Unified treatment of the potentiostatic reduction on an expanding mercury electrode of complexes at any concentration ratio of reacting species

The article describes a general treatment of voltammetry of equilibrium complexation reactions at an electrode expanding in accord with any power law, the stationary electrode and the dropping mercury electrode being special cases. Reversible, irreversible and quasireversible electrode reactions are embraced by the treatment. A general expression for the current—voltage characteristic was derived, which was applied to the case of the polarographic reduction of complex ion, where either the simple or complex ion is electrochemically active. An equation accounting for the surface concentration of all reacting species is derived, the solution of which under appropriate conditions gives the equations of Lingane, Matsuda and Ayabe, etc. Correlations of potential, current and wave shape with complexing agent concentration are presented and discussed. A criterion for the splitting of the polarographic wave in the absence of excess of the complexing agent is proposed. Experimental results of several papers are discussed in the light of these criteria and a satisfactory agreement between the results and the predictions is established.     

Amperometric titration of mercury with a stationary platinum electrode in stirred solutions

Amperometric titration of mercury (in the range of 0-4 to 3.5 mg) has been carried out with 2-mercaptobenzoxazole with a stationary electrode in stirred solution in a sodium acetate-acetic acid medium.     

A simple model for the electrochemical oscillations on a stationary mercury electrode in bromate solutions

A simple model for the electrochemical oscillations on a stationary mercury electrode in bromate solutions has been proposed and its mathematical simulation carried out on a qualitative level. We believe it can be added to the simplest mathematical models of chemical oscillators, since it consists of only four reaction components.

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Characteristics of an amperometric flow-through detector with a renewable stationary mercury electrode

The construction of the detector is described and its performance is assessed for 1,4-benzoquinone by flow-injection analysis. The detection limit is 0.5 ng and the relative standard deviation of the peak current is 0.4% for a 102-ng sample. The peak current is highly sensitive to temperature changes of the mercury electrode system. The relative temperature coefficient is 15% K^-1, whereas the corresponding value with respect to the mobile phase is 0.6% K^-1. The dependence of the peak current on the volume flow rate is described and explained by appropriate models.     

Determination of nitrate in natural waters by voltammetry at a stationary mercury drop electrode

Nitrate can be determined by second-sweep cyclic voltammetry at a stationary mercury drop electrode utilizing the autocatalytic effect of the hydroxyl ions formed at the surface of the electrode during the reduction of nitrate in the presence of an excess of trivalent cations. The reduction current in the second sweep with the same drop is proportional to the nitrate concentration in the range 1–1500 μmol l^?1 in natural waters. The humic substances present in natural waters have a favourable effect on the determination of nitrate. The method is applied to the determination of nitrate in drinking and river waters.     

Electrochemical reduction of tricarbonyl compounds: Reduction mechanism of alloxan on a mercury electrode

A study has been carried out on the reduction mechanism of alloxan over a DME corresponding to the first two-electron transfer over the pH range 0–12.Polarographic and voltammetric results show the reduction process to be kinetically controlled. Rate and equilibrium constant data for the prior chemical reaction have been evaluated and several reaction mechanisms for different pH zones have been proposed.     

Reduction mechanism of cobalt(II)-ammonia complexes on a dropping mercury electrode

The mechanism of the polarographic reduction of cobalt(II) complexes with ammonia at a dropping mercury electrode over a wide ligand concentration range was investigated. It was shown that the Co(II) aquo ion and the Co(NH₃)²⁺ and Co(NH₃²⁺₂ complexes participate in the electrode process. Transfer coefficients, α, for these species and the electrode reaction rates were evaluated. Stability constants of Co(II) complexes with ammonia in 0.5 M ammonium perchlorate were determined on the basis of the polarographic wave equation of totally irreversible reduction of complex specie.     

The adsorption of tetraethylammonium ions at a mercury electrode from perchlorate solutions

The adsorption of tetraethylammonium ions at the mercury-perchlorate solution interphase has been studied. Solutions at constant perchloric acid activity have been prepared and the use of several perchloric acid activities allows us to calculate the simultaneous adsorption of both ions: tetraethylammonium and perchlorate. An interphase model is described to explain co-adsorption of both ions.     

Reduction mechanism of benzoin on a mercury electrode

The electrochemical reduction of benzoin was studied in acid media at DME. Convolution potential sweep voltammetry was applied to obtain the charge transfer rate constants. The overall reduction corresponds formally to an irreversible two-electron, two-proton transfer. Deoxybenzoin was identified as the final product of the benzoin reduction. Low-temperature bulk electrolysis experiments showed the presence of an unstable enol intermediate. This compound undergoes an enol-keto transformation, whose acid catalyzed rate constants were determined by chronoamperometry. The kinetic results indicate a stabilization of the intermediate carbocation in the presence of perchlorate.     

Mechanism of the voltammetric reduction of phenolphthalein at the mercury electrode inDMF

The electrochemical reduction of phenolphthalein in dimethylformamide solution containing 0.1 mol dm^–3 tetraethylammonium perchlorate at the hanging dropping mercury electrode showed an irreversible two-electron voltammetric peak. It was found that the CV peak is diffusion-controlled at low concentrations (0.4 mmol dm^–3). At higher concentration (0.5 mmol dm^–3) a postpeak was developed besides the diffusion-controlled one which was assigned to the adsorbed depolarizer. Cyclic voltammetric studies indicate that phenolphthalein follows an ECEC mechanism. Convolution and deconvolution potential sweep voltammetry confirm that mechanism.

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Mechanismus der voltametrischen Reduktion von Phenolphthalein an der Quecksilberelektrode inDMF

Zusammenfassung Die elektrochemische Reduktion von Phenolphthalein an der tropfenden Quecksilberelektrode in Dimethylformamidlösung mit einem Gehalt von 0.1 mol dm^–3 an Tetraethylammoniumperchlorat zeigte ein irreversibles voltametrisches Maximum für zwei Elektronen. Es zeigte sich, daß der CV-Peak bei niederen Konzentrationen (0.4 mmol dm^–3) diffusionskontrolliert ist. Bei höheren Konzentrationen (0.5 mmol dm^–3) entwickelte sich ein nachkommendes Maximum neben dem diffusionskontrollierten, welches dem adsorbierten Depolarisator zugeordnet wurde. Untersuchungen mittels cyclischer Voltametrie zeigten, daß Phenolphthalein einem ECEC-Mechanismus folgt. Konvolutions-und Dekonvolutions-Potential-Sweep-Voltametrie bestätigten diesen Mechanismus.

     

Mechanism of the reduction of the alkaloids ofHaplophyllum at a dropping mercury electrode

Summary 1. The furanoquinoline alkaloids skimmianine, haploperine, and -fagarine are reduced at a dropping mercury electrode in 0.1 N (C₂H₅)₄NI and (C₂H₅)₄NOH in 80% ethanol forming tetrahydro or hexahydro derivatives according to the potential of the electrode.2. Carbostyryl and N-methylcarbostyryl are reduced under the same conditions to the corresponding dihydro derivatives.3. The proposed mechanism for the electrode processes is confirmed by a comparison of the UV and IR spectra of the substances before and after their electrolysis at a controlled potential with the spectra of synthetic compounds.Khimiya Prirodnykh Soedinenii, Vol. 3, No. 4, pp. 253–257, 1967     

Electrode reaction of zinc ions at a dropping mercury electrode in ammonium sulphate solutions

The electrode reaction of Zn(II) at a DME in aqueous solutions of (NH₄)₂SO₄ has been studied by d.c. and square-wave polarography at 25.0±0.1°C. The electrochemical kinetic parameters for the reaction are determined. The appearance of the second wave in the square-wave polarogram has been attributed to the low values of the rate parameter and the transfer coefficient.     

A study of superficial associations at a mercury electrode in the case of tetrabutylammonium halide solutions

The influence of the nature of the anion on the adsorption of the tetrabutylammonium cation from halide solutions at a mercury—solution interface has been studied. A series of measurements of the interfacial tension for such solutions also containing a non-adsorbable cation, have led to the evaluation of the adsorption of both the tetraalkylammonium cation and of the halide anion. The variation of global surface excesses of entropy and concentration with temperature have also been determined. The results obtained can be explained by the formation, at the surface of the electrode, of both paired and non-paired ions, the latter neutralizing the charge on the electrode. The role of hydrophobic forces whose importance has been recognized in the adsorption of neutral substances is stressed in the case of surfaceactive cations.     

Voltammetric reduction of finasteride at mercury electrode and its determination in tablets

Finasteride in hydroalcoholic solutions (ethanol/Britton-Robinson buffer, 30/70) exhibits cathodic response in a wide range of pH (-0.5 to 12) using differential pulse (DPP) and test polarography (TP). The reduction peak of finasteride at acidic pH, is a catalytic proton peak resulting from a mechanism involving a first protonation of finasteride followed by the reduction of the protons combined with finasteride in order to regenerate finasteride and liberate hydrogen. Based on the catalytic hydrogen wave, a novel method for the determination of finasteride can be proposed. For analytical purposes we selected DPP technique in an ethanol/0.0625 mol L(-1) H(2)SO(4) (30/70) solution medium. In this condition the I(p) varied linearly with finasteride concentration between 5 x 10(-5) and 5 x 10(-4) mol L(-1). Within-day and inter-day reproducibility's were adequate with R.S.D. values lower than 2%. The selectivity of the method was checked with both accelerated degradation trials and typical excipients formulations. The developed method was applied to the assay and the uniformity content of finasteride tablets and compared with the standard HPLC method. The DPP-developed method was adequate for the finasteride determination in pharmaceutical forms as that exhibited an adequate accuracy, reproducibility and selectivity. Furthermore, treatment of the sample was not required as in HPLC; the method is not time-consuming and less expensive than the HPLC ones.     

Interaction between a stationary mercury electrode and a hydrogen bubble in an aqueous sodium dodecyl sulphate solution

The stability and rupture of thin liquid films formed from an aqueous solution of Na₂SO₄ (0.05 mol dm⁻³) in the presence of 5 × 10⁻⁵ mol dm⁻³ sodium dodecyl sulphate between a stationary mercury electrode and a hydrogen bubble has been investigated as a function of electrode potential. The electrostatic component of disjoining pressure has been calculated using the results of capacity measurement for the mercury-solution interface. Special attention has been paid to films formed on positively charged mercury surfaces. In this case, despite the positive electrode polarization, the outer Helmholtz plane potential is found to be negative due to the high surface activity of the dodecyl sulphate anion. The van der Waals component of disjoining pressure has been calculated on the basis of a double sheath model of the two interacting surfaces, taking into consideration the orientation of the adsorbed surfactant layers at the interfaces. Calculations of the total disjoining pressure can explain film stability at negative mercury potentials, but do not explain film rupture when the polarization of the mercury is positive. The existence of a hydrophobic attractive interaction is postulated in the latter case.