DEVELOPMENT OF ELECTROANALYTICAL METHODS FOR TRACE ELEMENT ANALYSIS

Abstract

This thesis concerns the development of sensitive and selective electrochemical methods for trace element analysis which is important in industrial, environmental, biological and clinical studies. A rapid and sensitive differential-pulse adsorptive stripping voltammetric method, based on the formation and anodic oxidation of Al(III)-8-hydroxyquinoline complex, is presented for the determination of trace levels of aluminium. The detection limit for this method is 1.00 x 10-8 M A(III). A carbon paste electrode incorporating 8-hydroxyquinoline (HOx) was fabricated to preconcentrate aluminium(III) as the Al(III)-HOx complex. The differential pulse anodic peak current arising from this complex was then characterized for aluminium determination. A detection limit of 9.5 x 10-9 M Al(III) was obtained for a 30 second accumulation time. However. interferences from many metal cations were encountered. A HOx modified carbon paste electrode (HOx-MCPE) was employed to selectively accumulate TI(I) at open circuit. The accumulated TI(I), after reduction to metallic thallium at the electrode surface, was anodically stripped by differential pulse voltammetry. A detection limit of 4.90 x 10-9 M TI was found for a 120 s accumulation. A method was then developed for thallium speciation and analysis. based on the determination of TI(I) while masking Tl(III) with EDTA followed by chemical reduction of Tl(III) to TI(I) with hydroxylamine hydrochloride. As an extension to this, thallium(III) was selectively accumulated in an open circuit onto a HOx-MCPE. The ensuing measurement was carried out by differential pulse anodic stripping voltammetry after reducing the thallium(III) to metallic thallium. An enhanced detection limit of 2.30 x 10-10 M Tl was found for a 2-minute accumulation. A carbon paste electrode modified with zinc-diethyldithiocarbamate was used for the highly selective accumulation of mercury(II) based on the metal displacement principle. The accumulated Hg(II) was then reduced to metallic mercury in 0.10M KSCN-0.010 M HClO4 followed by differential pulse anodic stripping voltammetry. A detection limit of 8.0 x 10-10 M Hg(II) was obtained for a 15 minute accumulation. Electropolymerization of 3,3'-diaminobenzidine via C-N coupling at solid substrate electrode (platinum, gold and glassy carbon) gave stable and water-insoluble films under a wide range of pH. The poly(3,3'-diaminobenzidine) (PDAB) films were found to be electroactive in the acidic aqueous solutions but not in neutral or basic solutions. The PDAB film demonstrated different electrochemical response ability and permeability to Fe(CN)63-/4- with different solution acidity and Fe(CN)63-/4- ion concentration. Cyclic voltammetric behavior of p-dihydroxybenzene (H2Q) in 0.20 M KNO3 indicated that H2Q proceeded faster and simpler electron transfer on the PDAB-Au electrode compared with that on a bare Au electrode. The PDAB-modified gold electrode exhibited high selectivity for the open circuit accumulation of Se(IV) through complexation with the aromatic o-diamine groups attached to the polymer backbone to form the piaselenole. This property was exploited for the preconcentration of Se(IV), followed by differential pulse anodic stripping voltammetry for its determination. A detection limit of 9.9 x 10-9 M Se(IV) was realized for a 10 min accumulation. Finally, a potentiometric response behavior of gold electrode modified with PDAB film to selenium(IV) in solution has been examined. The calibration plots for the PDAB film-modified electrode gave an unexpected positive slope of 29-46 mV /decade at ambient temperature of 25 ± 1 °C, depending on the history of the electrode. Nevertheless, the aged electrode is preferred for application because of the enhanced reproducibility and longer term stability of the electrode response. A typical calibration plot useful for analysis gave a linear response for Se(IV) concentration from 2.00 x 10-6 M to 1.00 x 10-2 M (correlation coefficient 0.9999). The slope was 32.0 mV/decade and the estimated detection limit was 1.4 x 10-6 M Se(IV).