Effect of Bi(iii)-to-metal ion concentration ratios on stripping voltammetric response of bismuth-film glassy carbon electrodes

Abstract

The effect of Bi-to-metal ion concentration ratio (c_Bi/c_M ratio) on key evaluation indicators, including sensitivity, precision, and cathodic potential range, has been investigated for the determination of Cd and Pb at in situ prepared bismuth film electrodes. Unlike the usual recommendation of at least a 10-fold excess of Hg(II) for anodic stripping experiments at in situ prepared mercury film electrodes, it is found that the c_Bi/c_M ratios in the 1–10 range are sufficient to obtain a high determination sensitivity, but that the signal decreases significantly when the ratio exceeds 40. Further analysis shows that the precision of the analytical results is good when the c_Bi/c_M ratio is in the range of 5–10. The precision is even better in the range of 10–20, but too high a ratio cannot further improve the precision of the results. Therefore, it is recommended to keep the c_Bi/c_M in the range of 5 to 40 to balance the sensitivity and precision in detection. The study also finds that the optimum cathodic potential range is related to the total concentration of metal ions. Therefore, for metals susceptible to hydrogen evolution (e.g., zinc), additional consideration should be given to inhibiting the hydrogen evolution reaction when selecting the c_Bi/c_M ratio. This work is the first to investigate the effect of the c_Bi/c_M ratio on the morphology and thickness of deposits using EIS, SEM, and AFM. It is found that increasing the c_Bi/c_M ratio leads to an increase in the coverage and thickness of the bismuth film on the electrode surface, which enhances the sensitivity of the determination. However, this change is also accompanied by an increase in the electrode resistance, resulting in a significant decrease in signal when the ratio is too large. In addition, when the c_Bi/c_M ratio is <5, the precision of the bismuth film electrode is relatively poor due to the rapid increase of the bismuth coverage on the electrode surface. The uneven thickening of the deposit also affects the cathodic potential range. Based on these findings, standard curves with c_Bi/c_M ratios ranging from 5–25 are prepared and successfully applied to the analysis of river water and wastewater.