Transport and kinetics at microheterogeneous electrodes: Part V. The methylviologen/platinum system

Results are reported for the reaction of methylviologen radical cation, MV⁺ with platinum colloidal particles, studied by stopped flow spectrophotometry. The rate of the reaction depends on the gaseous pretreatment of the particles. For particles reduced by hydrogen, the kinetics are usually first order with respect to MV⁺. The reaction is also first order in the concentration of platinum, and is inhibited in a first order manner by the product MV²⁺. This inhibition suggests that MV²⁺ is adsorbed on the particle surfaces, and this has been confirmed by ac, ring—disc electrode studies on macroscopic platinum electrodes. At high concentrations of MV⁺ some deviation from first order kinetics is observed. These results are all explained by a kinetic model in which either the desorption of MV²⁺ or the adsorption of MV⁺ is the rate limiting process. The rate of consumption of MV⁺ on an oxidised surface is an order of magnitude faster than that on the reduced surface. Ring—disc studies show that this is because the MV⁺ is not producing H₂ but is reducing the surface oxide. The results are shown to fit a simple model which takes into account this titration of the oxide layer. The model also explains why the rate on partially oxidised surfaces will appear to have an order greater than one in [Pt].     

Pulsed titration sensors: Part 1: A breath-by-breath CO2 sensor

A novel electrochemical sensor for the determination of CO₂ in expired breath is described. The sensor works by generating from the reduction of O₂ in dimethyl sulphoxide (DMSO) in a generating pulse. There is a rapid titration reaction between the and any CO₂ present. In the recovery pulse the amount of unreacted is determined. The larger the concentration of CO₂ the less is found in the recovery pulse. The solubilities and diffusion coefficients of O₂ and CO₂ in DMSO have been determined using rotating disc voltammetry and rotation speed step experiments. The stoichiometry, the product, and the rate constant of the titration reaction have been determined using ring—disc voltammetry and laser Raman spectroscopy. The operation and the effect of adventitious water on the sensor are described. Results are presented which show that the sensor can indeed measure the breath-by-breath rhythm of expired CO₂ from a human subject.     

Electrochemical polymerization of poly(thiophene-3-acetic acid), poly(thiophene-co-thiophene-3-acetic acid) and determination of their molar mass

The electrochemical preparation and molar mass characterization of conducting polymers, poly(thiophene-3-acetic acid) (poly(TPAA)), and poly(thiophene-co-thiophene-3-acetic acid) (poly(TP/TPAA)) are reported. RRDE results revealed that, unlike other conducting polymers, current efficiency during the deposition of poly(TPAA) is by far less than 1 due to significant solubility of the polymer. Up to 48% of the oxidation products of the disc electrode can dissipate into the bulk solution. These dissipated species are polymers in oxidized form and reactive towards reductant originally present in the coating solution. The low current efficiency necessitates the use of relatively high monomer concentration and high polymerization potential to obtain a conducting poly(TPAA) film. Characterization with gel permeation chromatography (GPC) of molar mass and its distribution has been carried out. The results show that up to 65% to 85% of the whole polymers are comprised of big molecules containing over 2000 monomer units, which indicates the possibility of cross-linking of polymer chains. The remaining parts of the polymers are relatively short chains containing several to dozens of monomers. Increase of TPAA content in the copolymers leads to better solubility and higher molar mass. The latter is accompanied by greater electronic conjugation in the polymer chains.     

The tube electrode and e.s.r.

The convective diffusion equation for the downstream distribution of products and intermediates produced on a tube electrode under conditions of laminar flow is solved. For distances which are large compared to the thickness of the electrode, an analytical solution in real space can be found. The solution of the equation has been applied to an apparatus in which the intermediates and products are detected by e.s.r. The species are transported by laminar flow from the electrode through an e.s.r. cavity. Results are presented for two systems and are found to be in good agreement with the theory.     

Photogalvanic cells: Part X. The transparent disc electrode and the iron-thionine system

The kinetics of the iron-thionine system have been investigated using the transparent rotating disc electrode and flash electrolysis. Using the transparent disc electrode, from the results for the variation of photocurrent with rotation speed and irradiance, we can find the quantum efficiency for the production of semithionine, the efficiency for producing leucothionine from semithionine and the rate constant for the back reaction of leucothionine with Fe(III). This rate constant is also determined from the flash electrolysis experiments. The values of the kinetic parameters are found to be in good agreement with values determined by analysis of the photostationary state, flash photolysis and ac modulation at a transparent disc electrode. The relative merits of the different techniques for investigating photogalvanic systems are discussed.     

Amperometric enzyme electrodes: Part II. Conducting salts as electrode materials for the oxidation of glucose oxidase

Results are reported for the direct oxidation of the enzyme glucose oxidase on 7 different one-dimensional conducting donor acceptor salts. Experiments conducted with the enzyme in bulk solution are shown to be in good agreement with theory. Three salts, made of the cations tetrathiafulvalinium (TTF⁺), N-methylphenazinium or quinolinium with the anion tetracyanoquinodimethanide (TCNQ^?) had the lowest background currents and were used to make membrane sensors for glucose. Analysis of the variation of current with glucose concentration identified the rate limiting processes as transport of gluycose through the membrane and electrochemical kinetics under unsaturated and saturated conditions respectively. The electrochemical rate constants for these three materials were all greater than 10^?2 cm s^?1. TTF⁺TCNQ^? is the material of choice and linear calibration plots were obtained for glucose concentrations between 50 μmol dm^?3 and 10 mmol dm^?3.     

Amperometric enzyme electrodes: Part I. Theory

The theory of the steady state operation of an amperometric enzyme is derived. The reaction scheme includes diffusion of substrate and product through a membrane, the kinetics of the enzyme substrate reaction and the electrochemical regeneration of the enzyme. A simple diagnostic plot is derived which allows the rate limiting process to be identified, and the balance between the transport of substrate through the membrane and the enzyme and electrochemical kinetics to be determined. The effects of inhibition by the accumulation of product behind the membrane are also considered.     

Photogalvanic cells: Part IX. Investigations using the transparent rotating disc electrode

The transparent disc electrode has been used to investigate three possible photogalvanic systems — the iron-thionine system and two iron-ruthenium systems. Results have been obtained for the photocurrent both in the steady state and for the modulated photocurrent produced when the light source is modulated. In the latter case the phase shift and amplitude of the photocurrent are measured as a function of the frequency of modulation. The results are in good agreement with the theoretical equation devised for the transparent rotating disc electrode, and the values of the rate constants obtained agree with those obtained by more conventional techniques such as stopped flow or flash photolysis. Moreover values are obtained for the overall quantum efficiency, φ, for the production of the electroactive species. For the ruthenium systems φ～-1 but for the iron thionine system φ?0.8.