Supporting Electrolyte Concentration Research Articles

Effects of anion adsorption different from blocking surface sites deduced from instabilities in the oxidation of formic acid

Experimental evidence is presented supporting the conclusion that the role of anions in the electrocatalyzed oxidation of formic acid at Pd is more than simply blocking reactive surface sites. The effects of anion adsorption were probed in current-control experiments by varying the composition of the supporting electrolyte and the ratio of the supporting electrolyte concentration to that of the solute. The results are fundamentally different from those obtained in potential-control experiments conducted on the oxidation of small, oxygenated organic molecules. In the latter experiments, changing the supporting electrolytes produced quantitative differences that can be interpreted solely in terms of the blockage of surface sites by the adsorbed anions. In the current-control experiments, switching electrolytes caused drastic changes in the behavior exhibited by the oxidation of formic acid. Several experiments established trends that are opposite to those that would be predicted if the only effect of anions was to block the surface sites.

Reduction of an Ensemble of Chloride Aquacomplexes of Platinum(II): An Analysis in the Framework of Phenomenological Approach

A general approach is offered to a phenomenological analysis of experimental data on the kinetics of an electrode reaction complicated by chemical stages. The approach is based on a consideration of the way the process rate depends on the supporting-electrolyte concentration at a constant electrode charge. The possibilities of the approach are illustrated by the example of the reduction of chloride complexes of Pt(II) in the presence of chloride, indifferent (in the sense of inner-sphere substitution), and mixed supporting electrolytes. The performed analysis generalizes the Frumkin–Petrii relationship, which determines the average charge of the single reactant in the solution bulk. A system may contain an ensemble of reactants. In this case, in addition to the bulk properties of the system, similar relationships take into account the interface structure and the balance between partial rate constants for parallel reactions involving reactants of different chargeness.

Kinetics and Mechanism of Electroreduction of Palladium(II) Bisethylenediamine Complexes on a Dropping Mercury Electrode

Kinetics of electroreduction of complexes Pd(en)2+2(2 × 10–5M) on a dropping mercury electrode is studied in solutions with various concentrations of ethylenediamine and supporting electrolytes (NaF, NaClO4) at pH 4.5–11 and different instantaneous current recording times. Diffusion coefficients for reducing complexes and kinetic parameters of the slow electrochemical stage are determined. The concentration of supporting electrolytes is found to affect the half-wave potential, which points to an inner-sphere mechanism of electrochemical stage at supporting electrolyte concentrations of 0.005 to 0.03 M and to a predominantly outer-sphere mechanism at higher concentrations.

Kinetics and Mechanism of Electroreduction of Ammonia and Hydroxyammonia Complexes of Zinc(II) on a Dropping Mercury Electrode

It is found that the equilibrium potential of the Zn(Hg)/Zn(II) system depends on the concentration of ammonia molecules and solution pH. The dependence conforms to the literature data on the stability constants for ammonia and hydroxyammonia complexes of zinc. Their reduction on a dropping mercury electrode in solutions of pH 9.2–12 and [NH3] = 0.05–2 M yields one irreversible cathodic wave with a diffusion limiting current. In dilute supporting electrolytes, the plateau of the latter is preceded by a maximum due to accumulation of insoluble reduction products on the surface of the mercury drop. The pH and [NH3] dependences of the half-wave potential of waves that are undistorted by a maximum are analyzed with allowance made for a change in the composition of zinc(II) complexes in the bulk solution. According to the analysis, the slow two-electron electrochemical stage involves complexes Zn(NH3)2 2+ that form from complexes present in solution in preceding reversible chemical reactions. The effect the supporting-electrolyte concentration has on the electroreduction rate of zinc(II) complexes and the mechanism of the electrochemical stage is discussed.

Electron self-exchange reaction of viologen and its derivatives in poly(ethylene glycol)

The electron self-exchange rates ( k ex) of viologen and its derivatives are estimated by using microelectrode voltammetry in poly(ethylene glycol) films. The dependences of supporting electrolyte concentration and sizes of viologen and its derivatives on k ex and diffusion coefficients ( D) are discussed. Results show that k ex increases with the decrease of supporting electrolyte concentration and sizes of reactants.

Current oscillations of indium(III) at a dropping mercury electrode

Abstract Oscillations in currents observed at potentials around the polarographic maximum during the reduction of indium(III) in potassium nitrate solutions have been characterized. Effects of surfactants, supporting electrolyte concentration, mercury drop rate, resistors in series, and temperature are reported. The current–time series data of the oscillations obtained by holding the potential fixed are used to generate a one-dimensional return map. The minima of adjacent peaks of the oscillations are used for the return map. Analysis of the return map reveals the noise, repeatability of the oscillations, and enhances differences observed in the oscillations between potential settings.

Ohmic drop compensation in cyclic voltammetry at scan rates in the megavolt per second range: access to nanometric diffusion layers via transient electrochemistry

A new concept of a three-electrode potentiostat involving positive feedback compensation of ohmic drop is discussed. This potentiostat allows the electrochemical investigation of nanosecond time scales by allowing the recording of ohmic drop-free voltammograms at scan rates in the megavolt per second range. This range of scan rate corresponds to the development of diffusion layers having only a few nanometers thickness. The principle and properties of the potentiostat are first demonstrated analytically based on a simplified equivalent circuit for the conditions used in this study ( v <5 MV s −1 ). The validity of this simplified analytical approach is then tested and further investigated by precise simulations of the electronic properties of the real circuit, and then by experimental tests on RC dummy cells or on dummy cells equipped with a pseudo-faradaic impedance. These tests establish that the potentiostat behaves excellently up to slightly above 2 MV s −1 . These results were then confirmed by examination of the reduction voltammetry of anthracene in highly concentrated (0.9 M) supporting electrolyte to avoid interference with transport in the double layer, since usual supporting electrolyte concentrations would produce double layers of the same thicknesses as the diffusion layers that are created in this range of scan rates. These tests confirmed the results of the above investigations and finally demonstrated that this potentiostat allows the recording of undistorted voltammograms up to 2.25 MV s −1 .

Double-Layer-Capacitance Titration of Self-Assembled Monolayers of ω-Functionalized Alkanethiols on Au(111) Surface

The acid−base equilibrium of ω-functionalized alkanethiol monolayers on Au(111) has been studied using the change in double-layer capacitance accompanied by the protonation−deprotonation of ω-terminals. The pK of ω-carboxyl alkanethiols on Au(111) increases by four pH units. The shift becomes greater as the alkyl chain length increases. The same magnitude of the pK shift to the acidic side occurs in the monolayers of aminoethane thiol. The surface pK varies little with the electrode potential, whereas the increase in the supporting electrolyte concentration slightly diminishes the pK shift. The drawn-out shape of the titration curves is consistent with the mean-field model taking account of the repulsive interaction between adsorbed molecules, indicating the significance of the strong electrostatic repulsion between adsorbed thiol molecules in its charged states. The magnitude of the pK shift is, more than that predicted by the mean-field model, however, and suggests the considerable contribution from oth...

Electron Transfer Kinetics of Tris(1,10-phenanthroline)ruthenium(II) Electrooxidation in Aprotic Solvents

Double layer and solvent effects on the kinetics of the one-electron oxidation of Ru(phen)32+ (where phen = 1,10-phenanthroline) at platinum and gold ultramicroelectrodes were studied by ac-voltammetry. The standard rate constant of Ru(phen)32+ electrooxidation is independent of both electrode material and concentration of supporting electrolyte, indicating the absence of double-layer effects on the kinetics of electron transfer. The small variation of the formal redox potential of the Ru(phen)32+/3+ system with change of the supporting electrolyte concentration demonstrates the absence of a significant influence of ion pairing on the reaction rate. Therefore, the electrode process of Ru(phen)32+ oxidation is found to be ideally suitable for the study of solvent effects on electron transfer kinetics. The solvent dependence of the electron transfer rate constant was interpreted within the context of contemporary theory. The charge transfer process was found to be perfectly adiabatic. Since the crossing rate of the energy barrier is controlled by the dynamics of solvent relaxation, the rate of the electrode reaction primarily depends on the solvent's longitudinal relaxation time or, more approximately, on the viscosity of the solution.

Factors affecting the yield of polypyrrole colloids produced under electrohydrodynamic conditions

Several factors influencing the amount of polypyrrole colloid produced by electrohydrodynamic methods were investigated with the aim of increasing the yield of the process. The parameters investigated included solvent, pH, temperature, sonication, catholyte composition, cell design, flow rate, anode size and condition, monomer concentration and supporting electrolyte concentration. Particularly important in increasing the yield were the use of sonication, temperatures above room levels, adjustment of pH by alkaline catholyte, more efficient cell design, suitable flow rate and low monomer concentration. On the other hand, the amount of polymer deposited on the anode was not significantly reduced by any of the factors evaluated.

Electrochemical studies of heterogeneous reduction of tetracyanoquinodimethane in poly(ethylene oxide) electrolytes using ac impedance and cyclic voltammetry at an ultramicroelectrode

Electrochemical studies of the TCNQ0/TCNQ− couple have been carried out using ac impedance spectroscopy and cyclic voltammetry at platinum ultramicroelectrodes (UME). Liquid poly(ethylene oxide) (PEO) CH3–O–(CH2–CH2–O)4–CH3 has been used as the solvent with different concentrations of the TCNQ0/TCNQ− couple and LiClO4 as the supporting electrolyte. On the basis of the ac impedance results at the UME it has been found that the double layer capacitance and standard heterogeneous rate constant are independent of the presence of electroactive species and supporting electrolyte concentrations, indicating that adsorption of electroactive species onto the electrode is not significant. The standard heterogeneous rate constant ks was found to be 0.109 ± 0.005 cm s−1. A similar value of ks = 0.094 cm s−1 was obtained for the second reduction step TCNQ−/TCNQ2−. Diffusion coefficients of both TCNQ and TCNQ− are equal, D = 1.0 × 10−6 cm2 s−1 for a 0.5 M LiClO4 solution. Higher diffusion coefficients are obtained in less concentrated supporting electrolyte. Comparison is made between these results and those reported previously for PEO-400 HO–(CH3CH2O)8–OH. The different end groups significantly influence the viscosity and hence ks.

Surface forces between sphalerite and silica particles in aqueous solutions

Surface forces between a fractured sphalerite surface and a silica particle in supporting electrolyte solutions were measured using an atomic force microscope. Below pH 8, the long-range surface forces were found to be attractive, decreasing with increasing solution pH. The adhesion force varied from 18 mN/m at pH 4 to 7 mN/m at pH 8. At pH 10, an exponentially decayed repulsive force was measured and no adhesion was observed. The decay length was found to scale with the supporting electrolyte concentration. Although the repulsive force was reduced significantly when up to 0.5 mmol/l calcium ions were added, the surfaces remained non-adhering at contact. In contrast, an attractive force profile was obtained when 0.5 mmol/l cupric ions were added, accompanied by an adhesion force of 16 mN/m. In general, the measured force profile can be accounted for by considering classical van der Waals and electrical double-layer forces for constant surface charge density conditions. The measured force behavior correlates well with the observations in sphalerite-silica heterocoagulation tests.

Effect of cation adsorption on the kinetics of anion electroreduction

The phenomenon of the decrease of the accelerating action of small amounts of tetraalkylammonium cations on the electroreduction of anions, when the negative charge on the mercury electrode increases and when the supporting electrolyte concentration increases, is investigated. It is explained by the redistribution of the surface excesses of organic and inorganic cations in the electric double layer within the framework of the slow-discharge theory and the improved Stern–Grahame model considering forcing of organic ions out of the electric double layer with the increasing negative charge of the electrode surface. The ψ1 potential in the presence of adsorption of the surface-active organic cations is calculated and the corrected Tafel plots are constructed for solutions of different composition.

The Strength of Acids in Alcohols As Determined by Steady-State Voltammetry

Steady-state voltammograms for reduction of acids of various strengths in alcohols with excess supporting electrolyte and without any supporting electrolyte can be used to infer charge type and strength of the acid on the basis of the phenomenon of migration. For strong and moderately weak acids (K(a)/[Formula: see text] > 10(-)(3)) in alcohols, the ratio of steady-state transport-limited current to diffusion-limited current, corrected appropriately for ion-ion interactions, the presence of ionic impurities, and changes in viscosity, for hydrogen ion reduction without supporting electrolyte and with excess supporting electrolyte equals 2. For acetic acid, which is very weak (K(a)/[Formula: see text] < 10(-)(6)), the value of the steady-state transport-limited current is, under the experimental conditions applied here, independent of supporting electrolyte concentration. In the case of a homogeneous acid-base equilibrium, a novel analytical procedure yields diffusion coefficients of both hydrogen ion and undissociated weak acid molecules from the diffusional and migrational currents. Limiting currents obtained in alcohols with excess supporting electrolyte and without supporting electrolyte are compared by means of an extended formula that incorporates the ionic strength dependence of diffusion coefficients.

PH and supporting electrolyte concentration effects on the passive transport of cationic and anionic drugs through fixed charge membranes

The effects of pH and supporting electrolyte concentration on the passive transport of an ionized (cationic or anionic) drug through a thick fixed charge membrane have been theoretically studied. This system constitutes a simplified model for the pH controlled ion transport and drug delivery through membranes of biological and pharmaceutical interest. Calculations were carried out for different values of the membrane fixed charge, supporting electrolyte and drug concentrations covering a broad range of the conditions usually found in experiments. The theoretical approach employed is based on the Nernst–Planck flux equations, and all of the species present in the system (the neutral and ionized forms of the drug, the two supporting electrolyte ions and the hydrogen and hydroxide ions) have been taken into account without any additional assumption. It has been shown that the Goldman constant field assumption together with the total co-ion exclusion assumption provide good approximated solutions for high membrane fixed charge concentrations. The model predictions show that the internal pH within the membrane, the total drug flux and the membrane potential are very sensitive to the external pH values. Comparison of our results with available experimental data confirms the potential utility of the calculations for the analysis and design of experiments involving the pH dependent passive transport of ionized drugs through a fixed charge membrane, especially in the cases of thick biomembranes, biochemical sensors and pH-controlled drug delivery systems.

A parameter study on the impedance of poly(3-methylthiophene) film electrodes

Poly(3-methylthiophene) films (PMT), polymerised galvanostatically ( i=1.0 mAcm −2) in propylene carbonate +0.03 M TBAPF 6+0.2 M 3-methylthiophene solution, were characterised by cyclic voltammetry and coulometry combined with gravimetrical measurements and Electrochemical Impedance Spectroscopy (EIS). The dc electronic and ionic conductivities of PMT-films were measured on free-standing membranes as function of the electrode potential. From these measurements, the mobilities and diffusivities of electrons and ions were determined at T=298 K ( μ e=0.21 cm 2 V −1 s −1, μ i=6.3×10 −8 cm 2 V −1 s −1 and D e=5.6×10 −3 cm 2 s −1, D i=1.6×10 −9 cm 2 s −1). EIS measurements on Pt/PMT-film electrodes were carried out in the monomere free propylene carbonate solution in dependence of the following parameters: PMT-film thickness l, supporting electrolyte concentration c, temperature T and polarisation potential E. The low frequency part of the impedance spectra could be fitted by a simple cylindrical pore model. The dependence of the fit parameters on the system parameters favours the assumption that the PMT-film is porous and the pores are filled with the bathing electrolyte.

Electrode kinetics of Eu3+ in dimethylsulfoxide

Standard electrode reaction rate constants for the one-electron reduction of Eu(CF 3 SO 3 ) 3 on mercury in different concentrations of the supporting electrolyte tetrabutylammonium perchlorate in dimethylsulfoxide have been measured. Two techniques (i) ac polarography on the dropping as well as on the hanging mercury electrode and (ii) cyclic voltammetry in a two electrode system were employed. The evaluation of the ac data was carried out by the Randles and the Sluyters methods. The reaction rates from cyclic voltammetry were extracted via a simulation program. Physical properties such as the viscosities, resistivities and the double layer capacitances necessary for the evaluation of the data are given. The decrease in the standard electrode reaction rates with increasing supporting electrolyte concentrations was accounted for in the Frumkin model.

The Electrochemical Behavior of Iron(III) Incorporated in Zeolite Y-Modified Electrode and the Catalytic Oxidation of Ascorbic Acid

A new method is developed for the catalytic oxidation of ascorbic acid (AA) at a graphite-zeolite-modified electrode doped with iron(III) (Fe3+Y/ZCME), in which iron(III) exchanged in zeolite Y can act as catalyst to oxidize AA. First, the electrochemical behavior of iron(III) incorporated in the zeolite Y-modified electrode was studied. The results illustrate that diffusion controls the ferric/ferrous redox process at the Fe3+Y/ZCME. The anodic current of this process increases with the supporting electrolyte concentration of non-size-excluded cation and was suppressed when using size-excluded cation. Second, the catalytic oxidation of AA was explored at Fe3+Y/ZCME. With constant potential amperometric measurements optimal conditions for AA determination were obtained. The working potential was 0.55 V (vs. SCE); the supporting electrolyte was 0.1 M KNO3Or 0.05 M Na2HPO4-KH2PO4buffer solution (pH 6.8). The response current exhibits a linear relationship with the concentration of AA over the range of 1.58×10–6–2.15×10–2 M. When samples of urine, and fruit were analyzed with Fe3+Y/ZCME, we obtained AA recovery of 98–102%, and a relative standard deviation lower than 3%. Meanwhile, interferences from other ions and organic substances were examined.

Voltammetric studies of anthraquinone dyes adsorbed at a hanging mercury drop electrode using fast pulse techniques

Square wave voltammetric studies, involving adsorptive stripping methodology, of anthraquinone-based reactive dyes and of a typical anthraquinone dye Alizarin Red S are reported. The reduction of their anthraquinone moiety is reversible and fast and the sensitivity of these determinations can be improved if current sampling is made earlier after the pulse application. In order to try to modify the rate of the electrochemical process at the electrode, two approaches have been tested: the effect of adding surfactants and the influence of supporting electrolyte concentration. In both cases, a change in the results obtained is observed, owing, in the first case, to the modification of the electrode surface, and, in the second, to the importance of the resistance of the solution.

Volume and structure of humic acids studied by viscometry: pH and electrolyte concentration effects

Viscometry was used to evaluate the effects of pH and supporting electrolyte concentration on the intrinsic viscosities of eight humic acids and one fulvic acid. Two synthetic poly(acrylic acid) (PAA) samples of different molecular weight were also studied for comparison. Humic and fulvic acid molecules behave as flexible entities that can swell or shrink in response to changes in pH and ionic strength. An increase in the solution pH leads to the development of negative charges in the molecules with the consequent electrostatic repulsion between ionized groups and molecular swelling. Increasing the ionic strength increases the screening of charges and leads to molecular shrinkage. The pH dependence decreases with increasing electrolyte concentration and at 10 −1 M electrolyte the intrinsic viscosity is almost pH independent. The general behavior of PAAs is similar to that of the humics, though the effects of pH and electrolyte concentration are much larger for the PAAs. The degree of hydration of the humics differs for different samples. There are compact samples with low water content and swelling properties whereas other humics are more hydrated and flexible. All the studied humics have an internal structure that limits the expansion of the molecules when the electrolyte concentration is decreased. The latter is in accordance with the low values of the Mark–Houwink coefficient, α, of humics.