Linear sweep voltammetry at the tubular graphite electrode: Part II. Totally irreversible processes

The theory of linear sweep voltammetry at the tubular graphite electrode has been developed for irreversible processes. The convective diffusion differential equations have been transformed into an integral equation which is solved numerically. The current-potential curves have been calculated theoretically and verified experimentally. The dependence of the current-potential curves on velocity has been studied. A procedure for the determination of kinetic parameters, i.e. standard rate constant and transfer coefficient, is presented.     

Cyclic voltammetric studies of a thionine coated pyrolytic graphite electrode

Cyclic voltammetry has been used to study the spontaneous adsorption and oxidative coating of thionine onto basal plane pyrolytic graphite electodes. Spontaneous adsorption leads to a monolayer of flat lying molecules together with some weakly attached upper layers. Oxidation of an adsorbed thionine monolayer results in several products which appear to include polymeric species and the sulphoxide of thionine. Similar products are obtained by an oxidative coating procedure.     

The polished precipitate electrode: A new voltammetric method with the solid electrode

A new surface-renewal technique at the solid electrode has been developed, based on continuous polishing. Well-defined and reproducible current-voltage curves similar to those obtained in polarography are given by the "polished precipitate electrode" (PPE). The method can be used for the continuous determination of electroactive substances and for the study of electrode reaction mechanisms at the solid-liquid interface.     

A faradaic impedance study of the reduction of oxygen from aqueous alkaline solution at the dropping mercury electrode

The reduction of oxygen to hydrogen peroxide at a dropping mercury electrode in an aqueous solution of 1 M KNO₃+0.04 M KOH (pH=12.35) has been studied by means of impedance measurements as a function of frequency and d.c. potential. The reaction appears to be nearly reversible in the dc sense, but quasi-reversible in the ac sense. The impedance data obey the Randles' equivalent circuit with the following apparent values for the kinetic parameters: standard heterogeneous rate constant k_sh^a=0.035 cm s^?1 and cathodic transfer coefficient α^a_c=0.22. The results are interpreted in terms of a two-step charge transfer mechanism with the step O₂+eO₂^? being rate-determining.     

Supramolecular phenomena in organic redox films at electrodes: Part I. The methylene blue/leucomethylene blue redox couple at the platinum electrode

It is possible to generate conductive multilayer structures by reduction of methylene blue cations at the interface: clean surface of a platinum electrode/1 M aqueous electrolyte solution.The characteristics of the multilayer ordered phase generated in fluoride and nitrate solutions are presented and the mechanism of a fast charge transfer between electrode/organic film and organic film/aqueous solution interfaces is discussed. The conductivity of the film is interpreted by postulating formation of a mixed valence structure with the generation of a cation radical intermediate which is favoured in the solid state at characteristic potentials.     

Enzyme catalysed oxidation of ferrocene compounds

A facile transformation of ferrocene and a series of substituted ferrocenes to the corresponding ferricinium ions has been effected by hydrogen peroxide in the presence of the native or immobilized enzyme horseradish peroxidase.     

Catalytic effects of metal submonolayers formed by faradaic adsorption on the anodic oxidation of ascorbic acid at a platinum electrode

The catalytic effects of metal ions on the anodic oxidation of ascorbic acid on a Pt electrode in 1 M HClO₄ were studied by linear sweep voltammetry. The anodic peak due to a two-electron oxidation of ascorbic acid shifts to the negative potential side on the addition of Bi³⁺. This indicates the accelerating effect of Bi³⁺ on the oxidation of ascorbic acid. The presence of other metal ions, such as Pb²⁺, Hg²⁺, Tl⁺, Ag⁺ and Sb³⁺, also exerts similar effects. These metal ions were adsorbed on a Pt electrode at underpotentials and the adsorbed metals (denoted as M_ad) still remain on the electrode surface until the electrode potential goes up to and beyond the peak potential of the oxidation of ascorbic acid. On the other hand, metal ions forming no adsorbed layer on Pt, such as Co²⁺, Zn²⁺, Fe³⁺ and Ni²⁺, exhibit no catalytic effect. These facts suggest that the presence of a M_ad on Pt is essential for the promotion of the anodic oxidation of ascorbic acid. However, there is a difference in the catalytic action among the M_ad, for example, Cu_ad, Cd_ad, In_ad, Sn_ad and Mo_ad display no catalytic action.The catalytic activity depends on the degree of surface coverage by the M_ad. The maximal effect of the M_ad is attained in the submonolayer region. The effects of metal ions were discussed on the basis that the M_ad plays its major role in the removal of the adsorbed ascorbic acid occupying active sites on the electrode surface, and provides effective sites for the activation of adjacent water molecules. Furthermore, from the ¹³C NMR spectra for the oxidation products, the adsorbed water on the M_ad appears to function by promoting the subsequent hydration steps, following the electron-transfer step of ascorbic acid.     

Thermochemistry of fluorine compounds: IV. Rubidium tetrafluoroiodate

From measurement of the heat of hydrolysis, at 25°C , the enthalpy of formation of rubidium tetrafluoroiodate is derived: ΔH°_f [RbIF₄, cryst.]₂₉₈= ?191.12±4.43 kJ mol^?1. Heat capacity measurements for RbIF₄ over the range 273–303 K are also reported.     

Room-temperature fluorescence spectrometry of some pharmaceutical compounds

Room-temperature fluorescence of 66 pharmaceutical drugs are presented using a mixture of ethanol/water as solvent. The analytical figures of merit for the fluorescent species are reported with limits of detection ranging between 1 and 30 ng/ml for most of them and with average blank standard deviation of 7.6%. The spectral band half-widths are also reported.     

Electrochemical reduction of tricarbonyl compounds: Reduction mechanism of alloxan on a mercury electrode

A study has been carried out on the reduction mechanism of alloxan over a DME corresponding to the first two-electron transfer over the pH range 0–12.Polarographic and voltammetric results show the reduction process to be kinetically controlled. Rate and equilibrium constant data for the prior chemical reaction have been evaluated and several reaction mechanisms for different pH zones have been proposed.     

Operational comparison of various thermodynamic treatments of organic substance adsorption at the electrode/solution interface

Three isotherms are considered: Frumkin's Bennes' and Mohilner's. The relationship between the interaction parameters in the first two and the adsorbate activity coefficient in the last is discussed. The significance of various standard states for the definition of ΔG° _ads is analysed. The operational derivation of ΔG°_ads in the three cases and the relationship between the resulting three quantities are illustrated. The isotherms have been used to describe the adsorption of 1,4-dioxane on a polarized Hg electrode. The analysis has been carried out both at constant potential and constant charge. The picture emerging in each of the three cases is discussed in the light of the results obtained in the other two. It is concluded that the three approaches give the same qualitative information about the interfacial behaviour of dioxane, especially as far as the effect of the electric field and the particle-particle lateral interaction are concerned. The usefulness of the Frumkin isotherm for analysing promptly experimental adsorption data is thus maintained. Limitations of the present approach are discussed.     

Stripping voltammetry with collection at a rotating ring-disk electrode

A new electroanalytical technique is described, called "stripping voltammetry with collection." The technique involves the use of a rotating ring-disk electrode and is an improvement over traditional voltammetric stripping at a single electrode in that it is characterized by a lower limit of detection and that the period of deposition before stripping can be shorter. The use of the technique is illustrated by the determination of 10(-10)M Ag(+) in 0.1 M H(2)SO(4) by use of a ring-disk electrode having a disk electrode constructed of glassy carbon and a ring electrode constructed of platinum.     

Chemically-derivatized nickel surfaces: Synthesis of a new class of stable electrode interfaces

The interactions of protonic acids (HClO₄ and CF₃SO₃H) and of alkylating agents (CF₃SO₃Et and Et₃O⁺BF₄^?) with cyclic ethers and 1,3-dioxacycloalkane s (DCA) in CH₂Cl₂ have been studied by dc polarography (DCP) and differential pulse polarography (DPP). All the reductions were irreversible and kinetically controlled. There is no indication that HClO₄ reacts with THF or OXP; with CF₃SO₃H the decrease of the signals due to the acid may indicate the formation of a product (sec. oxonium ion or ester) or it may be due merely to a modification of the mercury surface by the oxacyclic compound which produces a reduction in the kinetic current; no new signals appear.For the various DCAs with CF₃SO₃Et, 1,3-dioxepan (DXP) gave the clearest indication of reactions occurring and the signals have been assigned (tentatively) to EtDXP⁺ and to the hemiformal ester EtO(CH₂)₄OCH₂OSO₂CF₃. For 1,3-dioxolan (DXL) the picture was less clear, and there was no evidence that 1,3,6-trioxan (TXA) was alkylated.Our study of the DCA with acids gave no evidence that DXL interacts with HClO₄ or CF₃SO₃H, which is due to the exceptionally low basicity of DXL. The signals obtained with DXP and 1,3,6-trioxocan (TXC) are assigned to a molecular complex H₂A⁺-DCA, to the sec-oxonium ion HDCA⁺, and to the hemiformal ester HO(CH₂)₄OCH₂OA.It is concluded that the polarographic behaviour of all the species involved is so complicated that our original aim of distinguishing by this technique between sec. and tert. oxonium ions is not feasible.The sec. oxonium ions formed from cyclic ethers or DCA in conc. H₂SO₄, whose existence was indicated by the PMR spectra, could not be detected polarographically as their reduction potentials were outside the “potential window”.     

Evaluation of a chloramine-T-selective electrode for catalytic titration of silver and mercury(II) based on iodide-catalyzed chloramine-T reactions

Semiautomatic methods are described for the catalytic titrimetric determination of microamounts of silver and mercury(II) using a chloramine-T-selective electrode as monitor. The methods are based on the inhibitory effect of Ag(I) and Hg(II) on the iodide-catalyzed chloramine-T-arsenite and chloramine-T-H₂O₂ reactions. Microamounts of silver in the range 0.2–200 μg (1 × 10⁻⁷−1 × 10⁻⁴ M) and of mercury(II) in the range 0.1–200 μg (2.5 × 10⁻⁸−5 × 10⁻⁵ M) were determined using the chloramine-T-As(III) indicator reaction. Mercury(II) in the range 4–2000 μg (1 × 10⁻⁶−5 × 10⁻⁴ M) was also determined using the chloramine-T-H₂O₂ indicator reaction. The accuracy and precision were in the range 0.1–1%.     

Voltammetric study of humic and fulvic substances: Part II. Mechanism of reaction of the Pb-fulvic complexes on the mercury electrode

A model is presented for the electrode process corresponding to the reduction of Pb(II) complexed by fulvic substances. The various factors which may affect the polarographic results are discussed and a mathematical expression is given for the polarographic wave. The influence of various parameters was tested. It was found that the formation of complexes both present in solution and adsorbed at the surface of the electrode is the predominant factor which affects the potential of the wave. Moreover, the diffusion coefficient of the complexes is much lower than that of free lead. We discuss the nature of the complexes adsorbed at the electrode surface and the implications of the findings of this work when using the polarographic methods for the determination of the complexing ability of waters.     

Tunnel electrode: Part II. Electron-transfer theory at an n-type semiconductor tunnel electrode

The formula for the cathodic tunnel current at an n-type semiconductor tunnel electrode was derived from the double adiabatic perturbation theory considering the vibration in the first coordination sphere. In the low-temperature limiting case, the apparent difference between the normal and the abnormal regions was observed in both the transfer coefficient α and the activation energy E* as a function of potential. It was concluded that the semiconductor tunnel electrode has advantageous characteristics for investigation of the electron-transfer mechanism at high overvoltage.