Reversible Half-wave Potentials Research Articles

Electrode reaction of zinc ions at a dropping mercury electrode in concentrated chloride solutions

The electrode reaction of Zn(II) at a dropping mercury electrode has been studied by d.c. and square-wave polarography in concentrated aqueous solutions of lithium, sodium, and magnesium chlorides. The electrochemical rate parameter continuously increases with increasing concentration of MgCl 2, while the parameter first decreases, passes through a minimum, and then increases with increasing concentration of LiCl or NaCl. The analysis of reversible half-wave potential and rate parameter suggests that the reaction kinetics is mainly governed by the ion-ion interaction between zinc and chloride ions.

Polarographic Behavior of Tris(acetylactonato)ruthenium(III) in Aqueous and Acetonitrile Solutions

Abstract In tetraethylammonium perchlorate-acetonitrile solution, [Ru(acac)3] was reversibly reduced at the dropping mercury electrode to [Ru(acac)3]− with a conditional electrode reaction rate-contsant estimated to be ca. 0.2 cm s−1 or larger. The reduced form was stable in the solution under an inert gas atmosphere. In aqueous solutions, [Ru(acac)3] was reduced reversibly when the depolarizer concentration was less than 0.07 mol m−3 at 25 °C. At higher depolarizer concentrations, [Ru(acac)3] molecules were adsorbed onto the mercury surface, and the free molecules in solution were reduced through the adsorbed layer at a smaller rate; the adsorbed layer was removed at a certain negative potential, and the reversible reduction took place normally at the bare surface. The difference between reversible half-wave potentials observed in acetonitrile and aqueous solutions was discussed in terms of solvation energies of the oxidized and reduced forms.

A coulostatic double-pulse method for a fast electron-transfer process followed by a fast chemical reaction

A new technique is proposed for measuring the kinetic parameters of electrochemical electron-transfer reaction whose reductant or oxidant species is quite unstable and undergoes an irreversible first-order reaction in solution. The principle of this procedure is in quick generation of the unstable reactant and quick measurement of overpotential before decomposition occurs. The working electrode placed in a solution containing one of the reactants is polarized at a potential where no current flows. Then, by applying a coulostatic pulse to the cell, the potential of the working electrode is stepped to a potential beyond the reversible half-wave potential determined in advance by a separate experiment. At the very moment when the decaying potential reaches a potential chosen near the reversible half-wave potential, the second coulostatic pulse with reversed polarity is applied to the cell. Analysis of the potential vs. time curve after the end of the second pulse is performed in the same way as in the modified coulostatic pulse method. The technique makes it possible to measure the standard rate constant for the electron-transfer reaction involving an unstable reactant whose lifetime is as short as 10−5 s. It is applied to the reaction Co(en)33++eCo(en)22++en (en=ethylenediamine) in acidic and basic solutions. The standard rate constant of 0.5 cm s−1 and the transfer coefficient of 0.5 are obtained in the presence of a follow-up reaction whose first-order rate constant is as large as 104 s−1. They are in excellent agreement with the values obtained under the experimental conditions where no follow-up reaction occurs.

A coulostatic double-pulse method for a fast chemical reaction following an electron-transfer process

A new technique is proposed for measuring the rate of fast homogeneous chemical reaction following an electron-transfer process. Consider an electrochemical redox reaction whose reductant species is unstable and undergoes an irreversible chemical reaction in solution. An electrode placed in a solution containing the oxidant species is polarized at a potential where no current flows. Then a sharp coulostatic pulse is applied to the cell and the potential is stepped to another potential where the reductant species is generated at a diffusion-controlled rate. After an appropriate time, the second coulostatic pulse with the polarity opposite to the first one is applied to the cell and the electrode potential is stepped back to a potential near the initial one where the reductant species is reoxidized at a diffusion-controlled rate. The cell is put under the condition of open circuit after each end of the coulostatic pulses. The potential vs. time curve after the end of the second pulse is recorded and analyzed to obtain the first-order rate constant of the homogeneous chemical reaction. This technique has a particular advantage in measuring the rate of fast homogeneous chemical reactions with rate constants ranging from 10 3 to 10 6 s −1 . The method is applied to the reaction Co(en) 3 3+ + e Co(en) 3 2+ →Co(en) 2 2+ +en (en=ethylenediamine) in acidic solution. The observed value for the first-order rate constant, (1.0±0.1)×10 4 s −1 , is in fairly good agreement with the values obtained polarographically by assuming that the true reversible half-wave potential for the above electrode reaction is identical with the experimental halfwave potential in basic solution where no decomposition of Co(en) 3 2+ occurs.

Influence of adsorption of electroactive species on the interfacial admittance measured in a.c. polarography

The parameters describing the electrode admittance in the case of a reversible electrode reaction with reactant and/or product adsorption, are expressed as functions of the d.c. potential, assuming, that the adsorption obeys the Langmuir isotherm. On the basis of these expressions theoretical a.c. polarograms are predicted. It is shown that for relatively weak adsorption the polarographic peaks increase and shift away from the reversible half-wave potential. When the adsorption is stronger, the peaks split. When only one component is adsorbed, the observed behaviour is mainly due to the low-frequency capacity C LF. When both components are equally adsorbed, the high frequency capacity C HF is the most important.

A coulostatic double-pulse method for a fast electron-transfer process followed by a fast chemical reaction

A new technique is proposed for measuring the kinetic parameters of electrochemical electron-transfer reaction whose reductant or oxidant species is quite unstable and undergoes an irreversible first-order reaction in solution. The principle of this procedure is in quick generation of the unstable reactant and quick measurement of overpotential before decomposition occurs. The working electrode placed in a solution containing one of the reactants is polarized at a potential where no current flows. Then, by applying a coulostatic pulse to the cell, the potential of the working electrode is stepped to a potential beyond the reversible half-wave potential determined in advance by a separate experiment. At the very moment when the decaying potential reaches a potential chosen near the reversible half-wave potential, the second coulostatic pulse with reversed polarity is applied to the cell. Analysis of the potential vs. time curve after the end of the second pulse is performed in the same way as in the modified coulostatic pulse method. The technique makes it possible to measure the standard rate constant for the electron-transfer reaction involving an unstable reactant whose lifetime is as short as 10 −5 s. It is applied to the reaction Co(en) 3 3++ eCo(en) 2 2++en (en=ethylenediamine) in acidic and basic solutions. The standard rate constant of 0.5 cm s −1 and the transfer coefficient of 0.5 are obtained in the presence of a follow-up reaction whose first-order rate constant is as large as 10 4 s −1. They are in excellent agreement with the values obtained under the experimental conditions where no follow-up reaction occurs.

A coulostatic double-pulse method for a fast chemical reaction following an electron-transfer process

A new technique is proposed for measuring the rate of fast homogeneous chemical reaction following an electron-transfer process. Consider an electrochemical redox reaction whose reductant species is unstable and undergoes an irreversible chemical reaction in solution. An electrode placed in a solution containing the oxidant species is polarized at a potential where no current flows. Then a sharp coulostatic pulse is applied to the cell and the potential is stepped to another potential where the reductant species is generated at a diffusion-controlled rate. After an appropriate time, the second coulostatic pulse with the polarity opposite to the first one is applied to the cell and the electrode potential is stepped back to a potential near the initial one where the reductant species is reoxidized at a diffusion-controlled rate. The cell is put under the condition of open circuit after each end of the coulostatic pulses. The potential vs. time curve after the end of the second pulse is recorded and analyzed to obtain the first-order rate constant of the homogeneous chemical reaction. This technique has a particular advantage in measuring the rate of fast homogeneous chemical reactions with rate constants ranging from 10 3 to 10 6 s −1. The method is applied to the reaction Co(en) 3 3++ eCo(en) 3 2+→Co(en) 2 2++en (en=ethylenediamine) in acidic solution. The observed value for the first-order rate constant, (1.0±0.1)×10 4s −1, is in fairly good agreement with the values obtained polarographically by assuming that the true reversible half-wave potential for the above electrode reaction is identical with the experimental halfwave potential in basic solution where no decomposition of Co(en) 3 2+ occurs.

Kinetics of reduction, composition and the stability constants of the zinc–maleic acid complexes in aqueous medium

Polarographic reduction of the zinc-maleic acid complexes has been studied at the dropping mercury electrode ( dme) in aqueous medium at 30° and ionic strength 2.0 Gelling's method is used for determining the reversible half-wave potentials and the kinetics parameters (α and K s ) for the quasireversible reduction of the complexes. The value of α and K s varied from 0.41 to 0.35 and 2.58 × 10 −3 to 2.21 × 10 −3 cm/s respectively with the ligand concentration changing from 0.00 to 0.60 M. Analysis of the polarographic characteristics ,of the system by the DeFord and Hume method gave the following values of the overall stability constants: β 1 = 50±2; β 2 = 160± 10 and β 3 = 2250 ± 50. The overall stability constants of the zinc-maleic acid complexes have also been calculated using the recently developed method of Mihailov. The resulting values are: β 1 = 29.9, β 2 = 301.4 and β 3 = 2025. Percentage distribution of the complex species present in the system is shown as a function of log/ligand concentration.

ChemInform Abstract: ELECTROCHEMICAL REDUCTION OF SYM‐DIBENZOCYCLOOCTATETRAENE, SYM‐DIBENZO‐1,5‐CYCLOOCTADIENE‐3,7‐DIYNE, AND SYM‐DIBENZO‐1,3,5‐CYCLOOCTATRIEN‐7‐YNE

AbstractDie reversiblen Halbstufenpotentiale der drei Titelverbindungen, die Geschwindigkeiten des heterogenen Elektronentransfers und die Transferkoeffizienten werden mit Hilfe von ′cyclischen Voltammogrammen und Digital‐Simulationen von Voltammogrammen bestimmt, und die Kinetik der chemischen Folgereaktionen wird untersucht.

Electrochemical reduction of sym-dibenzocyclooctatetraene, sym-dibenzo-1,5-cyclooctadiene-3,7-diyne, and sym-dibenzo-1,3,5-cyclooctatrien-7-yne

A study of the electroreduction of sym-dibenzocyclooctatetraene (DBCOT), sym-dibenzo1,5-cyclooctadiene-3,7diyne (DBCOD) and sym-dibenzo-l,3,5-cyclooctatrien-7-yne (DBCOM) in DMF-TBAP solutions was carried out to investigate the effects of differences in the structure of the central eight-membered ring on the electrochemical behavior. The reversible half-wave potentials ( E , 1 2 9 , heterogeneous electron-transfer rate constants ( k s ) , transfer coefficients, and pseudo-firstorder rate constants of the following chemical reactions were determined by cyclic voltammetric-digital simulation techniques. The results are consistent with reduction of tub-shaped DBCOT to a planar radical anion and dianion and of planar or almost planar DBCOM and DBCOD to planar radical anions. Estimates of the energy of the conformational change were obtained by comparison of E 119 to calculated energies of the lowest unoccupied molecular orbital and from the k , values. The electrochemical behavior of cyclooctatetraene (COT) and related compounds and the electron spin resonance (ESR) of the associated radical anions have been the subjects of numerous investigations.2-8 The general picture which emerges is that reduction of the tub-shaped COT produces a planar or nearly planar radical anion (COT.-); the large change in molecular geometry leads to a high activation energy and hence slow electron-transfer rates. The second reduction step to the dianion involves only small changes in geometry and hence more rapid electron transfer. A more detailed explanation of the experimental results requires taking account of solvation changes, ion pairing effects, and following protonation reactions. Recently, Wong, Garratt, and Sondheimerg reported the synthesis of sym-dibenzo-l,5-cyclooctadiene-3,7-diyne (DBCOD) and sym-dibenzo1,3,5-cyclooctatrien-7-yne (DBCOM). The central eight-membered ring in these com-

ChemInform Abstract: STUDIES ON THE ELECTRODE PROCESSES OF OXOVANADIUM(IV). II. ELECTROLYTIC REDUCTION OF VANADYL ACETYLACETONATE IN ACETONITRILE SOLUTION AT MERCURY ELECTRODE

The electrolytic reduction of vanadyl acetylacetonate in acetonitrile solution was investigated by means of conventional and AC polarography, cyclic voltammetry and controlled-potential electrolysis. A well-defined polarographic cathodic wave was obtained in a solution containing excess acetylacetone. The wave-form was outwardly a single step with two-electron transfer. However, logarithmic analysis revealed that it consists of two steps. The first step is slightly irreversible, being accompanied by V–O bond breaking, and the second one is controlled by the rate of coupled chemical reactions. The reaction mechanism is discussed in detail. The reversible half-wave potentials of the species [VO(acac)2] and [V(acac)3] were determined to be −1.93 and −1.78 V vs. Ag/0.1 M AgClO4, respectively, from the logarithmic analysis.

Studies on the Electrode Processes of Oxovanadium(IV). II. Electrolytic Reduction of Vanadyl Acetylacetonate in Acetonitrile Solution at Mercury Electrode

Abstract The electrolytic reduction of vanadyl acetylacetonate in acetonitrile solution was investigated by means of conventional and AC polarography, cyclic voltammetry and controlled-potential electrolysis. A well-defined polarographic cathodic wave was obtained in a solution containing excess acetylacetone. The wave-form was outwardly a single step with two-electron transfer. However, logarithmic analysis revealed that it consists of two steps. The first step is slightly irreversible, being accompanied by V–O bond breaking, and the second one is controlled by the rate of coupled chemical reactions. The reaction mechanism is discussed in detail. The reversible half-wave potentials of the species [VO(acac)2] and [V(acac)3] were determined to be −1.93 and −1.78 V vs. Ag/0.1 M AgClO4, respectively, from the logarithmic analysis.

Convolution potential sweep voltammetryV. Determination of charge transfer kinetics deviating from the butler-volmer behaviour

Convolution potential sweep voltammetry is applied to the analysis of charge transfer kinetics. The possibility of using the method without assuming a priori the validity of a given kinetic law, e.g. the Butler-Volmer law as usually done, is illustrated on the example of the reduction of tert -nitrobutane in acetonitrile and dimethylformamide. It is found that the post-factum experimental kinetics indeed deviate significantly from the Butler-Volmer behavior, involving a dependence of the transfer coefficient α upon the electrode potential. It is shown that the experimental variations of α match the orders of magnitude predicted by the Marcus theory. A procedure is proposed and applied for determining the reversible half-wave potential for redox couples in which one species is not quite stable chemically by combining the cyclic voltammetry and CPSV data.

Convolution potential sweep voltammetry V. Determination of charge transfer kinetics deviating from the Butler-Volmer behaviour

Convolution potential sweep voltammetry is applied to the analysis of charge transfer kinetics. The possibility of using the method without assuming a priori the validity of a given kinetic law, e.g. the Butler-Volmer law as usually done, is illustrated on the example of the reduction of tert-nitrobutane in acetonitrile and dimethylformamide. It is found that the post-factum experimental kinetics indeed deviate significantly from the Butler-Volmer behavior, involving a dependence of the transfer coefficient α upon the electrode potential. It is shown that the experimental variations of α match the orders of magnitude predicted by the Marcus theory. A procedure is proposed and applied for determining the reversible half-wave potential for redox couples in which one species is not quite stable chemically by combining the cyclic voltammetry and CPSV data.

Standard electrode potentials of R-/R+ redox couples and medium effects in non-aqueous solvents.

The standard electrode potentials of R-/R+ redox couples may be estimated from polarographic reversible half-wave potentials of R/R+ and R-/R couples. When R+ and R- are large ions, similar in size and structure, the standard potential may be assumed as a solvent-independent point of electrode potential in various solvents. A few possible candidates for the reference redox couple have been discussed.

On the validity of the polarographic determination of stability constants of metal ions in the case of adsorbable electroactive species

Summary The idea that anion-induced adsoprtion of the electroactive species might interfere with the determination of stability constants via shifts in the half-wave potential is critically examined. It is shown that this adsorption occurs only if the standard thermodynamic potential at the electrode-solution interface differs from that in the solution. The final conclusion is that the position of the reversible half-wave potential is not influenced by the adsorption of the electroactive species if the reactions between the complexes, and all adsorption phenomena, proceed infinitely fast.

Logarithmic analysis of two overlapping D.C. Polarographic waves: IV. Quasireversible electrode processes

Summary The application of logarithmic analysis of two quasireversible overlapping d.c. polarographic waves is discussed. In this case, only the reversible part of the first quasireversible wave and the irreversible part of the second quasireversible wave can usually be separated from the original experimental logarithmic curve. One of the quasireversible waves can be completely constructed only in the case when the other is relatively small. The logarithmic analysis can also be applied to the anodic-cathodic quasireversible wave and a method for the determination of both transfer coefficients, reversible half-wave potential, and the corresponding rate constant of the electrode reaction is proposed.

On the estimation of the reversible halfwave potential

On the estimation of the reversible halfwave potential

On the estimation of the reversible halfwave potential

On the estimation of the reversible halfwave potential

Ac Polarography and its application to overcome the problem of Dc polarographic maxima in the study of complex ions

Dc polarograms of many species electro-active at the dropping mercury electrode exhibit marked maxima which make the theoretical analysis of the current/voltage curve difficult. For studies on complex ions of the electro-active species a knowledge of reversible half-wave potentials and diffusion currents is required. These parameters cannot conveniently and reliably be evaluated from polarograms exhibiting maxima and to make any useful calculations, gelatin or other maximum suppressors must be added. The technique of using a maximum suppressor is however unsatisfactory in the study of complex ions, as electrode reactions are frequently altered by its addition. More satisfactory approaches are here suggested, using ac polarographic methods. The chloride complexes of Bi(III) and Sb(III) have been investigated, as both these electrode reactions exhibit pronounced maxima in chloride media. Further difficulties in the use of the polarographic method of evaluation of the chloride complexes of these systems are discussed in detail. Four complexes were found for both systems and the values of the stability constants are reported.