A study on the determination of chromium as chromate at a carbon paste electrode modified with surfactants

A procedure for the determination of chromium is described based on synergistic pre-concentration of the chromate anion at a carbon paste electrode modified in situ with quarternary ammonium salts such as 1-ethoxycarbonylpentadecyltrimethylammonium bromide (Septonex^®), cetyltrimethylammonium bromide (CTAB) or cetylpyridinium bromide (CPB). The proper electrochemical detection utilises the reduction Cr(VI) → Cr(III) performed in the differential pulse cathodic voltammetric mode. In discussion, considerable attention has been paid to the accumulation mechanism at the carbon paste electrode in the presence of surfactants. Furthermore, after optimising the corresponding experimental conditions (0.1-0.3 M HCl + 0.1 M NaCl as the supporting electrolyte, 2.5-25 μM as the total concentration of modifier, pre-concentration at +0.7 V versus Ag/AgCl and the stripping from +0.7 to −0.4 V), the analytical performance of the method has been evaluated. The signal of interest was reproducible within ±8% and proportional to the concentration in a range of 0.5-50 μM CrO₄²⁻, with a limit of detection (S/N = 3:1) of about 5×10⁻⁸ M CrO₄²⁻ (with accumulation for 300 s). Interference studies were focused mainly on the species capable of forming ion-pairs with the modifier; i.e., TlCl₄⁻, AuCl₄⁻, PdCl₄²⁻, PtCl₆²⁻, VO₄³⁻, MnO₄⁻ and I⁻. Practical applicability of the method was tested on model solutions via the recovery rates (typically 90-110%) or using selected certified reference materials (tea, bush leaves, clover) and two samples of black tea when the respective results were compared to those obtained by the reference determinations with ICP-AES.