T. P. Aleksandrova

Voltammetric studies of the activity of an electrode made of a graphite-epoxy composite in redox reactions of ascorbic acid and hydroquinone

The potentials of the anodic peak of ascorbic acid oxidation and the potential differences of anodic and cathodic peaks (ΔEp) of the hydroquinone/benzoquinone redox system at an electrode made of a graphite-epoxy composite are determined in weakly acidic and neutral supporting electrolytes by direct and cyclic voltammetry. The results obtained are compared with thermodynamic values and with the available values of these parameters at different solid electrodes for the above-mentioned redox systems. The effect of aging of the surface of electrodes made of graphite-epoxy composites on the potentials and peak currents of the anodic oxidation of ascorbic acid are studied. It is demonstrated that the regeneration of the electrode surface by mechanically cutting thin layers is important for reducing the δEp value of the hydroquinone/benzoquinone redox system down to 28–30 mV in supporting electrolytes with pH 2.0 and 7.0. This value is typical of thermodynamically reversible electrode reactions involving two-electron transfer at 20–25°C.

Voltammetric determination of dihydroxybenzenes at a mechanically renewed electrode made from a graphite-epoxy composite

Differences of potentials of anodic and cathodic peaks (ΔEp) are determined in cyclic voltammograms of dihydroxybenzene/p-benzoquinone redox systems at an electrode made of a graphite-epoxy composite in a wide pH range. The data obtained (ΔEp = 29 ± 1 mV) are close to the thermodynamic values for two-electron reversible reactions. This indicates that the electrode mechanically renewed by cutting a 0.5-μm surface layer directly in a test solution exhibits a high activity in such electrochemical reactions. The potentials of anodic and cathodic peaks are proportional to the pH of the supporting electrolyte solution in the range from 1.0 to 9.0. A change of 58 ± 1 mV in Ep per unit pH for all isomers shows that the first stage of the oxidation of each dihydroxybenzene isomer involves one electron and is accompanied by the detachment of one hydrogen ion, that is, an intermediate oxidation product, semiquinone, is formed. Despite the closeness of the potentials of hydroquinone and pyrocatechol peaks (ΔE = 100 mV), a scheme is proposed for the selective voltammetric determination of dihydroxybenzene isomers in a 0.1 M HCl solution in hydroquinone-pyrocatechol, pyrocatechol-resorcinol, and hydroquinone-resorcinol binary mixtures. The concentrations of hydroquinone and pyrocatechol are found from cathodic peaks and that of resorcinol, from the anodic peak. The results are well reproducible and contain no systematic error.

Stripping voltammetry of bromide and iodide ions at a renewal silver electrode

The behavior of bromide and iodide ions at a silver electrode renewed by cutting off a thin 0.5-μm surface layer was studied. The advantage of this method of electrode renewal over some variants of mechanical renewal was demonstrated. It was shown that bromide and iodide ions can be determined in concentration ranges from 10-6 to 10-3 M and from 10-7 to 10-3 M, respectively.

Electrochemical behavior of hydrazine at mechanically renewed solid electrodes

The electrooxidation of hydrazine was studied at metallic wire electrodes made of Co, Ni, Ag, Cu, and a graphite-epoxy composite by anodic and cyclic voltammetry with a linear potential sweep in alkaline-supporting electrolytes. Electrode working surfaces were regenerated by mechanically cutting a thin 0.5-μm layer in situ before each polarization cycle. The effects of the electrode material and the renewal of its surface on the parameters of anodic voltammograms of hydrazine were demonstrated. Hydrazine anodic peaks obtained at clean surfaces of electrodes fabricated from Ni, Ag, and a graphite-epoxy composite and also peaks at the potentials of the oxide formation on Ni and Cu electrodes can be used as analytical signals for the voltammetric determination of hydrazine.

High-voltage electrodeposition conditions in the stripping voltammetry of metal ions: A mechanism and analytical prospects of the process

It was found that the exaltation phenomenon in the parallel reduction of hydrogen ions from weak acids or water is responsible for the acceleration of the electrodeposition of metals under high-voltage conditions. Under these conditions, the rate of electrodeposition can be increased by 2 to 3 orders of magnitude compared to the rates of electrodeposition under standard conditions. At a constant high voltage of electrodeposition, the stripping peak currents of metals are proportional to the deposition time and the bulk concentration of analyte ions. Exaltation phenomena can be used in analysis not only for intensifying the process, but also for lowering the limit of detection, improving the selectivity, and decreasing the adverse effect of surfactants.

Peculiarities of electrodialysis processing of mineralized human exometabolites

The possibilities of the electrodialysis method in processing solutions of mineralized human exometabolites, where the dialysate is used for watering plants and the concentrate is subjected to further processing to afford common salt, are considered. The effect of the degree of solution purification from organic compounds on the electrodialysis rate is studied. Solutions are purified on carbon sorbent Sibunit which can be repeatedly recovered. The optimal parameters of the sorption stage of purification, namely, solution pH 2–4; contact time 1.5–2 h, specific consumption of sorbent 300 g/L, are determined. It is shown that the preliminary enzymatic degradation (enzymolysis) of urea by urease and, especially, the sorption cleaning of solution considerably accelerate the transmembrane mass transfer, remove complications of this process (precipitation), and favor both the dialysis with the preset degree of desalination with respect to sodium ions (which inhibit the plant growth) and the formation of the organic-free concentrate that can be used in the further extraction of dietary salt.

High-voltage accumulation of metals as a tool to decrease the detection limit of anodic stripping voltammetry

High-voltage accumulation (HVA) of metals on electrodes is shown to increase the rate of metal deposition as well as allow a substantial decrease of the detection limit of anodic stripping voltammetry (ASV). Using silver accumulation as an example, it is shown that advantages go along with keeping the reproducibility of analytical signal, its concentration dependence and dependence on accumulation time as usual for stripping voltammetry.

Correlation of the electrochemical reversibility and resolution of the anode and cathode peaks of the electrode processes of hydroquinone and metol on graphite and platinum

Cyclic and direct voltammetry with linear potential sweep was used to study the potential difference of the anode and cathode peaks for hydroquinone and metol in the cyclic voltammograms (CVA) of their individual solutions on the main factors affecting the reversibility of the electrochemical process on electrodes of a graphite-epoxy composite (GEC), namely, the state of the surface of the indicator electrode, the pH of the buffer solution, and the mode of polarization. The surface state of the GEC electrode was affected by its passivation in air for various periods of time; the reversibility of the electrode process was judged by the difference between the anode and cathode potentials on the CVAs of hydroquinone and metol. A correlation was found between the degree of reversibility and the difference of peak potentials for similar electrode processes of hydroquinone and metol on the GEC electrode and other solid electrodes made of graphite materials and platinum.

Electrochemical behavior of the system ferricyanide-ferrocyanide at a graphite-epoxy composite electrode

Cyclic and direct voltammetry with linear potential sweep has been used for the investigation of the dependence of the reversibility and reduction current in the system Fe(CN)63−/Fe(CN)64− on the concentrations of LiCl, NaCl, KCl, and CsCl solutions. The electrode was made of a graphite-epoxy composite and activated by mechanically cutting a surface layer directly in the solution and deactivated by the long-term storage in the air. The selected type of the graphite electrode and the method used to activate its surface provides the reversibility and diffusion control of the electrode process in the system Fe(CN)63−/Fe(CN)64− regardless of the composition of the supporting solution. In the case of the deactivated electrode, the degree of irreversibility of this process depends on the form and concentration of metal chloride in the supporting electrolyte and the diffusion transfer is complicated by the adsorption of compounds formed between the ferricyanide and the cation of the supporting solution.

Exaltation effects in the metal electrodeposition from acid electrolytes

The exaltation of mass transfer of discharging cations, caused by the concurrent hydrogen evolution from acidified solutions, results in a sharp increase in the metal deposition rate. In this case the limiting process rate depends linearly on the hydrogen evolution current density; it depends but weakly on the solution agitation and temperature. Under the electrolysis of solutions containing weak acids as a supporting electrolyte, the higher the acid formation constant, the more pronounced is the dependence of the electrodeposition limiting rate on the hydrogen current density. When microelectrodes are used, the varying of the background acid nature may affect the hydrogen evolution rate markedly, while the metal electrodeposition rate mainly depends on the cell voltage. When metals are electrodeposited from complex anions, the parallel hydrogen evolution hinders the mass transfer; this process depends on the anion stability constant and charge, all other conditions being the same. The found peculiarities can be used in the inversion voltammetry for the lowering of the metal detection limit, improving of the analysis selectivity, and time saving.