Gyula Svehla

Caesium-selective poly(vinyl chloride) membrane electrodes based on calix[6]arene esters

Hexameric calix[6]arene ester compounds have shown an ability to extract large alkali metal ions such as caesium and rubidium in phase transfer studies. A potentiometric selectivity study has been made using two ester containing hexameric calix[6]arenes, p-tert-butyl hexaethyl ester (1a) and the unsubstituted hexaethyl ester (1b), as the ionophores for caesium ions. Six electrodes were prepared with 2-nitrophenyl octyl ether as the solvent mediator and various amounts of potassium tetrakis(p-chlorophenyl)borate as the ion exchanger in a poly(vinyl chloride) matrix. The ligands examined produced electrodes with near-Nernstian slopes; the selectivity coefficients for caesium with respect to rubidium, potassium, sodium, lithium, calcium, magnesium, hydrogen and ammonium were determined. Lifetimes of at least 1 month were estimated.

Calixarene-based potentiometric ion-selective electrodes for silver.

Four lipophilic sulphur and/or nitrogen containing calixarene derivatives have been tested as ionophores in Ag(I)-selective poly (vinyl chloride) membrane electrodes. All gave acceptable linear responses with one giving a response of 50 mV/dec in the Ag(I) ion activity range 10(-4)-10(-1)M and high selectivity towards other transition metals and sodium and potassium ions. This ionophore was also tested as a membrane coated glassy-carbon electrode where the sensitivity and selectivity of the conventional membrane electrode was found to be repeated. The latter electrode was then used in potentiometric titrations of halide ions with silver nitrate.

Sodium-selective membrane electrode based on p-tert-butylcalix[4]arene methoxyethylester

Twelve calixarenes have been studied as prospective ionophores for sodium. Five of these have shown promising results in the liquid membrane form. All five have been successfully incorporated into poly(vinyl chloride) membranes, and applied as ion-selective electrodes for sodium. Electrode responses, selectivities towards alkali and alkaline earth metals as well as towards hydrogen and ammonium ions, temperature coefficients and response times have been studied. Based on these results p-tert-butylcalix[4]arene methoxyethylester has been chosen as the best ionophore for the preparation of sodium-selective electrodes. Details of electrode construction are presented.

Symmetrical, unsymmetrical and bridged calix[4]arene derivatives as neutral carrier ionophores in poly (vinyl chloride) membrane sodium selective electrodes

The complexing ability of a range of 19 symmetrical, unsymmetrical and bridged calix[4]arene derivatives having ester, ketone, amide, amine and thioether functionalities were determined by the picrate extraction method. On incorporating these calix[4]arene derivatives as neutral carrier ionophores in sodium-selective poly (vinyl chloride) membrane electrodes the performance was assessed on the basis of the sensitivity and selectivity over the alkali, alkaline earth metals and hydrogen and ammonium ions. The temperature dependence, response times and lifetimes were also determined. Four ionophores in particular gave excellent sensitivity and selectivity and lifetimes of > 200 days. These electrodes were then tested without additional lipophilic additives and one ionophore was incorporated into poly (vinyl chloride) membrane electrodes with plasticizing solvents of varying polarity.

An ASIC-based system for stripping voltammetric determination of trace metals

The work presented here is part of an overall voltammetric design system with a silicon based disposable component for the determination of copper, cadmium, lead and zinc. Voltammetric (the measurement of current as a function of potential) methods of analysis are attractive for determinations of trace metals. They offer a combination of simple low cost instrumentation with excellent sensitivity and detection limits, comparable to those obtainable with instruments costing orders of magnitude greater. The system consists of electrodes, an application specific integrated circuit (ASIC) and software control. Screen-printed silver/silver chloride and carbon electrodes are used in the preparation of mercury film electrodes on which anodic stripping voltammetry is performed for the detection of the four heavy metals listed above. Work has also been carried out to investigate calibration problems caused by the formation of intermetallic complexes.

Ionophore/ionomer films on glassy carbon electrodes for accumulation voltammetry. Investigation of a lead(II) ionophore

The use of ionophores in voltammetry with modified glassy carbon (GC) electrodes is described. A lead(II) ionophore, methylenebis (N, N-diisobutyldithiocarbamate) was dissolved in a solution containing 0.1% Nafion, and this mixture was applied to the GC electrode. Solvent evaporation gave the GC–Nafion–ionophore modified electrode. The composition of the film coating was studied with respect to the mass of Nafion and amount of ionophore; the best composition was found to comprise 20 µg Nafion and 10 nmol ionophore. Accumulation of lead(II) from 0.1 mol l–1 acetate buffer (pH 4) followed by voltammetry gave a quasi-reversible reduction peak at about –0.85 V (versus SCE), with halfwave potential, E1/2=–0.57 V and peak-to-peak separation, ΔEp= 0.51 V. This reduction potential is 0.25 V more negative than the reduction potential in the absence of the ionophore where E1/2=–0.48 V and ΔEp= 0.14 V. Accumulation of 1 × 10–5 mol l–1 lead(II) for 60 s at the GC–Nafion–ionophore modified electrode gave a lead(II) surface coverage of 1.3 × 10–9 mol cm–2. The film exhibited thin-film voltammetric behaviour. Detection of lead(II) down to 1 × 10–7 mol l–1 with a 600 s accumulation time was possible. The effects of preconcentration time and solution composition were evaluated. The accumulation process appears to consist of two steps: ion exchange followed by complexation in the film.

Incorporation of hydroxamic acid ligands into Nafion film electrodes

This paper describes studies on Nafion-coated glassy carbon electrodes incorporating hydroxamic acids. Desferrioxamine, a trihydroxamic acid, and glycine hydroxamate, a monohydroxamic acid, can be incorporated by ion exchange into the Nafion film. These ligands were detected in the film visually, after reaction with iron(III), and electrochemically, by voltammetric oxidation. The complex of iron(III) with desferrioxamine was electrochemically active at the iron(III) centre in the Nafion film. The electrochemistry of this complex in the film agrees well with the solution electrochemistry reported by other workers: irreversible at low pH and reversible at high pH. The ability of the modified electrode to act as a sensor for iron(III) was assessed. Complexation of iron(III) from dilute solution into the polymer film was possible, and 5 × 10–7 mol l–1 iron(III) could be detected after a 10 min preconcentration, with differential-pulse voltammetric detection in pH 10 ammoniacal buffer.

Evaluation of equivalence points in the potentiometric titration of mixtures of halides

The accurate and precise location of the equivalence points in the potentiometric titration of halide ions can be achieved numerically by using Grans method. The results have been shown to be better by this method than those obtained on the same titration data by the differential methods of Kolthoff, of Fortuin and of Hahn.

An Asic-based System For Stripping Voltammetric Determination Of Trace Metals

The work presented here is part of an overall voltammetric design system with a silicon based disposable component for the determination of copper, cadmium, lead and zinc. Voltammetric (the measurement of current as a function of potential) methods of analysis water are attractive for determinations of trace metals. They offer a combination of simple low cost instrumentation with excellent sensitivity and detection limits, comparable to those obtainable with instruments cost orders of magnitude greater. The system consists of electrodes, an application specific integrated circuit (ASIC) and software control. There is increasing pressure on industry and regulatory bodies to monitor industrial and domestic pollution. A particular concern is the contamination of water supplies with heavy metals. The aim of this work is to develop a method of heavy metal analysis using a device that can be manufactured by means that are compatible with microelectronics processes giving a low cost solution to trace metal analyses for water quality applications.