Formal Standard Potentials Research Articles

Interfacial potential difference for the liquid/liquid ion partition process

Abstract Studies of the equilibrium interfacial potential difference at the liquid/liquid interface and the kinetics of the ion partition process are presented. The standard formal potentials of TMA + (+ 0.044 V), TEA + (−0.051 V), picrate − (+0.042 V), ClO 4 − (−0.087 V), and MnO 4 − (−0.080 V) ions are determined by a new method and are consistent with the available published values. A theoretical model is described to account for the rate of change of the interfacial potential difference during the ion partition process. Such rates are dependent upon a number of factors such as the initial concentrations, the transfer rate constants, and the diffusion coefficients of transferring ions. A numerical method is used to calculate the interfacial potential values as a function of time according to several convolution equations. The measured interfacial potential-time curves are similar to the theoretically calculated ones. Thus, the model helps the understanding of mechanisms of interfacial ions transfer and the rate of change of interfacial potential difference. The ion transfer process is started by a fast interfacial ion transfer stage, followed by a transfer/diffusion combined stage, and ended by a slow diffusion stage.

Copper(I), silver(I) and gold(I) solvation in nitriles

The formal standard potentials of the Cu/Cu+, Cu/Cu2+, Ag/Ag+ and Au/Au+ couples have been measured in acetonitrile, propionitrile and benzonitrile in 0.1 mol dm−3 tetraethylammonium perchlorate medium at 25°C. The emf data were used to estimate the Gibbs free energies of Cu+, Cu2+, Ag+ and Au+ transfer from water to nitriles. Determined transfer energies are negative for Cu+, Ag+, and Au+ and positive for Cu2+. The disproportionation constant KD=[Cu2+]/[Cu+]2 has been calculated for all nitriles studied. The results are discussed in terms of ionic solvation.

Current-reversal chronopotentiometry at a dropping mercury electrode

A theoretical study for a slow charge-transfer reaction, in current-reversal chronopotentiometry at a DME, using a preceding blank period, is presented. When t 1⪢ t, the equations obtained here become those reported in the literature at stationary plane electrodes. In addition, an easy method of determining the kinetic parameters of the electrode reaction and the formal standard reduction potential is presented.

A spectroelectrochemical study of carboxymethylated cytochrome- c

Direct electrochemical reduction of dicarboxymethylated cytochrome c [(Cm)-cyt c] has been carried out to investigate the effect of the displacement of methionine 80 (Met 80) from the sixth coordination position on the redox potential of heme iron. Differential pulse and cyclic voltammetry were employed in the present study. At gold microelectrodes or gold-coated RVC electrodes, the reduction process occurs in the absence of mediators. The rate of heterogeneous charge transfer for (Cm)-cyt c appears to be greater than that operative in native cytochrome c. A value of −0.218 versus s.h.e. at neutral pH was determined for the formal standard potential ( U′ 0,7) from spectroelectrochemical measurements. The large change in U′ 0 of the modified cytochrome c with respect to the native protein (Δ U ⋍0.5 V) is in agreement with rearrangement of the tertiary structure induced by rupture of the Met 80-heme iron bond, which leads to a high degree of heme exposure to the solvent. In addition, the results support the important role of Met 80 in assuring stability, through its bond with the heme iron, to the close crevice structure.

Electrode Potentials of Metal Chlorides in Binary and Ternary Fused Salt Solutions with Alkali Chlorides

The standard formal electrode potentials of several metals have been determined from 700–900 °C in NaCl, KCl, CsCl or the equimolar mixtures of these salts using galvanic or redox cells with a silver-silver chloride reference electrode. The activity coefficients and other partial molar properties of these solutions were calculated as a function of temperature from the emf measurements.

Determination of standard potentials and electron-transfer rates for halobiphenyls from electrocatalytic data.

Electron-transfer properties of halogenated biphenyl (PCB's and PBB's) are important in their synthetic, environmental, and biological transformations, many of which involve radical pathways. The electrocatalytic method was used to obtain standard formal potentials (E/sup 0/') and homogeneous electron transfer rate constants (k/sub ss/) for five halobiphenyls of representative structures. A nonlinear regression method was developed to obtain the best fit of theory to the log k/sub 1/ vs. E/sub PQ//sup 0/ data and yielded optimized values of parameters. Precisions of 2-7 mV in E/sup 0/' and 0.1-0.4 in log k/sub ss/ were improved over those obtained graphically. A correlation was observed between E/sup 0/' of the halobiphenyls and biological activity. 25 references, 1 figure, 8 tables.

Electrochemical Properties of Adriamycin Adsorbed on a Mercury Electrode Surface

Abstract The electrochemical behavior of adriamycin has been studied by means of cyclic d.c. and a.c. voltammetry with a hanging mercury drop electrode, quinizarin being used as a model compound. Both adriamycin and quinizarin are strongly adsorbed on the mercury electrode with their aromatic ring planes oriented parallel to the electrode surface, and give two sets of reduction waves. The first wave, due to the quinone moieties of the adsorbed adriamycin and quinizarin, has been explained on terms of a two-step one-electron surface redox reaction. The formal standard redox potentials, semiquinone formation constants, and charge transfer rate constants of the surface redox reaction of the quinone moieties have been determined. Although the reduced form of adriamycin is not very stable chemically, it is converted to a stable and electrochemically active form for a few minutes at pH 4.54. The second wave appearing at more negative potential would be due to a kinetic or catalytic process.

Electrochemical and structural studies of tera-heme proteins from desulfovibrio —standard potentials of the redox sites and heme-heme interaction

The macroscopic standard (formal) potentials of the four hemes in cytochromes c 3 from Desulfovibrio vulgaris Miyazaki ( DvM) and Hildenborough ( DvH), which Show reversible voltammetric responses, are determined by a least-square fitting to the analytical equation for the differential pulse polarograms. The potential differences between the macroscopic formal potentials for the interdomain redox sites, which are Heme-3 and Heme-4, are only −18 mV for cytochrome C 3 from DvM and −8 mV for that from DvH in 0.03 M phosphate buffer at pH 7.0 and 25 °C. These small potential differences suggest, at least, an attractive interaction between these hemes. The π-π interaction between Heme-3 and Heme-4 through Phe-20 and His-25 leads to the shift of the formal potential of the proximate hemes.

Electrochemistry of uranium(IV) in acidic AlCl3 + N-(n-butyle)pyridinium chloride room-temperature molten salts

The electrochemistry of uranium in acidic AlCl 3 + N -(n-butyl)pyridinium chloride was investigated. In 2:1 melts, the reduction of U(IV) to U(III) on glassy carbon electrodes is irreversible (kinetic parameters for this electrode reaction are reported). Measurements of the formal standard potential of the U(IV)/U(III) redox couple as a function of the melt acidity indicate that U(III) exists as U 3+ while U(IV) is present as chlorocomplexes, UCl x (4− x )+ with 3≥ x ≥1 (in 2:1 melts, UCl 3+ is the major U(IV) species. The oxidation of U(IV) to U(V) on glassy carbon electrodes is irreversible and strongly dependent on the melt acidity. Spectroscopic measurements indicate that U(V) in those melts exists as a chlorocomplex with the metal atom in an almost regular octahedral field. In 2:1 melts, a second anodic wave for the oxidation of U(V) to U(VI) is observed at potentials more positive than the thermodynamic potential limit of the solvent; this wave merges with the chlorine evolution reaction if the acidity of the melt is decreased.

Electrochemistry of uranium(IV) in acidic AlCl 3+ N-(n-butyl)pyridinium chloride room-temperature molten salts

The electrochemistry of uranium in acidic AlCl 3+ N-(n-butyl)pyridinium chloride was investigated. In 2:1 melts, the reduction of U(IV) to U(III) on glassy carbon electrodes is irreversible (kinetic parameters for this electrode reaction are reported). Measurements of the formal standard potential of the U(IV)/U(III) redox couple as a function of the melt acidity indicate that U(III) exists as U 3+ while U(IV) is present as chlorocomplexes, UCl x (4− x)+ with 3≥ x≥1 (in 2:1 melts, UCl 3+ is the major U(IV) species. The oxidation of U(IV) to U(V) on glassy carbon electrodes is irreversible and strongly dependent on the melt acidity. Spectroscopic measurements indicate that U(V) in those melts exists as a chlorocomplex with the metal atom in an almost regular octahedral field. In 2:1 melts, a second anodic wave for the oxidation of U(V) to U(VI) is observed at potentials more positive than the thermodynamic potential limit of the solvent; this wave merges with the chlorine evolution reaction if the acidity of the melt is decreased.

A correlation between the half-wave potential and the micelle-solubilization equilibrium of ferrocene in cationic micellar solutions

The reversible half-wave potential of the ferricinium/ferrocene redox couple in aqueous 0.2 M Li 2SO 4, (0.165±0.005) V vs. SCE, is determined directly by cyclic voltammetry of ferrocene, in spite of its poor solubility. The reversible half-wave potentials of this couple in cationic micellar solutions of alkyltrimethylammonium bromide are independent of the ferrocene concentration, showing that the pseudo-phase model or the Poisson distribution is applicable to these micellar solutions. The relation between the half-wave potentials in the water and the micellar solution is formulated on the basis of the micelle-solubilization equilibrium. The solubility data and the above half-wave potential data of ferrocene together offer corroborating evidence for the validity of this relation. This fact means that the voltammetric technique can be applied to a complicated micellar system to yield a clear-cut conclusion, and can be used in conjunction with the solubility measurement to determine the formal standard potential of a substance which is insoluble in water but can be solubilized in micellar solution.

Electrode reaction schemes and standard potential of solvated electron electrode in methylamine

Potential-sweep and cyclic voltammetric studies on solutions of LiCl, NaI, KI and CsI in methylamine indicate that the cathodic reaction is the dissolution of electrons from the electrode metal into the solution and/or the deposition of alkali metal M on to the electrode, depending on the solubility of M. On the basis of these findings, the electrode reaction schemes for the respective salt solutions are written in terms of the solvated electron species: free solvated electron e s − , ion pair (M + ·e s − ) and alkali anion M − . These species involved in the prevailing reaction paths agree with those proposed by pulse radiolysis studies on alkali metal methylamide solutions in methylamine. The formal standard potential of the metal/e s − electrode found is −2.90±0.02 V vs. Ag/AgNO 3 (0.02 M ) at −50°C. From this value the chemical solvation energy for e s − is evaluated as −1.26 eV.

Electrode reaction schemes and standard potential of solvated electron electrode in methylamine

Potential-sweep and cyclic voltammetric studies on solutions of LiCl, NaI, KI and CsI in methylamine indicate that the cathodic reaction is the dissolution of electrons from the electrode metal into the solution and/or the deposition of alkali metal M on to the electrode, depending on the solubility of M. On the basis of these findings, the electrode reaction schemes for the respective salt solutions are written in terms of the solvated electron species: free solvated electron e s −, ion pair (M +·e s −) and alkali anion M −. These species involved in the prevailing reaction paths agree with those proposed by pulse radiolysis studies on alkali metal methylamide solutions in methylamine. The formal standard potential of the metal/e s − electrode found is −2.90±0.02 V vs. Ag/AgNO 3 (0.02 M) at −50°C. From this value the chemical solvation energy for e s − is evaluated as −1.26 eV.

Electrochemical Oxidation and Reduction of Flavin Mononucleotide Adsorbed on a Mercury Electrode Surface

Abstract The electrochemical redox reaction of flavin mononucleotide (FMN) adsorbed on a hanging mercury drop electrode was studied in a pH 6.9 phosphate buffer and a pH 4.9 acetate buffer by means of cyclic d.c. and phasese lective a.c. voltammetry. Both the oxidized and reduced forms of FMN arc strongly adsorbed on the electrode surface, and a stable adsorption layer of FMN((FMN)ad) is formed. The cyclic d.c. and a.c. voltammetric behavior of (FMN)ad is explained by equations for a two-step one-electron surface redox reaction. The formal standard redox potentials, semiquinone formation constants, and charge transfer-rate constants of the surface redox reaction of FMN at the mercury electrode were determined.

Electrochemical Oxidation and Reduction of Ferredoxin Adsorbed on Mercury Electrode Surface. Phase-selective A. c. Polarography at DME

Abstract The electrochemical behavior of Clostridium pasteurianum ferredoxin (Cl.Fd) was studied by a.c. polarography with the dropping mercury electrode. The a.c. polarograms observed in acidic media (pH<7) were assigned to RSH/RSHg redox reaction of the cysteine residues of apoferredoxin, whereas those observed in basic media (pH>8) were assigned to a two-step one-electron transfer (EE mechanism) to the iron-sulfur clusters of Cl.Fd adsorbed on the mercury electrode surface ((Cl.Fd)ad). The a.c. polarographic behavior of (Cl.Fd)ad observed in basic media was interpreted by theory of a.c. polarography of a two-step surface redox reaction. The formal standard oxidation-reduction potential of (Cl.Fd)ad was determined to be −0.33 V vs. NHE at pH 7.0, and its pH dependence was −30 mV/pH between pH 8 and 10.

Electrode Kinetics of Solvated Electrons in Liquid Ammonia

AbstractElectrode reaction of ammoniated electrons was investigated with voltammetry, potentiometry and chronoamperometry. Experiments were conducted on Pt, Au and W electrodes in 0.5 M LiI, NaI, KI, CsI and KBr solutions of liq. NH3. The results in dilute solutions (< 10<−4 M) do not reflect the differences in alkali metal, electrode metal and supporting electrolyte and are explained by a simple one‐electron transfer process whose transferring species is a free solvated electron and its ion‐pair with a metal cation in a rapid equilibrium. The formal standard potential is −2.155 ± 0.005 V vs. Pb/Pb(NO3)2 (0.05 M) at −40°C in liq. NH3 involving 0.5 M alkali metal iodide. The lower limit of the averaged diffusion coefficient is 0.7 × 10−4 cm2 s−1. At higher metal concentrations (> 10−2 M), a two‐electron transfer process tends to prevail, which is explained by considering the dimer of ion‐pairs as the diffusing species.

Das Redoxverhalten von Bisbiphenylchrom(I)-jodid in Wasser

Bisbiphenylchromiumiodide [BBCr(I)J] is widely used as a “reference ion” in comparing redox potentials in different solvents. The voltametric behaviour in aqueous solutions however, is complicated by adsorption phenomena of the oxidized as well as the reduced form. Classical polarography, A—C polarography, cyclic voltametry, large scale electrolysis and pulsevoltametric methods have been employed to obtain information about the nature of the electrode reaction, its reversibility and the solubility of the reactants. Based on these data it is shown that the formal standard redox potential corresponds to the reactionBBCr(I)ads/BBCr(O)ads and has to be regarded as a “surface redox potential”.

The reversible redox systems R 6Sn 2/R 3Sn + on platinized platinum electrode

Electrochemical reversibility has been found for organometallic redox couples R 6Sn 2/R 3Sn +. Formal standard potentials E o at a platinized platinum electrode have been determined for the systems having R = Me, Et, Bu and Ph in methanol and in methanol/benzene. The presence of R 3Sn radicals at the platinized platinum electrode-solution interface seems to be responsible for the reversibility. The E o values obtained under different conditions, are discussed.

Potentiostatic determination of kinetic parameters of electrode reactions with generation of a reactant in situ

It is shown that kinetic parameters of simple, fast electrode reactions of the type O + ne = R, where R is soluble in the solution or in the mercury electrode, can be determined by the potentiostatic method with a solution initially containing only the substance O, the substance R being generated in situ during electrolysis, provided that the electrode reaction involves only a single rate-determining step. Current-time curves are recorded with a fast response, electronic potentiostat and an oscilloscope upon applying a potential step from the zero-current potential to various potentials on the ascending part of the current-potential curve. The forward rate constant k f at a given potential is calculated from the current at zero time found by extrapolation of the linear portion of the plot of current against square root of time, while the backward rate constant k b is calculated indirectly from the slope of the same straight line. Plotting log k f and log k b against potential allows a simultaneous determination of the formal standard rate constant k s, both cathodic and anodic transfer coefficients, the number of electrons involved in the rate-determining step and the formal standard potential of the system being studied. This method is considerably simpler than the well-known Gerischer-Vielstich method, and it should be particularly advantageous when R is highly reactive or forms an amalgam which is unstable in air. The upper limit of k a that can be determined by this method is the same as that determinable by the Gerischer-Vielstich method. The kinetic parameters found by the present method for the electrode reactions zinc ion-zinc amalgam in 1 M potassium nitrate, copper II ion-copper amalgam in 1 M potassium nitrate and cadmium ion-cadmium amalgam in 0.5 M sodium sulphate were in fair agreement with the values reported in the literature.

A potentiometric study of the semiquinone of n,n'-diphenyl- p-phenylenediamine

N,N'-Diphenyl- p-phenylenediamine was oxidized at 25° coulometrically at constant current using electrolytically generated bromine in 90% glacial acetic acid at a pH of —0.1. The course of the oxidation with time was followed potentiometrically using a platinum indicating electrode and a saturated silver chloride reference electrode. Only one inflection point was observed. This corresponded to a one-electron step, providing definite evidence of the existence of a semiquinone. The formal standard reduction potential, E 1°', for the first step was found to be 0.563 V vs. N.H.E., and E 2°' for the second step was 0.781 V vs. N.H.E. The value of 5250 ± 50 for the semiquinone formation constant, K, corresponding to the equilibrium O + R = 2S was determined.