Apparent Standard Potential Research Articles

Spectroscopic and Electrochemical Study of the Adsorption of [Co(en)2Cl2]Cl on γ-Alumina: Influence of the Alumina Ligand on Co(III)/(II) Redox Potential

UV-visible and Raman spectroscopies as well as electrochemical techniques have been used to characterize cis- and trans-[Co(III)(en)2Cl2]Cl (en=ethylenediamine) complexes and the gamma-alumina-supported cis-Co((III)) complex. It is shown that the electrochemical reduction of these complexes occurs according to a multistage mechanism involving two electrochemical steps, with the formation of a dimer that was characterized by UV-visible spectroscopy (intervalence band at 670 nm). The apparent standard redox potential for each step has been determined, and experimental results reveal that cis and trans complexes present similar electrochemical characteristics. It is also shown that the deposition of trans-[Co(III)(en)2Cl2]+ on gamma-alumina leads to an inner-sphere complex (ISC) in a cis configuration in which Cl- ligands are substituted by OH or O- surface groups of alumina. These changes in the coordination sphere of the complex induce a substantial decrease of its apparent redox potential since it is -0.63 V/SCE (saturated calomel electrode) for the gamma-alumina-supported cis-Co(III) complex, whereas values of -0.17 and -0.35 V/SCE were determined in dimethyl sulfoxide (DMSO) for the trans and cis precursor complexes, respectively.

Dual-thermodynamic estimation of stoichiometry and stability of solid solution end members in aqueous–solid solution systems

The dual-thermodynamic (DualTh) approach is shown to provide a useful alternative to other methods in: (i) forward modelling of equilibrium speciation, activities, and element partitioning in a heterogeneous system involving several variable-composition phases, such as the aqueous–solid solution system; (ii) estimation of interaction parameters of a non-ideal mixing model from known bulk compositions of coexisting aqueous and solid-solution phases; and (iii) retrieval of unknown stoichiometries and apparent standard chemical potentials of trace solid-solution end-members. Inverse-modelling tasks (ii) and (iii) can be performed when the solid solution of interest is shown experimentally to co-exist with the aqueous phase either in the equilibrium or at the minimum stoichiometric saturation state. DualTh calculations exploit the ability of Gibbs energy minimisation (GEM) algorithms to find simultaneously two numerical solutions of the isobaric–isothermal chemical equilibrium speciation problem: (1) primal solution x — a vector of amounts of components (species) in phases; and (2) dual solution u — a vector of chemical potentials of stoichiometry units (usually chemical elements and charge). Conversely, the chemical potential of a phase component can be found in two complementary ways: (i) primal via its standard-state potential, concentration and activity coefficient (the latter two are functions of the x vector); and (ii) dual through its formula stoichiometry multiplied by the u vector. The DualTh methods compare primal and dual values of the chemical potential in simple and straightforward equations that can be easily computed in a spreadsheet, or implemented in GEM geochemical modelling codes.

Thermochemical properties of lanthanides (Ln = La, Nd) and actinides (An = U, Np, Pu, Am) in the molten LiCl–KCl eutectic

The electrochemical reduction of actinides (U, Pu, Np and Am) and lanthanides (La and Nd) chlorides was investigated by cyclic voltammetry and chronopotentiometry at different temperatures in LiCl–KCl eutectic. The diffusion coefficients of these metallic cations were estimated as well as their apparent standard potentials. These values of potentials are compared with existing data measured also by transient electrochemical techniques or e.m.f. measurements.

Electrochemistry of Uranium in Molten LiCl-KCl Eutectic

The Institute for Transuranium Elements (ITU) is building up an accurate database of actinide behavior in chloride melts in support of its nuclear fuel reprocessing development program. The electrochemical properties of uranium, dissolved in LiCl-KCl eutectic melt, were investigated by transient electrochemical techniques, such as cyclic voltammetry (CV) and chronopotentiometry on an inert tungsten electrode. It was shown that is reduced to by a two-step mechanism corresponding to and transitions. In the range, the diffusion coefficients of and were similar and equal to: and . The apparent standard potentials of and redox systems were vs. and vs. , respectively. Some thermochemical properties of uranium solutions were also derived from the electrochemical measurements. The Gibbs free energies of dilute solution of and in the LiCl-KCl were determined to be: and in kJ , respectively. In the range, the activity coefficients γ of and range between and , respectively.

An electrochemical study of hemoglobin in water–glycerol solutions

The effect the composition of a water–glycerol mixture has on the electrochemical properties of hemoglobin (Hb) is studied. With the increased glycerol concentrations, the peak-to-peak separation of hemoglobin is found to increase from ∼40 to 200 mV, with the apparent standard potential of hemoglobin negatively shifted, which demonstrate that the electron-transfer activity of hemoglobin will decrease at relatively high glycerol concentrations and the oxidized state of hemoglobin will be more stable with the increasing glycerol concentrations. Meanwhile, the electrocatalytic activity of hemoglobin to hydrogen peroxide, as well as the binding of ligands or effectors to hemoglobin in the presence of glycerol, are also been investigated. Our studies indicate that glycerol will decrease the electrocatalytic activity of hemoglobin, while have little effect on the microenvironment around the heme site.

A new film for the fabrication of an unmediated H 2O 2 biosensor

A novel and stable film made from polyethylene glycol (PEG) on pyrolytic graphite (PG) electrode was presented in this paper for incorporating horseradish peroxidase (HRP) to study the direct electrochemistry of the enzyme. In PEG film, HRP showed a thin-layer electrochemistry behavior. The apparent standard potential ( E°′) was −0.379 V versus SCE at pH 7.2. Moreover, the PEG–HRP modified electrode exhibited excellent electrocatalytical response to the reduction of H 2O 2 with a calibration range between 2.0×10 −6 and 6.0×10 −4 M and a good linear relation from 2.0×10 −6 to 1.0×10 −4 M, on which an unmediated H 2O 2 biosensor was based. The detection limit of 6.7×10 −7 M was estimated when the signal-to-noise ratio was 3. The relative standard deviation (R.S.D.) was 4.7% for six successive determinations at a concentration of 4.0×10 −5 M. The apparent Michaelis–Menten constant ( K m app) of the sensor was found to be 1.38 mM. Epinephrine, dopamine, and ascorbic acid did not interfere with the sensitive determination of H 2O 2.

Carbonate‐Melt Oxidized Iridium Wire for pH Sensing

AbstractA new approach to the preparation of an iridium oxide film for pH sensing is demonstrated. A thick, uniform and dense ceramic oxide layer was grown on the surface of an iridium wire by oxidizing the wire in a molten alkali metal carbonate at high temperature. The alkali metal ion from the carbonate melt was incorporated into the oxide, resulting in a highly stable oxide compound. After treated in acid solution, the oxide layer became hydrated. SEM, XRD, TGA and element analysis were carried out to characterize the oxide film, and a possible formula of the oxide was computed as Li0.86IrO2.34(OH)0.76⋅0.39H2O. The electrode made with this new type of oxide film exhibited good pH sensitivity and stability, even in strong acid/base solutions, or in strong corrosive solution such as hydrofluoric acid. Furthermore, the electrode showed excellent long‐term stability over the test period of two and a half years. In addition, individual electrodes prepared from the same batch exhibited remarkable agreement with respect to potential/pH slopes and apparent standard electrode potentials. The performance of the electrodes depends on the properties of the oxide film, such as composition, hydration state and oxidation state.

Electrochemical and electrocatalytic properties of myoglobin and hemoglobin incorporated in carboxymethyl cellulose films

Protein-CMC films were made by casting a solution of myoglobin (Mb) or hemoglobin (Hb) and carboxymethyl cellulose (CMC) on pyrolytic graphite electrodes. In pH 7.0 buffers, Mb and Hb incorporated in CMC films gave a pair of well-defined and quasi-reversible cyclic voltammetric peaks at about −0.34 V vs. SCE, respectively, characteristic of heme Fe III/Fe II redox couples of the proteins. The electrochemical parameters such as apparent standard heterogeneous electron transfer rate constants ( k s) and formal potentials (E°′) were estimated by square wave voltammetry with nonlinear regression analysis. In aqueous solution, stable CMC films absorbed large amounts of water and formed hydrogel. Scanning electron microscopy of the films showed that interaction between Mb or Hb and CMC would make the morphology of dry protein-CMC films different from the CMC films alone. Positions of Soret absorbance band suggest that Mb and Hb in CMC films retain their secondary structure similar to the native states in the medium pH range. Trichloroacetic acid, nitrite, oxygen, and hydrogen peroxide were catalytically reduced at protein-CMC film electrodes.

Oxygen Reduction on Uncatalyzed Carbon-PTFE Gas Diffusion Cathode in Alkaline Medium

The reduction of to hydroperoxide ion on a commercial uncatalyzed carbon-polytetrafluoroethylene (PTFE) gas diffusion cathode has been studied in aqueous solutions with KOH concentrations ranging between 1.0 and 6.0 mol dm−3. The couple behaves as reversible in open circuit and its apparent standard electrode potential, calculated from the Nernst equation, becomes less negative when raising the concentration. Under linear sweep voltammetric conditions, the process is independent of both and concentrations and a first-order reaction with respect to feed is found. In each alkaline medium the apparent activation energy for reduction at several potentials has been determined from Arrhenius-type plots at temperatures ranging between 5 and 45°C. While the Tafel slope is proportional to absolute temperature, the cathodic transfer coefficient is always close to unity. A reaction mechanism involving the initial reduction of on a located active site of the carbon black surface by a reversible one-electron transfer, followed by the irreversible protonation of the resulting adsorbed superoxide ion by water as the rate-determining step, is proposed. © 2002 The Electrochemical Society. All rights reserved.

Effect of dimethyl sulfoxide on the electron transfer reactivity of hemoglobin

Hemoglobin, after being treated with dimethyl sulfoxide, exhibits a direct electrochemical response at a pyrolytic graphite electrode. The apparent standard potential ( E°′) of hemoglobin is −0.119 V (vs. NHE). Meanwhile, since no electrochemical mediator is required for its direct electrochemistry, this work provides a convenient way to perform electrochemical research on this protein.

A pH Electrode Based on Melt-Oxidized Iridium Oxide

Fabrication and characterization of a novel potentiometric pH electrode based on melt-oxidized iridium oxide film is presented. The oxide film produced in a lithium carbonate melt has the composition of and shows high chemical stability. The electrode based on this oxide film exhibits very promising pH sensing performance, with an ideal Nernstian response in the tested pH range of 1 to 13. The potential response is fast, with a 90% response time obtained in less than 1 s for all pH changes. The open-circuit potential of the electrode is almost drift-free, with an average variation over time in a pH 6.6 solution as small as 0.1 mV/day. Furthermore, the potential/pH slopes and the apparent standard electrode potentials show excellent agreement among electrodes from the same batch. A comparison is made of the present electrode and those reported in the literature with respect to fabrication method and pH sensing characteristics. © 2001 The Electrochemical Society. All rights reserved.

Electrochemistry of 9,9′-spirobifluorene derivatives: 2-acetyl- and 2,2′-diacetyl-9,9′-spirobifluorene. Preparation of stereoisomeric 2,3-bis(9,9′-spirobifluoren-2-yl)butane-2,3-diols

2-Acetyl- and 2,2′-diacetyl-9,9′spirobifluorene 1 and 2 were studied by cyclic voltammetry in dimethylformamide. The corresponding anion radicals show remarkable persistency in aprotic DMF. The (apparent) standard potentials are E°=–1.77 V (SCE) and E°=–1.75 V for the (quasi-reversible) reduction of 1 and 2 to the anion radicals, respectively. Preparative electrolysis of 1 in DMF–Et4NClO4(0.1 mol dm–3), with excess acetic acid as proton donor, furnished alcohol 3 and the diastereoisomeric pinacols 5 and 6, which were isolated and characterized. The diastereoisomeric excess, de, as evaluated (NMR) on the electrolyzed solution was only slightly in favour of the (±) compound. Spectroscopic properties of compounds 1–6 are, inter alia, the 13C NMR chemical shift for the spiro-carbon at δ= 65.9 (TMS), and the fragmentation patterns in the mass spectra, with the 100% relative abundance of the molecular-ion M˙+ in the case of the aromatic ketones 1 and 2. Some comments on the influence of conformations of 5 and 6 and of the presence of an intramolecular hydrogen-bond in some intermediates and products are also presented.

Considerations of the pH dependence of hydrous oxide films formed on iridium by voltammetric cycling

AbstractThe electrochemical behavior and the analytical utility of hydrous oxide films, formed on iridium by voltammetric cycling, have been studied by evaluating the dependence of the apparent Standard Electrode Potential (E°′) and the ratio of protons to electrons (p/e) participating in the potential determining equilibria on the charge storage capacity. A model based on electron exchange involving protons in solution and conductive sites on the hydrous oxide surface is proposed and it is shown that an increase in population of the conductive sites can be correlated with an enhancement in proton exchange capacity. The influence of the pretreatment is shown to be an important factor in determining both E°′ and p/e. The implications of these results for the use of hydrous iridium oxide electrodes as potentiometric pH sensors are considered.

Determination of the apparent standard potential of the Ti/Ti(III) system in the LiFNaFKF eutectic using voltammetry, chronopotentiometry and open-circuit potentiometry

For metal/metallic ion reversible systems, logarithmic analysis of semi-integral of voltammograms, logarithmic analysis of cathodic chronopotentiograms and open-circuit potential measurements are interesting techniques to determine precisely the equilibrium potential of metal in solutions of its ions. These methods were used for the determination of the titanium equilibrium potentials in various TiF 3− 6 ions solutions in the LiFNaFKF eutectic at 700°C. From these data and Nernst's law we calculated the apparent standard potential of the Ti/Ti(III) system in fluoride melts at 700°C.

Electrochemical determination of lead and lead ion transfer at liquid-liquid interfaces

The transfer behavior of lead (II) ion at a liquid-liquid interface, facilitated by neutral carriers (polyethylene glycols), is studied by an electrochemical method. The transfer process is discussed in terms of the formation of complexes in two phases and attributed to the transfer of a complex ion across the interface. The apparent standard transfer potential, ΔwoφPbL2+o, apparent standard Gibbs energy of transfer, ΔwoGPbL2+o, and the dissociation constant of the complex, KPbL2+, in the aqueous phase are obtained from the experimental data. The results suggest a new electrochemical method for the determination of lead.

Reactions of horseradish peroxidase on a platinum cathode

Thin layer spectroelectrochemistry was used to observe the electrochemical reactions of horseradish peroxidase on a platinum cathode. When there is no oxygen dissolved in the solution, the reduction of ferriperoxidase into ferroperoxidase is very slow, but it can be accelerated by using flavin adenine dinucleotide (FAD) as redox mediator. The electrochemical re-oxidation of ferroperoxidase is much more rapid, even in the absence of mediator. Constant potential electrolysis makes it possible to prepare ferri- and ferroperoxidase mixtures and to determine the apparent standard potentials of the system at different pHs. The measurements were made for peroxidase and for two isoenzymes. With dissolved oxygen and depending on the potential sweep range, linear sweep voltammetry resulted in the appearance of compound III alone or mixed with compound I or II, depending on the pH of the solution. At more negative potentials, all the compounds were de-oxygenated electrochemically to return to the native peroxidase. In the presence of peroxidase, the electrochemical reduction of oxygen produced, depending on the potential, the superoxide anion or hydrogen peroxide that were responsible for the formation of compound III and of compounds I and II, respectively.

Reactions of horseradish peroxidase on a platinum cathode

Thin layer spectroelectrochemistry was used to observe the electrochemical reactions of horseradish peroxidase on a platinum cathode. When there is no oxygen dissolved in the solution, the reduction of ferriperoxidase into ferroperoxidase is very slow, but it can be accelerated by using flavin adenine dinucleotide (FAD) as redox mediator. The electrochemical re-oxidation of ferroperoxidase is much more rapid, even in the absence of mediator. Constant potential electrolysis makes it possible to prepare ferri- and ferroperoxidase mixtures and to determine the apparent standard potentials of the system at different pHs. The measurements were made for peroxidase and for two isoenzymes. With dissolved oxygen and depending on the potential sweep range, linear sweep voltammetry resulted in the appearance of compound III alone or mixed with compound I or II, depending on the pH of the solution. At more negative potentials, all the compounds were de-oxygenated electrochemically to return to the native peroxidase. In the presence of peroxidase, the electrochemical reduction of oxygen produced, depending on the potential, the superoxide anion or hydrogen peroxide that were responsible for the formation of compound III and of compounds I and II, respectively.

Nonlinear regression analysis of steady‐state voltammograms obtained at microelectrodes

AbstractNonlinear regression was applied to the analysis of steady‐state voltammograms obtained at carbon fiber microelectrodes. Reversible regression models were used to analyze data for oxidation of ferrocene in acetonitrile with and without added electrolyte. This analysis constitutes a test for reversibility of the redox couple. An estimate of cell resistance in highly resistive media was obtained by including ohmic drop in the model for reversible electron transfer. This allows computation of „resistance‐free”︁ voltammograms obtained in that medium. A general model for steady‐state voltammograms of quasi‐reversible electrode reactions was used to estimate simultaneously apparent standard heterogeneous electron transfer rate constants (k0′), formal potentials, and electrochemical transfer coefficients for ferrocyanide and the Co(II)/Co(I) couple of vitamin B12r. Precision for ferrocyanide with a k0′ of 0.023 cm/sec was about ±30%. Results on ferrocene suggest an upper limit for estimating k0′ of about 1–2 cm/sec when the signal/noise ratio (S/N) is large. Data for the quasi‐reversible Co(II)/Co(I) couple of vitamin B12r showed that results of reasonable precision and accuracy can be obtained for systems with poor S/N.

Ion transfer of dyes across the liquid—liquid interface

The transfer behavior of both acidic and basic dyes at the interface between water and some organic solvents has been studied in detail by electrochemical methods, and a transfer mechanism is proposed for both acidic and basic dyes. The equations of interfacial half-wave potentials for both dyes are deduced in terms of the mechanism and are consistent with the experimental data, respectively. Apparent standard transfer potentials Δ w 0ϕ 0 and Gibbs energies Δ w G 0 are calculated. The effect of dye structure and the nature of organic solvent on the transfer of dye are discussed in detail and a linear empirical relationship between interfacial half-wave potential and dielectric constant of organic phase is inferred for both acidic and basic dyes.

Effects of electron donors on Ca 2+-dependent K + transport in one-step inside-out vesicles from the human erythrocyte membrane

The interactions between reducing agents and Ca 2+ in the activation of Ca 2+-dependent K + transport have been studied in one-step inside-out vesicles. The artificial electron donor system ascorbate + phenazine methosulphate increases the apparent sensitivity to Ca 2+ by about 5-times over control values (half activation constant, about 5·10 −8 M) while oxidized cytochrome c decreases the sensitivity to about 1 3 of the controls. Using redox buffers at a fixed p Ca it is shown that the shift from the low to the high-affinity state can be accounted for by the reduction of a membrane component accepting two electrons and with an apparent standard redox potential (pH 7.5) of 47 mV. The electrons can be transferred directly from reduced PMS or to oxidized cytochrome c, but not from ascorbate, NADH or reduced glutathione.