Electrochemical redox of arsenic (III) and Cu (II) mixtures with ultraflat Au(111) thin films in water

Abstract

The ability to detect trace concentrations of arsenite, As (III), in real water solutions is impacted by co-contamination of other metals and co-occurring ions. The presence of copper (II) ions are the most likely co-contaminant in natural waters to interfere with electrochemical As (III) detection, due to the close oxidation potentials of Cu (II) and As (III). The use of well-oriented ultraflat Au(111) thin film electrodes provided increased peak separation and sensitivity for electrochemical deposition and oxidation of Cu (II) and As (III) in 0.5 M sulfuric acid compared to an Au wire electrode. However, mixtures of Cu (II) and As (III) altered the oxidation peak positions during both cyclic voltammetry (CV) and linear stripping voltammetry (LSV) analysis. Calibration curves using the standard additions method in trace concentrations were conducted for Cu, As, and Cu & As solutions. Sweeping the potential at 10 mV s−1 during CV in Cu & As mixtures resulted in a sequential deposition condition where a layer of Cu inhibited co-deposition of As to the electrode. In contrast, the rapid potential stepping of LSV to a potential where Cu and As reduction simultaneously occurs produced a peak profile different from Cu or As alone. A larger oxidation peak during LSV was also observed when both Cu and As were present. X-ray photoelectron spectroscopy indicates a Cu-As alloy is formed on the surface after LSV deposition. SYNOPSIS: Analysis of Cu (II), As (III) and Cu (II) & As (III) mixtures suggests that a Cu3As intermetallic phase is formed during LSV which impacts trace As detection even in trace Cu conditions. This has important implications for the ability to determine As concentrations near the MCL of 10 µg L−1 in natural water systems which may contain Cu (II) and other co-contaminants.