Janet. Osteryoung

Square-wave voltammetry applied to the totally irreversible reduction of adsorbate

Abstract The totally irreversible reduction of adsorbed material is studied by square-wave voltammetry. The model compound chosen is midazolam, 8-chloro-6-(2-fluorophenyl)-1-methyl-4 H -imidazo[1,5- a ][1,4]benzodiazepine. Experimental voltammograms are compared directly with the exact numerical solution for the boundary value problem via COOL, a numerical method yielding estimates of transfer coefficient and rate constant for the process. The agreement between experimental and calculated voltammograms is excellent over a wide of experimental conditions. Artifacts of measurement such as uncompensated solution resistance are readily detected. The limitations of previous empirical descriptions of the response are shown by comparison with these results.

Gold Electrocrystallization on Carbon and Highly Oriented Pyrolytic Graphite from Concentrated Solutions of LiCl

The early stages of the electrolytic deposition of gold on carbon electrodes from concentrated LiCl electrolytes have been investigated by voltammetry, chronoamperometry, and microscopy. The analysis of current-time transients according to existing theories indicates that this process occurs by multiple three-dimensional nucleation, followed by diffusion-controlled growth of nuclei. Diffusion coefficients calculated on the basis of nucleation theory were found to be higher than those Irom rotating disk experiments (RDE), suggesting that gold species are adsorbed on the electrode surface. Gold nucleation from 6 M LiCl cannot be classified as either instantaneous or progressive nucleation. The kinetic parameters for nucleation, aN o (nucleation rate) and N o (number density of active sites on the substrate surface), were estimated by using a general Poisson nucleation law. Both quantities were found to vary with potential and with concentration of gold. The potential dependence of the nucleation rate, aN o , was interpreted according to classical and atomistic theory. For low gold concentrations, where adsorption of AuCl or AuCl 3 might occur prior to nucleation, the number of atoms in the critical nucleus (N c ) was less than unity over the entire potential range analyzed. For high gold concentrations the number of atoms forming the critical nucleus depends on overpotential. In all solutions studied nucleation takes place on a limited number of active sites

Voltammetric reduction of hydrogen ion in solutions of polyprotic strong acids with and without supporting electrolyte

Abstract Voltammetric reduction of strong polyprotic acids in solutions with excess and without supporting electrolyte was studied both theoretically and experimentally. A theoretical model based on the transport (diffusion and migration) equations and the electroneutrality principle was used to compute voltammograms by finite difference simulation under both steady state and transient conditions. Simulated voltammograms are compared with experimental reduction curves obtained at platinum microelectrodes without and with excess electrolyte for two strong polyprotic acids, sulfuric (H2SO4) and tungstosilicic (H4W12SiO40) acids. Perchloric acid (HClO4) was used as the reference for comparison. The experimental results agree well with the calculated voltammetric curves both without and with excess supporting electrolyte. The dependence of voltammetric response on the concentration of acids is also discussed.

Methods for determining the intrinsic and effective charges on spherical macroions.

The intrinsic number of charges per particle, Z, on particles in a suspension of monodisperse sulfonated polystyrene latex is found by measuring steady-state voltammetric transport-limited currents for the reduction of hydrogen counterion at a Pt disk microelectrode in a suspension containing excess supporting electrolyte. Limiting currents measured in deionized latex suspensions yield a corresponding effective charge, Z*. Electrostatic binding of an inner layer of counterions to the particle renders Z* < Z. Voltammetrically determined charges agree with the intrinsic and effective Stokes charges as determined by titration and electrophoresis, respectively. ζ- Potentials calculated from the measured electrophoretic mobility of the particles yield the Loeb charge number, which agrees more closely with Z than with Z*. For existing data on spherically charged macroions, Z*/Z decreases with increasing ratio of intrinsic charge to macroion radius, Z/a. This finding is supported by effective charge values calculated from the cell model using the nonlinear Poisson-Boltzmann equation with the convention that Z* is the charge found between the c(r)/ 〈c〉 = 1 boundary and the cell wall.

A Novel Gold Electroplating System: Gold(I)‐Iodide‐Thiosulfate

The electrodeposition of gold from a new electroplating system for gold that contains iodide and thiosulfate has been studied using rotating disk voltammetry. The cathodic electron transfer is slow, with a transfer coefficient of α = 0.76 ± 0.02, and is coupled with a preceding chemical reaction, the formation of the complex ion Au(S 2 O 3 ) 2 3 . The magnitude of the current contribution from the chemical step can be made negligible by using high concentrations of thiosulfate and by the proper choice of supporting electrolyte. High quality gold deposits with a very high aspect ratio have been obtained through a photoresist mask from this plating bath under optimized conditions. The choice of univalent salt as supporting electrolyte affects markedly the electroreduction of Au(I).

Conditions of Strict Voltammetric Reversibility of the H+/H2 Couple at Platinum Electrodes

Cyclic voltammetric curves obtained at Pt electrodes for the hydrogen couple, H(+)/H(2), fit very well the Shuman theory, as corrected, for reversible electrode processes of other than 1:1 stoichiometry. Good agreement was obtained for acid concentrations in the millimolar range and for normal scan rates, which minimize the effect of the adsorption peaks. An error in Shumans equation for potential is corrected. Voltammograms obtained at Pt microelectrodes fit well the theoretical simulated data.

Voltammetric, Optical, and Spectroscopic Examination of Anodically Forced Passivation of Cobalt‐Tungsten Amorphous Alloys

Passivation and corrosion of thin layers of amorphous Co-W alloys were examined using potentiometry with linear current change, voltammetry, and anodic current pulsation. The state and the composition of the anodized alloy surface were determined by X-ray diffraction, electron microprobe spectroscopy, and optical microscopy. Investigations performed in noncomplexing electrolytes indicated that passive layers on the alloy are easily formed and are stable in acidic solutions. They can be formed also in alkaline solutions, under conditions of larger current. In neutral solutions an effective protecting layer cannot be obtained during anodization. In this pH range, selective oxidation of cobalt takes place, and amorphous tungsten is formed on the surface. The tungsten layer is loose and does not protect the base metal against further dissolution. Prolonged application of anodic current to the amorphous layers, at any pH, leads to appearance of cracks on the surface. The behavior of amorphous and polycrystalline, homogeneous alloys were compared also.

Steady-State Limiting Currents Determined by Coupled Diffusion, Migration, and Chemical Equilibrium

A simple theoretical model is presented for the reduction of a singly charged cation under conditions where migration is important and the cation is coupled to a neutral species through a chemical equilibrium, AB = A(+) + B(-). Only the steady-state transport-limited current, I(l), is considered. Simple algebraic equations describe the ratio of I(l) to the diffusion-limited current, I(d), as it depends on the degree of dissociation, determined by the ratio of equilibrium constant to formal concentration, K(AB)/C*(AB). The ratio I(l)/I(d) is found to depend on the ratio of electrolyte to equilibrium concentration of A(+) in bulk solution just as for the well-known result for the case without the equilibrium (i.e., K(AB) → ∞). The results are in accord with published experimental data for weak acids. Agreement and disagreement with other theoretical treatments of this problem are discussed. The main results are for 1:1 supporting electrolytes; extensions are made to 2:1, 1:2, and 2:2 supporting electrolytes.

Chronoamperometry of strong acids without supporting electrolyte

The steady-state reduction current of the hydrogen ion in a 1:1 strong acid without adding supporting electrolyte at a microelectrode is known to be independent of the diffusion coefficient of the anion, although the anion diffuses to the electrode together with the hydrogen ion in order to maintain electroneutrality. This paper aims at resolving this inconsistency of the diffusion of the anion by investigating transient reduction currents of the hydrogen ion without supporting electrolyte. The time-dependent diffusion equation associated with migration was solved under the condition of the potential step toward the limiting current-domain at a hemi-spherical electrode. The theoretical transient current has a linear relation with the inverse square root of the time. The anion behaves as if it were electroactive. The slope of the line is expressed by an average of diffusion coefficients of the hydrogen ion and the interacted anion. Chronoamperometric measurements were made in hydrochloric acid including various concentrations of supporting electrolyte.

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.