Development and application of chemically modified electrodes for analysis (1992)
AuthorsXu, Shuanghuashow all
This thesis presents a study on development and application of chemically modified electrodes (CMEs) for analysis, especially for analysis of aluminium(III). A parallel study involved flow injection analysis with indirect amperometric detection. The electrochemistry was studied for several electroactive ligands which bind strongly to aluminium(III). The effect of pH on redox behaviour was investigated. The complex formation between aluminium(III) and these ligands was examined under different conditions such as pH, electrolyte and temperature. For quantitative determination of aluminium(III), either the increase in peak height due to the redox processes of the formed aluminium(III)-ligand complex, or the decrease of the peak height for free ligand redox processes, was used. Several CMEs were prepared using polymer coatings, chemisorption and by mixing ligand into the electrode material. The use of CMEs consisting of charged polymers for preconcentration of analytes was investigated. The accumulation of aluminium(III) in polyxylylviologen coated electrodes as anionic phenolic complexes was studied. The measurement of aluminium(III)(4-nitrocatechol) complexes which have an overall negative charge resulted in a sensitive method for analysis of aluminium(III). A 1,2-dihydroxyanthraquinone(alizarin)-modified graphite electrode was used in the voltammetric determination of aluminium(III). Alizarin was immobilised on a solid electrode by chemisorption. The electrode was applied in determination of exchangeable aluminium(III) in soils. This provided a simple, rapid method for monitoring aluminium in the environment. Ligands mixed into electrode materials (carbon paste and graphite epoxy) were studied. Graphite epoxy modified with 4- nitrocatechol showed good results for the determination of aluminium(III). In CME studies, it was found that the loss of ligand from CMEs due to irreversible oxidation of ligand and other reasons limits electrodes to single-use. The method for determination of aluminium(III) by adsorptive stripping voltammetry of its SVRS complex was modified by room temperature reaction at a higher pH, followed by accumulation in acidified solution. This method provided good results with high sensitivity and reproducibility and significantly shorter analysis time. Aluminium(III) was determined in a flow injection system involving the formation of the aluminium(III)-1,2- dihydroxyanthraquinone-3-sulphonic acid (DASA) complex and amperometric measurement of excess DASA at +0.50 V on a gold electrode. Electrode fouling by adsorption of ligand oxidation products was minimized by use of a double pumping system and cathodic/anodic voltage cycling. The method was applied to soil extracts. A simple flow system for magnesium(II) determination in natural waters and serum was developed. This involved complexation of magnesium(II) with the redox active ligand eriochrome black T (EBT). Analysis of human blood serum samples was effected by two methods: (1) direct injection of serum after dilution and (2) after dialysis to effect separation of magnesium(II) from acidified serum in the flow system. Electrode fouling by adsorption of EBT oxidation products and serum in the flow system was minimized by use of several surfactants and a dialyzer.