Speciation of Cr(VI) and Cr(III) in Water Samples by Adsorptive Stripping Voltammetry in the Presence of Pyrogallol Red Applying a Selective Accumulation Potential

An adsorptive stripping voltammetric procedure for the speciation of Cr(VI) and Cr(III) in the presence of pyrogallol red (PGR) is presented. The method is based on the previous reduction of Cr(VI) to Cr(III) at the electrode surface, its complexation with PGR, and the later reduction of Cr-III-PGR to Cr-II-PGR at -0.85 V. The effects of various operational parameters such as pH, ligand concentration (C-PGR), and accumulation potential and time (E-ads, t(ads)) were optimized. These studies were carried out using individual Cr(VI) and Cr(III) solutions and also mixtures of them. The results showed that the proper choice of the adsorptive potential produces selectivity for Cr(VI) in the presence of Cr(III). At an E-ads of 0.00 V only Cr(VI) produces the reduction signal of CrIII-PGR at -0.85 V, while at an E-ads of -0.68 V both Cr(VI) and Cr(III) produce this signal. Total chromium was determined after oxidation of Cr(III) to Cr(VI) by UV radiation in the presence of H2O2. The concentration of Cr(III) was evaluated as the difference between total chromium and Cr(VI). Under the best experimental conditions (pH 4.5; C-PGR 0.25 mu mol L-1; E-ads -0.68 V and t(ads) 60 s), the peak current is proportional to the total Cr concentration up to 20.0 mu g L-1, with a 3 sigma detection limit of 0.05 mu g L-1. The relative standard deviation for a Cr(VI) solution (9.8 mu g L-1) was 1.8 % for six successive assays. The method was validated using synthetic sea water (ASTM D665) spiked with Cr(VI) and Cr(III), and with a certified reference water (NCS ZC76307). In this reference material total chromium was determined as Cr(III) other aliquot of reference sample was oxidized and the total chromium determined as Cr(VI). Finally, the method was applied to the determination of Cr(VI) and Cr(III) in sea water samples