William T. Yap

Chronoamperometry of polymer-modified electrodes: Charge transport by diffusion and migration

Abstract A theoretical analysis of charge-transport phenomena due to both diffusion and electrostatic migration in a polymer-modified electrode is presented for chronoamperometry. Concentration distributions and current-time curves were derived for the simple case of a constant electric field in the polymer film.

Effects of resistance and capacitance on the chronoamperometry of polymer-coated electrodes as modeled by a finite elements digital simulation

A finite elements digital simulation of chronoamperometry with polymer-coated electrodes is presented. The model takes account of mass transport by diffusion and migration, and the effects of the uncompensated resistance and capacitance of the system. A graphical method for the analysis of experimental data was introduced and applied to electrodes coated with poly[Ru(II)(bipy)2(4-vpy)2]+PF−6.

THERMODYNAMICS OF THE HYDROLYSIS OF PENICILLIN G AND AMPICILLIN

Apparent equilibrium constants and calorimetric enthalpies of reaction have been measured for the beta-lactamase catalyzed hydrolysis of penicillin G(aq) and ampicillin(aq) to penicillinoic acid(aq) and to ampicillinoic acid(aq), respectively. High-pressure liquid-chromatography and microcalorimetry were used to perform these measurements. The results for the reference reactions at T = 298.15 K and Im = 0 are: Ko = (9.4 +/- 3.1) x 10(-7), delta rGo = (34.4 +/- 1.0)kJ mol-1, delta rHo = -(73.7 +/- 0.4)kJ mol-1, and delta rSo = -(363 +/- 4) J K-1 mol-1 for penicillin G-(aq) + H2O(1) = penicillinoic acid2-(aq) + H+(aq); Ko = (6.0 +/- 3.0) x 10(-6), delta rGo = (29.8 +/- 1.7) kJ mol-1, delta rHo = -(70.0 +/- 7.5) kJ mol-1, and delta rSo = -(335 +/- 26) J K-1 mol-1 for ampicillin-(aq)+ H2O(1) = ampicillinoic acid2-(aq)+H+(aq). Calorimetric enthalpies of reaction for the beta-lactamase catalyzed hydrolysis of cephalosporin C have also been measured but the reaction products have not been identified and the measured enthalpies cannot be assigned to a specific reaction. Acidity constants for ampicillin, penicillin G, ampicillinoic acid, and penicillinoic acid are also reported. A strain energy of 116 kJ mol-1 for the beta-lactam ring is obtained from thermochemical data.

Spectroelectrochemistry of a system with product deposition

Abstract An analysis of the spectroelectrochemistry of systems containing a species which deposits onto the electrode is presented. In this model, in addition to the Nernst equation, a thermodynamic condition for phase equilibrium between the deposited and solution species of the product is assumed. Relations between the variables of the Nernst plot were derived and procedures for obtaining the formal potential, number of electrons involved, and the maximum equilibrium concentration of the product are presented. These results are applied to the experimental data on the aqueous solution of the laser dye, 1-methyl-4-(5-phenyl-2-oxazolyl) pyridinium p -toluenesulfonate.

Chronocoulometry of a system with deposition of the product on the electrode

Abstract The charge-time relation for double potential-step large-amplitude chronocoulometry was developed for a system with electrodeposition of the product. The model assumes that the concentration of the product near the electrode cannot be larger than the solubility of the product. This model indicates that the Qr(t>τ) vs. θ plot has a constant slope which is smaller than that of the Q(t τ) vs. θ plot increase linearly with √τ.

Mathematical analyses of ac voltammetry of a surface confined redox process

Analytical expressions were derived for the harmonics of the current response to an ac voltage input, when the rate-determining process is a redox reaction of a couple confined to the electrode surface. Some input/output phase relations such as functions of the heterogeneous electron transfer rate and the re-organization energy are illustrated.

Electrochemical investigation of N-methyl-4-(5-phenyl-2-oxazolyl)pyridinium p-toluenesulfonate: A laser dye with product deposition

Abstract The electrochemical and spectroelectrochemical behavior of N -methyl-4-(5-phenyl-2-oxazolyl)pyridinium p -toluenesulfonate was studied to provide an evaluation of the electrochromic properties of this laser dye. Preliminary cyclic voltammetric experiments provided evidence for the deposition onto the electrode of the product of the electrochemical reduction process. Double potential-step chronocoulometry and spectroelectrochemistry at an optically transparent thin-layer gold-minigrid electrode were used to elucidate the reaction mechanism further. Equations were derived to interpret the spectropotentiostatic Nernst plots which exhibit discontinuities as a result of product precipitation onto the electrode. By fitting the experimental data to these expressions, it was possible to calculate the formal potential (−1.017 V vs. SCE) and number of electrons (1.1) for the redox process plus the solubility of the product (0.27 mmol/L). When this solubility value was used in equations derived for the chronocoulometry model for this system, a value of 5 × 10 −6 cm 2 /s was obtained for the diffusion coefficient of the reduced species. From the forward chronocoulometric step, the diffusion coefficient for the oxidized form, the laser dye cation, was calculated to be 6.8 × 10 −6 cm 2 /s. For redox systems in which there is product deposition, the mathematical treatment described permits the calculation of various thermodynamic and transport properties based on spectropotentiostatic and chronocoulometric data.

Electron transefr to and from molecules with interacting multiple redox centers

Abstract A theoretical analysis is given for electron transfer in molecules with interacting multiple redox centers such as dimers, oligomers and polymers, and for voltammetric techniques such as de polarography and rotating disk voltammetry. The analysis takes accout of the interactions between nearest-neighboring centers only. The determination of the interaction energies from the experimental current-potential curves is illustrated.

On the Applications of Discontinuous Bessel Integrals to Chronoamperometry

Discontinuous Bessel integrals are applied to a boundary value problem related to chronoamperometry, with zero concentration at the disk satisfied on the average and the zero flux at the shroud satisfied approximately only. Current functions are derived, series expansion at long time and asymptotic expansion at short times are given. Plots of numerical calculations of current functions are presented.

Analysis of biochemical reactions in equilibrium: Hydrolysis of penicillin and ampicillin

A general equilibrium model for a class of biochemical reactions was formulated, using the formalism of binding polynomials for the evaluations of the various species. Thermodynamic equilibrium constant was obtained from the observed equilibrium total concentrations of reactants at specified pH and certain ligand concentrations. Results for the hydrolysis of penicillin and ampicillin were analyzed with this equilibrium model. Predicted fractions of hydrolysis products at various pH values were calculated.