Determination of arsenic and antimony in seawater by voltammetric and chronopotentiometric stripping using a vibrated gold microwire electrode

Abstract

The oxidation potentials of As0/AsIII and Sb0/SbIII on the gold electrode are very close to each other due to their similar chemistry. Arsenic concentration in seawater is low (10–20nM), Sb occurring at ∼0.1 time that of As. Methods are shown here for the electroanalytical speciation of inorganic arsenic and inorganic antimony in seawater using a solid gold microwire electrode. Anodic stripping voltammetry (ASV) and chronopotentiometry (ASC) are used at pH≤2 and pH 8, using a vibrating gold microwire electrode. Under vibrations, the diffusion layer size at a 5μm diameter wire is 0.7μm. The detection limits for the AsIII and SbIII are below 0.1nM using 2min and 10min deposition times respectively. AsIII and SbIII can be determined in acidic conditions (after addition of hydrazine) or at neutral pH. In the latter case, oxidation of As0 to AsIII was found to proceed through a transient AsIII species. Adsorption of this species on the gold electrode at potentials where SbIII diffused away is used for selective deposition of AsIII. Addition of EDTA removes the interfering effect of manganese when analysing AsIII. Imposition of a desorption step for SbIII analysis is required. Total inorganic arsenic (iAs=AsV+AsIII) can be determined without interference from Sb nor mono-methyl arsenious acid (MMA) at 1.6<pH<2 using Edep=−1V. Total inorganic antimony (iSb=SbV+SbIII) is determined at pH 1 using Edep=−1.8V without interference by As.The methods were tested in samples from the Irish Sea (Liverpool Bay). AsIII was determined on-board ship immediately after sampling. AsIII concentrations were found to range from 0.44 to 1.56nM and were higher near the coast. SbIII was below the detection limit (<0.1nM SbIII), iAs was comprised between 8 and 25nM while iSb varied from 0.5 to 1.7nM.