg(II) ions and subsequent reaction of Hg(I) with chloride ions. This insoluble nonelectrochemically generated calomel is formed under open circuit conditions and, not being reduced during the cathodic polarization, is reduced at more anodic potentials. The most stable and reproducible responses of this system for ASV of Cd 2 and Pb 2 is obtained when the mercury is removed from the electrode surface by electrochemical oxidation at the end of each anodic scan.

Determination of Mercury by Anodic Stripping Voltammetry with a Gold Nanoparticle‐Modified Glassy Carbon Electrode

“…The difference between peak potentials is 54 mV, close to the value expected for a reversible one-electron process. The hypothesis is in agreement with the conclusions of Nolan et al [42], who studied the behavior of Hg 2þ at carbon electrodes, and of Ribeiro et al, who investigated the redox mechanism of methylmercury at carbon microelectrodes [43].…”

Voltammetric Studies on the Electrochemical Reduction of Methylmercury in HCl Aqueous Medium at a Carbon Microelectrode

“…experimental conditions of Figure 1a, two anodic peaks, peak A 1 , at þ 0.14 V, and a small peak A 1b , at þ 0.28 V, were recorded. A decrease in the lower limit of the applied potential from À 1.3 to À 0.8 V leads to a CV, Figure 1b, which shows a well-defined pair of peaks, C 1 /A 1 , at þ 0.067 and þ 0.14 V, respectively, an anodic plateau between À 0.4 and 0 V and a cathodic one between À 0.4 and À 0.8 V. The anodic peak, A 1 , presents a shape characteristic of the stripping processes, i.e., the oxidation of Hg(0) to Hg(II), with the most probable subsequent formation of the calomel, (Hg 2 Cl 2 ) [28]. A decrease in the upper potential limit to lower values, i.e., 0 V, leads to the CV given in Figure 1c.…”

Electrochemical studies of the interaction between a modified activated carbon surface and heavy metal ions

“…Generally, besides the c 0 cathodic peak, a second cathodic peak/wave (c 1 ) and a broad, double anodic peak (a 1 ,a 2 ) is observed for all samples. The following reactions may explain the observed cathodic peaks and waves [31,37,38]: …”

“…The contact of active carbon materials with an aqueous solution containing the tested cations may lead to the adsorption of cations and metal-containing species (i.e. MeOH + ), partial reduction of adsorbed cations and the formation of an imperfect metallic layer deposited on the carbon surface (especially for mercury) [37,39]. Modification of the carbon surface chemistry strongly influences the extent and mechanisms of metal cation adsorption (Table 2) and the electrochemical activity of the cations present in the system studied here (Figures 8-10).…”

“…Generally speaking, the electrochemical activity (magnitude of the metal oxidation-reduction peaks) of cation/activated carbon surface systems in nitrate solution depends on the ion uptake, as Figures 8-11 clearly show. The electrochemical behaviour of a mixture of cations in solution on a commercial carbon electrode (GC) (Figure 11(a)) is close to the literature data [30][31][32][33][34][35][36][37], but for studied PACE depends on the surface chemistry of modified carbon samples and correlates with their adsorption ability. For this particular mixture of cations in solution the dominant electrochemical process is connected with mercury oxidation-reduction reactions (Figure 11(b)), which can be explained by the highest adsorption of this cation on the carbon surface ( Table 2).…”