Electroanalytical stripping methods, which involve initial preconcentration of the chemical species at an electrode, are simple and inexpensive. Anodic stripping voltammetry (ASV) has been used extensively to determine metals in environmental samples. Trace metals, such as cadmium and lead, are reduced onto a hanging mercury drop electrode, and are then determined oxidatively by scanning to more positive potentials. Determinations can be made at even lower concentrations by using a (rotating) mercury film electrode (MFE) where the metals cannot diffuse far into the mercury from the surface. Cathodic stripping voltammetry (CSV) originally involved oxidizing mercury to form an insoluble mercury salt of the determinand: the mercury ion was reduced during the negative-going potential scan

Glassy carbon electrodes were coated with films of poly(glutamic acid) (PG), and the modified electrode proved to be very effective in the oxidation of caffeic acid. The performance of the film was also tested with ascorbic acid, coumaric acid, ferulic acid, sinapic acid and chlorogenic acid. At pH 5.6, all the hydroxycinnamic acids yield a higher peak current intensity when oxidized after incorporation in the PG-modified electrode, and only the oxidation of ascorbic acid exhibits overpotential reduction. At pH 3.5 only caffeic and chlorogenic acid are incorporated in the modified electrode and exhibit a well-defined oxidation wave at +0.51 V and +0.48 V, which is the base for their determination. Linear calibration graphs were obtained from 9 × 10−6 mol L−1 to 4 × 10−5 mol L−1 caffeic acid by linear voltammetric scan and from 4 × 10−6 mol L−1 to 3 × 10−5 mol L−1 by square wave voltammetric scan. The method was successfully applied to the determination of caffeic acid in red wine samples without interference from other hydroxycinnamic acids or ascorbic acid.

The detection limit (about 0.017 mu g mL(-1)) for voltammetric determination of iodide (peak at +0.87 V vs. Ag/AgCl at pH 2) at a glutaraldehyde-cross-linked poly-L-lysine modified glassy carbon electrode involving oxidation to iodine was found to be several orders of magnitude lower than that for the voltammetric determination on a bare glassy carbon electrode. This method was applied successfully to the determination of iodide in two medicinal formulations. Idoxuridine was determined indirectly at the same electrode by accumulating it first at -0.8 V vs. Ag/AgCl. At this potential the C-I bond in the adsorbed idoxuridine is reduced giving iodide, which is then determined at the modified electrode. The method was successfully applied to the determination of idoxuridine in a urine sample.

A film of poly-L-lysine (PLL) adheres better to the surface of a glassy carbon electrode when the PLL is partially cross-linked by means of glutaraldehyde. A film composition of 97.5% PLL/2.5% glutaraldehyde gives good adhesion and retains the anionic exchange capability of the PLL. The performance of the film was tested with hexacyanoferrate(III) using electrochemical and nonelectrochemical accumulation.

A voltammetric method for the determination of free and total sulfur dioxide in beer is described. First, volatile aldehydes (mainly acetaldehyde) are purged with nitrogen from a beer sample diluted in alkaline medium, collected in an appropriate electrolyte trapping solution and determined, after derivatization with hydrazine, by voltammetry using a hanging mercury drop electrode. Then, the remaining beer solution is strongly acidified and (total) sulfur dioxide is purged with nitrogen, collected in an appropriate electrolyte trapping solution and determined by voltammetry. The free sulfur dioxide concentration is calculated by difference between (total) sulfur dioxide and acetaldehyde concentrations. The proposed method has a relative standard deviation of about 2.1% and 4.4%, respectively for (total) sulfur dioxide and free sulfur dioxide concentrations normally found in beer, and results are in good agreement with those obtained by the p-rosaniline reference method.

Clotrimazole was shown to react at room temperature in Britton Robinson buffer pH 2 with the reactive dye Procion Red HE-3B. The product exhibited a differential pulse polarographic peak at −0.38 V, which was well separated from the peaks of the reactive dye at −0.08, −0.80 and −0.95 V, and this allowed the indirect determination of clotrimazole in the presence of excess of the reactive dye. The method has been applied satisfactorily to the determination of clotrimazole in pharmaceutical formulations, calibration graphs are rectilinear up to at least 40 μg ml−1. The detection limit was calculated to be 2.6 μg ml−1 (3 σ).

Procion red HE-3B (RR120) is an example of dye currently used in affinity purification. A method is described for determining trace amounts of RR120 dye contaminant in human serum albumin by cathodic stripping voltammetry. The method is based on a measure of a well-defined peak at −0.58 V, obtained when samples of HSA protein (0.01–2% w/v) containing dye concentrations are submitted to a heating time of 330 min at 80 °C in NaOH, pH 12.0 and the samples are removed to a solution containing Britton–Robinson buffer, pH 4.0. Using an optimum accumulation potential and time of 0 V and 240 s, respectively, linear calibration curves were obtained from 1.0×10−9 to 1.0×10−8 mol l−1 for RR120 dye. Leakage/hydrolysis of reactive red 120 from an agarose support (e.g. at pH 2 or 12) can also be conveniently determined at very low levels (sub-μg ml−1) by means of cathodic stripping voltammetry, which involves adsorptive accumulation of the dye onto the hanging mercury-drop electrode.

A number of dyes exhibit genotoxic or ecotoxic properties leading to the need for sensitive and selective methods for their determination. Because of the easy reducibility of dyes, modern polarographic and voltammetric methods (differential pulse polarography on classical dropping mercury electrode, differential pulse voltammetry on hanging mercury drop electrode or adsorptive stripping voltammetry) are suitable for the determination of trace amounts of these substances in the general environment in the vicinity of production plants. The scope and limitations of these methods is reviewed and optimum conditions for recently developed methods are summarized. It is shown that the sensitivity of newly developed polarographic and voltammetric methods is sufficient even for the most demanding applications and their selectivity can be increased by their combination with preliminary separation using thin layer chromatography or liquid extraction.

Scope and limitations of modem polarographic and voltammetric techniques on mercury electrodes are discussed and many practical examples of their applications in practical analysis are given to demonstrate that even in the third millennium polarography and voltammetry at mercury electrodes can be very useful analytical tools, which in certain cases can successfully compete with modem spectroscopic and separation techniques.

A sensitive method is described for the determination of cefaclor by cathodic stripping voltammetry at the hanging mercury drop electrode. cefaclor is accumulated at the electrode surface as a mercury salt, which is reduced at -0.67 V. The optimum accumulation potential and accumulation time were +0.15 V and up to 180 s, respectively. Linear calibration graphs were obtained between 3.9 mu g.L-1 to 39 mu g.L-1 and the limit of determination was evaluated to be 1.9 mu g.L-1. The method was applied successfully to the determination of cefaclor in pharmaceutical formulations.

Remazol brilliant orange 3R shows only a voltammetric peak for the reduction of the azo group. No peak was observed for the reduction of the sulfatoethylsulfone or vinylsulfone reactive groups. The reduction of a pre-protonated azo group involving a two-electron process, gives a hydrazo derivative in acidic solution. In alkaline solution the reduction process occurs at more negative potential with the formation of an unstable hydrazo compound which decomposes via HN–NH bond cleavage and loss of a sulfato group. Optimum conditions are given for the cathodic stripping voltammetric determination of the dye in aqueous solution. The optimum accumulation potential and time were 0V and up to 60s, respectively. Linear calibration graphs were obtained from 30 to 300ngml−1 in pH 4 and 6.2 to 62ngml−1 in pH 10. The limit of determination obtained was 1.5ngml−1 (pH 10). The coefficient of variation was 2.6% (n=7) at 62ngml−1 of the reactive dye.

With the introduction of PC-driven analytical voltammetric instrumentation, the investigation of several useful voltammetric characteristics of electroactive compounds has become much easier, increasing the possibilities of discrimination in analysis. In this work some suggestions are given for the development of strategies to apply in the systematic study of compounds adsorbed on a hanging mercury drop electrode (HMDE). This would establish their behaviour under different experimental conditions for analytical purposes.Although indications of reactant adsorption can be easily detected with normal-pulse voltammetry (NPV), this technique is not as appropriate as square-wave voltammetry (SWV) for analytical purposes. The different effects of SW frequency and amplitude on signals obtained with two quinoxalines were studied. Based on their different electrochemical reversibilities (dimethylquinoxaline (DMQ) reversible, and hydroxyquinoxaline (HOQ) irreversible), these two compounds could be determined simultaneously, in spite of their similar peak potentials. A marked effect of the surfactant tetraphenylphosphonium chloride (TPPC) on the peak potential of the azo colouring matter, azorubine, was also observed. A noticeable increase in peak height occurs at relatively low frequencies, in contrast to the (expected) reduced peak height at high frequencies.Finally the possibility of avoiding the time consuming need to remove oxygen is also considered. At high SW frequencies the interference of oxygen is drastically reduced. DMQ at concentrations as low as 10(-7) M could be determined without oxygen removal by using a SW frequency higher than 100 Hz. A direct application was made to the voltammetric determination of diacetyl in brandy: this involved prior derivatization of the compound with o-phenylenediamine to form DMQ. (C) 1999 Elsevier Science B.V. All rights reserved.

Square wave voltammetric studies, involving adsorptive stripping methodology, of anthraquinone-based reactive dyes and of a typical anthraquinone dye Alizarin Red S are reported. The reduction of their anthraquinone moiety is reversible and fast and the sensitivity of these determinations can be improved if current sampling is made earlier after the pulse application.In order to try to modify the rate of the electrochemical process at the electrode, two approaches have been tested: the effect of adding surfactants and the influence of supporting electrolyte concentration. In both cases, a change in the results obtained is observed, owing, in the first case, to the modification of the electrode surface, and, in the second, to the importance of the resistance of the solution. (C) 1999 Elsevier Science B.V. All rights reserved.

Josef Cvaĉkaa, Jir̂ı́ Barek*a, Arnold G. Foggb, Josino C. Moreirac and Jir̂ı́ Zimaa aUNESCO Laboratory of Environmental Electrochemistry, Department of Analytical Chemistry, Charles University, Hlavova 2030, 128 43 Prague 2, Czech Republic b Department of Chemistry, Loughborough University, Loughborough, Leicestershire, UK LE11 3TU c CESTEH/ENSP/FIOCRUZ, Rua Leopoldo Bulhoes 1480, Manguinhos, 21041-210 Rio de Janeiro, Brazil

J. Cvačka, J. Barek, J. Zima, A. G. Fogg and J. C. Moreira, Analyst, 1998, 123, 9R DOI: 10.1039/A705097F

Preliminary studies of the feasibility of monitoring by cathodic stripping voltammetry the hydrolysis of two further types of reactive dyes have been made. The azo reduction peak in differential pulse cathodic stripping voltammograms of the 2,3-dichloroquinoxaline reactive dye, Reactive Red 41, and in those of its hydrolysis product are sufficiently separated for the hydrolysis of Reactive Red 41 to be followed using the heights of these peaks. In the case of the 1,4-dichlorophthalazine reactive dye, Reactive Red 96, the azo peaks of the reactive and hydrolysed dyes are too close to be used to monitor the hydrolysis reaction, but peaks associated with reduction of the 1,4-dichlorophthalazine group are present which could be used to monitor the hydrolysis of Reactive Red 96.