Portugaliae
Electrochimica Acta

A PVC membrane containing the tetra ethyl ester of p-tert-butyl calix[4]arene as ionophore was prepared and tested for sodium ion in solutions with and without albumin (BSA), after confirmative experiments with respect to Nernstian behaviour, limit of detection, selectivity to K+, reproducibility and lifetime. Measurements of Na+ have been taken in solutions of albumin containing constant 0.01 mol dnT" NaCl solution. The protein concentration ranged from 0 to 100 g dm"3 in increments of 20 g dm"3. The reference electrodes were of two types, hypertonic (saturated calomel electrode) and isotonic (Ag, AgCl, 0.15 mol dm"3 KC1). Variation of potential with albumin concentration is larger than expected [2] and bigger for hypertonic than for isotonic reference electrodes. Explanation was found in the presence of sodium contamination of the protein.

Good laboratory practice means among other things, specifically for pH measurements, that measurements are done correctly and to have evidence that this has been done correctly. The several components of the pH System have to be considered. The pH meter has to provide raw data (mV) as well as the sample results. This is especially important during the calibration. The choice of the electrode is important as well as the sample type is critical. Once proved and validated the correct electrode and its correct filling solution the refilling procedure should be a part of the correct SOP (Standard Operating Procedures). Calibration points and frequency together with maintenance procedures also need to be included in the SOP's. Calibration standards must be traceable to international Standards, and therefore identifiable and dated. The various factors to affect the pH need to be understood. In particular, the temperature has different effects which need to be worked out on the proper way to arrive at a reproducible and accurate result. In particular, the patented Ross technology compensates several of the temperature effects, using an innovative galvanic pair, which is proven to have a faster answer to temperature changes and is more precise on the readings. Finally, recording and submitting data in proper records is very important in GLP environments as adherence to these procedures enhance the relevance of documentation.

The electrochemical oxidation of molinate, a thiocarbamate herbicide, has been studied in order to determine it in a phytopharmaceutical product. A square wave voltarnmetry electroanalytical method has been developed and the results were in good agreement with those obtained by the AO AC reference method using gas chromatography. The recovery is 100.2%.

Square wave adsorptive voltammetry at the hanging mercury drop electrode has been applied to the determination of folic acid in pharmaceutical preparations including complex mixtures of vitamins. Folic acid was extracted with an alkali solution in a single step process. One aliquot of this solution was analysed in a boric acid/sodium hidroxide buffer, pH 9.75, sweeping the potential from -0.50 to -0.90 V vs AgCl/Ag at pulse amplitude of 40mV and pulse frequency of 50Hz. The accumulation potential was -0,50V while the accumulation time was 10s. When other substances that might adsorve on the HMDE were present the accumulation potential was -0,75V and the time was 30s. The determination was carried out by using the standard additions method. Recoveries results between 96.0 and 99.7% were obtained.

A method for the determination of oc-ketoacids was developed, involving derivatization with o-phenylenediamine and the differential-pulse polarographic determination of the resulting hydroxyquinoxalines. The method has a detection limit of about 5x10"7 M a concentration that is lower than the usual concentrations of the a-ketoacids in fermented products; it is also possible to distinguish glyoxylic acid from pyruvic acid and a-ketoglutaric acid, with no need for a separation procedure. Using the method developed in the direct analysis of a white wine, practically no glyoxylic was found and a total concentration of 3xl0-5 M was obtained for the whole of pyruvic acid and a-ketoglutaric acid.

This paper describes the determination of salicylic acid in pharmaceutical preparations by flow injection analysis (FIA) using amperometric detection. The developed system allows the direct determination of the specie on aqueous solution after simple dilution of the formulations in phosphate buffer at pH 7. The determination of salicylic acid in pharmaceutical preparations were carried out in a single channel FIA manifold, giving recoveries between 99 and 103% with a sampling rates of 150 samples/h.

An immunosensor based on indirect amperometric immunoassay was developed for the determination of LPS. The immunosensor employed horseradish peroxidase labelled antirabbit Ig-G. Peroxidase is a heme containing enzyme that catalyzes the oxidation of a variety of organic and inorganic compounds by hydrogen peroxide [1,2]. In this work the reaction between hydrogen peroxide and hydroquinone was catalyzed by HRP and the electrochemical reduction of the oxidised benzoquinone [3] yielded a current that was related to the concentration of LPS used. LPS was immobilized on an IMMobilon AV membrane, incubated with anti-LPS rabbit serum and, in a second incubation step, with anti-rabbit Ig-G - HRP. Capping and washing procedures were made in between, similarly to what is done in ELISA. The membranes thus prepared were applied to an US biosensor electrode base, similar to what has been used before [4]. A potential of -150mV was applied from an Amperometric Biosensor Detector and the current corresponding to the reduction of benzoquinone was recorded. Different incubation times and antibody dilutions were tested. The immunosensor was found to be sensitive to LPS and have good reproducibility.

The simultaneous determination of Tl, In and Pb by differential pulse anodic stripping voltammetry was carried out using multivariate calibration methods. The ability of this methodology to overcome the problem of overlapping signals is demonstrated. The method was applied to analysis of ternary mixtures of these ions with successful results.

The square wave adsorptive stripping voltammetry of cobalt and nickel has been investigated at a wall-jet mercury thin film disc electrode in continuous flow. Following a study of the stripping response, experimental parameters were optimised and a sensitivity enhancement of an order of magnitude over similar differential pulse procedures was found. Applications are discussed.

The redox reaction of dopamine adsorbed layer on gold is studied. The adsorption takes place at open circuit condition. The adsorbed substance presents a reversible oxidation of the OH groups on the ring, and an irreversible oxidation at higher potentials. The dopamine metal bond is weak and the substance may be removed by a physical procedure. Moreover, multilayers are formed over the first layer. An estimation of the dopamine co-area over gold is reported.

Ti-supported electrodes of nominal composition Ti/Iro.3Ti(o.7-x) Cex02 (0

The formation of conducting polymer films on electrodes by electropolymerisation of activated pyrrolic esters together with their characterisation is presented. Electrochemical evidence for the formation of by-layer polymer films is also reported Modification of the films by reaction on the activated carbonyl esters is described.

This paper reports on the electrochemical characterisation of the oxide film grown on niobium in a borate buffer solution of pH 8.8 at room temperature. The formation of a duplex oxide, probably NbOx(x= 1.2) / Nb205, is suggested by cyclic voltammetry results. Peak currents and scan rates showed a linear dependency when plotted in the log-log plane, exhibiting a unity slope indicating control by a surface process. Electrochemical impedance spectroscopy confirmed the voltammetry results. Impedance spectra were conducted within the potential domains of both anodic peaks after 10 cycles at 200 mVs"1. Bode plot analysis and simulation by the equivalent circuit approach suggested an extra time constant when cycling was extended to include the second peak. The stability of the oxide film was tested in 0.5 M sodium chloride solutions at a low scan rate (0.2 mVs"1) showing no breakdown up to 2500 mV (SCE).

A uniformly rich molybdenum film was deposit on aluminium, without the use of external polarisation, using a buffer and a complexant. Potential - time curves showed that the coating is fully-grown within 60 seconds of immersion. Protective films formed within a narrow range of pH, from 4 to 6. Lower pH values, near those used for Cr treatment, gave rise to hydrous blue Mo(V) compounds with little or no protecting ability whilst higher pH values did not allow the coating to be formed probably due to insufficient thinning of the air formed alumina film. Stability of the produced coatings was assessed by polarising the samples to breakdown in a 0.5 M NaCl solution with added carbonate/bicarbonate. Optimisation of the coating produced a pitting resistance comparable to that of chromate conversion coatings. The mechanism of film formation is briefly discussed.

Tungsten can be successfully implanted on aluminium forming a solid solution. SIMS, XPS and AES analysis showed that almost no tungsten was found in the passive film formed on the alloy in 0.6M NaCl solution. The beneficial effect of tungsten in the increase of the pitting potential of the alloy is due to the ennobling of the pitting process caused by the presence of metallic tungsten. This element modifies the thermodynamics and kinetics of the dissolution and facilitates hydrogen evolution inside the incipient pits, diminishing the ability of the metal to maintain the critical pH associated to stable pitting.

This communication follows a previous work concerning indirect electrochemical reduction of dyes [1] and its application on dyeing control with sulphur dyes. The study concerns the cathodic peak intensity variation with sulphur dye C.I. Leuco Sulphur Black 1 (Black Diresul RDT liq.) concentration, in the presence of a mediator, iron/triethanolamine (FeWTEA), in alkaline media.

The electrochemical reduction of azo dyes in aqueous alkaline solutions is investigated. Cyclic voltammetry, controlled potential electrolyses, chronoamperometry and steady state voltammetry at microelectrodes were used to provide insight into the reduction process. With basis on the data obtained from the experimental techniques mentioned above, a mechanism for the reduction reaction is proposed.

A comparative study of heme proteins horse heart cytochrome c and cytochrome c3 of Desulfovibrio vulgaris (strain Hildenborough) using different electrochemical techniques (linear scan and cyclic voltammetry, differential pulse and square wave voltammetry) with platinum and gold macro and microelectrodes is presented. The electrochemical response was evaluated analysing the reversibility of the electrode process and the formal potentials determined. Diffusion coefficients for both proteins were also calculated from the voltammetric data. The electrochemical behaviour at macro and microelectrodes was analysed and the advantages of microelectrodes pointed out.

The electrochemical behavior of lignin, in alkaline medium, was studied by cyclic voltammetry and compared with that of some lignin model compounds e. g. guaiacol, eugenol, vanillic alcohol and vanillin, in order to elucidate the process involved in electrode reactions. Galvanostatic electrolyses of lignin were carried out using different electrode materials in order to investigate the influence of electrode material on the product distribution.