ELECTROCHEMICAL STRIPPING ANALYSIS OF TRACE METALS AT CARBON-BASED ELECTRODES

Abstract

Electrochemical stripping determination of trace metals at carbon-based electrodes was carried out in this work. The stripping methods used here include anodic stripping analysis, cathodic stripping analysis and adsorptive stripping analysis. The main stripping technique used here was differential pulse stripping voltammetry. although cyclic voltammetry was also used for characterization of the electrode processes. The working electrodes used are glassy carbon electrodes, chemically modified graphite-tube epoxy composite electrodes and chemically modified carbon paste electrodes. Glassy carbon electrodes were used in the preliminary stages of this program for the studies on the cathodic stripping of adsorbed 2-mercaptobenzothiazolemercury(II) complex. This slightly soluble complex was found to be adsorbed at the surface of the glassy carbon electrode. On the cathodic scan in the differential pulse mode, a stripping peak with peak potential at -0. 15V (vs Ag/Ag Cl, saturated KCl) could be observed. Factors affecting this cathodic response were systematically investigated. A linear calibration plot was obtained for the Hg(II) in the concentration range from 1.00 x 10-7 M to 1.00 x 10-5 M. The detection limit for a 5 minute accumulation, was estimated to be 1.6 x 10-8 M (S/N = 3 ). For the same accumulation time, 8 replicate determinations of Hg(II) at concentrations of 1.00 x 10-6 M and 1.00 x 10-7 M gave relative standard deviations of 2.0% and 2.9% respectively. Of 15 metal ions examined for possible interferences, only Ag(I) and Cu(]]) interfered. This work was followed by using a novel method for the fabrication of a 2- mercaptobenzothiazole bulk-modified composite electrode for the preconcentration and stripping analysis of mercury(II). In this method, a preformed, porous graphite tube was impregnated with a modifier by thorough soaking in a solution containing the modifier. After drying, the modifier was sealed in place in the pores with epoxy. For the purpose of demonstrating the usefulness of the electrode, 2- mercaptobenzothiazole was chosen as the modifier and the modified electrode was used to preconcentrate mercury(Il), followed by voltammetric stripping analysis. The fabricated electrode possessed good stability and had an extended lifetime. Precision for 10 consecutive differential pulse stripping experiments, for a 2 min preconcentration, gave relative standard deviations of 1.9% and 2.6% for Hg(Il) concentrations of 1.00 x 10-6 M and 1.00 x 10-8 M, respectively. Detection limit was estimated to be 3.0 x 10-9 M (S/N = 3, 3 min preconcentration). A certified aqueous sample, USEPA WP 386. together with various samples, were employed for method testing. The work on the analysis of mercury(ll) using the 2-mercaptobenzothiazole modified graphite-tube electrode was further extended to include the simultaneous determination of mercury(II), bismuth(III) and copper(II). Complete resolution of peaks was obtained and the detection limits were estimated to be 4.2 x 10-9 M, 3.6 x 10-9 M and 9.5 x 10-8 M (S/N = 3, 3 min preconcentration) for mercury(Il), bismuth(III) and copper(II), respectively. The relative standard deviations for 10 consecutive runs each of 5.00 x 10-7M and 1.00 x 10-8 M Hg(II) were 1.4% and 2.6%. The relative standard deviations for 10 consecutive runs each of 5.00 x 10-7M and 1.00 x 10-8 M Bi(III) were 1.1% and 2.4%. The relative standard deviations for 10 consecutive runs each of 5.00 x 10-6M and 5.00 x 10-7 M Cu(II) were 1.5% and 2.7%. Various samples were tested using the optimized procedure. A carbon-paste electrode modified with 1-(2-pyridylazo)-2-naphthol was fabricated and used to selectively accumulate cobalt(II) at open circuit. After medium exchange, the accumulated cobalt(ll) was electrochemically oxidized and cathodically stripped using differential pulse voltammetry. Various factors influencing the accumulation and stripping were studied. An optimized procedure was then developed for the determination of cobalt(II). Under optimum conditions, a detection limit of 5.9 x 10-9 M (S/N = 3, 3 min preconcentration) was obtained. The relative standard deviations obtained for 10 successive measurements each of 1.00 x 10-6 M, 1.00 x 10-7 M and 1.00 x 10-8 M cobalt(II) were 2.82%, 5.14% and 9.67%, respectively. V(II) and Ce(III) interfered with the determination. These interferences were eliminated by masking with sodium citrate. It has been previously reported in the literature [157] that the differential pulse cathodic stripping voltammetric determination of manganese at the 1-(2-pyridylazo)- 2-naphthol modified carbon paste electrode was useful for the Mn(II) determination. In this work, it was found that by adding a suitable surfactant to the accumulation and stripping media, the detection limit could be significantly lowered. Because the effect of a surfactant may depend on its ionic charge, a systematic study was carried out on cationic, anionic and nonionic surfactants. After optimization of conditions, it was found that the detection limit could be lowered by as much as 10 times, to 7.07 x 10-10 M (S/N = 3, 3 min preconcentration). The relative standard deviations obtained for 10 successive measurements each of 1.00 x 10-7 M, 1.00 x 10-8 Mand 1.00 x 10-9 M Mn(ll) were 3.33%, 4.76% and 5.89%, respectively. It was found that the selectivity of Mn(II) determination was also enhanced when surfactants were added. When no surfactants were added, Hg(II), Co(II) and Fe(III) interfered in the determination. After adding surfactants, only Hg(Il) interfered with the analysis. Interference from Hg(Il) could be eliminated by masking with sodium diethyldithiocarbamate (DDTC).