Graham A. Wright

A potassium-ion selective electrode with valinomycin based poly(vinyl chloride) membrane and a poly(vinyl ferrocene) solid contact

The construction of an ion-selective electrode for potassium, employing valinomycin as carrier in a poly(vinyl chloride) membrane and a poly(vinyl ferrocene) solid contact between the membrane and the metallic substrate, is described. The performance of this electrode is compared to electrodes in the coated-wire configuration and the conventional electrode with an aqueous internal reference electrolyte. Nernstian responses were obtained for the three types of electrodes and the selectivities and the detection limits are very similar for all electrodes. The stabilities of these electrodes were investigated over the first 20 days of use. The solid-contact electrode showed a significant improvement for stability compared to the coated-wire electrode and it approaches the performance of the conventional one.

Photoelectrochemistry and spectroscopy of substituted polyanilines

Abstract The properties of four substituted polyanilines, poly(N-methylaniline), poly(m-methylaniline), poly(o-methoxyaniline), and poly(o-ethoxyaniline) have been examined by photoelectrochemical and spectroscopic techniques (Raman and XPS). The middle peak in cyclic voltammograms for poly(N-methylaniline) was ascribed to hydrolysis products such as benzoquinone, and in the case of poly(o-methoxyaniline) to branched forms. The photocurrent profiles of the substituted polyanilines resembled those of polyaniline to a large extent, with prolonged cathodic and anodic photocurrents given for the polymer in a state of high conductivity. With poly(o-ethoxyaniline) only short-lived photocurrents were obtained pointing to a more highly insulating material, also shown by the limited growth of this polymer. Raman spectra of the substituted polyanilines taken in situ revealed typical polyaniline Raman bands, while the largest shift in the υCN band (35 cm−1) occurred with poly(N-methylaniline). A deconvolution of the N 1s signal in XPS scans showed nitrogens consistent with oxidised forms of the polymers: a high proportion of positively charged nitrogens for poly(N-methylaniline) and of imine nitrogens for poly(m-methylaniline).

Photoelectrochemical and spectroscopic studies of sulfonated polyanilines. Part I. Copolymers of orthanilic acid and aniline

Conducting films of sulfonated polyaniline were produced by copolymerization of orthanilic acid (o-aminobenzenesulfonic acid) with aniline on a platinum electrode. The degree of sulfonation and the oxidation states of the elements were examined using X-ray photoelectron spectroscopy. The copolymer structure was further investigated by in situ Raman spectroscopy (514 nm excitation). Photocurrent profiles on a millisecond time scale in response to a light-flash perturbation were taken, producing exclusively anodic photocurrents for the copolymer in the conductive state. Partial phase diagrams for the sulfonated copolymer were constructed, mapping the conductive regions as a function of pH and electrode potential. Both conductor-insulator boundaries moved by 60 mV per pH unit, consistent with the internal redox processes of the copolymer, while the sulfonated polyaniline was soluble in neutral pH solutions.

Nucleation and growth of anodic oxide films on bismuth—I. Cyclic voltammetry

The early stages in the formation of a continuous anodic layer of bismuth oxide on a solid bismuth electrode, in the pH range 5–14, were studied. The oxide covered the surface by the simultaneous thickening and spreading of patches. The metal surface was classified into two different areas with different overvoltage for oxide nucleation. The ratio of the two areas varied according to the history of the surface. The thickening of the newly formed layer (final thickness ∼20 nm) followed the high-field growth law i = Aexp(BE) where E is the field in the oxide layer, with B = (2.0 ± 0.5) × 10−6 V−1 cm. This value of B gives an activation distance for high-field ion transport of 0.2 nm, comparable to the radius of a lattice site and much smaller than that obtained previously, for much thicker films. Dissolution of the film, giving breakdown of the oxide layer and pitting of the metal, occurred for pH<8. The thickness of the anodic film was thus limited to only 4 nm at pH 5. Cathodic reduction of the anodic oxide resulted in a porous metal surface. The current—voltage curve for the reduction often had a complex shape, which was related to the morphology of the original anodic layer.

The kinetics of the anodic formation and reduction of phase silver sulfide films on silver in aqueous sulfide solutions

Abstract The kinetics of the potentiodynamic formation and reduction of silver sulfide films on a silver rotating disc electrode in aqueous sulfide solutions were investigated. Due to the inherently fast reaction between silver and the HS − species, the physical processes of migration and diffusion of ions in solution were frequently rate limiting. Only when the rate of HS − transport in solution was very high, did the rate of film growth become limited by the solid-state electromigration of silver ions through the silver sulfide film. By utilizing the low field approximation for field-assisted electromigration, the ionic conductivity of these films was found to be 5.9 × 10 −4 S cm −1 . Silver sulfide film reduction occurred by the injection of electrons into the film to the film/electrolyte interface. All of the results have been supported by scanning electron microscope investigations.

The potentiodyanmic formation and reduction of a silver sulfide monolayer on a silver electrode in aqueous sulfide solutions

Abstract A potentiodynamic study of silver electrodes in aqueous sulfide solutions, carried out to form phase silver sulfide films, revealed that a monolayer of silver sulfice forms as a distinct and separate stage of film growth at an underpotential of about 0.12 V. The monolayer peak (and its cathodic counterpart) was also characterized by a linear relationship between peak current density and potential sweep rate and a constant charge density of about 0.2 mC/cm2. The potentiodynamic E/i curves for the silver sulfide monolayer were simulated by computer on the basis of a mechanism of the initial adsorption of HS− on the silver surface in a fast equilibrium step followed by a rate determining electron transfer step to form AgHS as a surface intermediate. The AgHS species then rapidly diffuses on the surface and joins a growing two-dimensional silver sulfide monolayer nucleus. Under the experimental conditons studied here, the formation and reduction of the silver sulfide monolayer was found to be of intermediate kinetic reversibility.

The anodic dissolution of nickel—1: Perchlorate and fluoride electrolytes

Abstract A potentiostatic sweep technique has been used to study the anodic dissolution of nickel in acidic perchlorate, acetate and fluoride solutions. At slow potential sweep rates a prepassive film exists throughout the anodic region in perchlorate and acetate electrolytes. By the use of fast sweeps, or by the addition of F − , film formation and growth is sufficiently reduced to reveal a linear anodic. Tafel region. The rate of active dissolution, which is independent of [H + ] and [F − ], obeys the following rate law; i = 2 Fka w exp [β FE/RT ], with β = 0·53. The following mechanism is proposed for active dissolution, with the first step rate-determining: (1) Ni + H 2 O → NiOH ads + H + + e − , (2) NiOH ads → NiOH + + e − , (3) NiOH + + H + ⇌ Ni 2+ + H 2 O. Prepassivation is thought to occur from the intermediate NiOH ads through a solid state mechanism.

THE KINETICS OF SILVER BROMIDE FILM FORMATION ON THE SILVER ANODE

Abstract The anodic potentiodynamic formation of an AgBr film on an Ag rotating disc electrode was studied in aqueous bromide solutions. As the electron transfer step (1) is intrinsically fast, and as the film remains porous throughout its growth, the rate of film growth is limited by physical parameters such as ionic diffusion and migration in the solution. The anodic E / I curves for AgBr film formation were calculated quantitatively by computer on the basis of the following model of film growth. Film formation occurs initially by the nucleation of islands of film to a critical thickness, and then these islands spread laterally until only small pores remain between them. As these pores become small, the resistance of the solution within them becomes rate limiting (at the anodic current peak). After the peak, the concentration of bromide ions at the pore base falls to zero and current is then controlled by the diffusion of bromide ions into the lengthening pores of the film. This lateral spreading mechanism and the subsequent retention of the porous film morphology has been substantiated by comprehensive Scanning Electron Microscope investigations.

Photoelectrochemical and spectroscopic studies of sulfonated polyanilines Part II. Copolymers of orthanilic acid and substituted anilines

Abstract Copolymerization of orthanilic acid ( o -aminobenzenesulfonic acid) with m -methylaniline ( m -toluidine) on a platinum electrode led to the formation of conducting films of a sulfonated poly (methylaniline), Anodic photocurrents for the conducting copolymer were seen in response to a light-flash perturbation on a millisecond time scale, allowing a partial conductivity phase diagram to be constructed. The profile was similar to that obtained for sulfonated polyaniline, but with a smaller conductive range (0.25 V) at any particular pH value. Only a thin insulating layer was produced by Copolymerization of orthanilic acid with either o -methoxyaniline or o -ethoxyaniline. The structure of the copolymers was further investigated using X-ray photoelectron spectroscopy and in situ Raman spectroscopy (514 nm excitation).

Structure and electronic properties of bismuth anodic oxide films—I. Photoeffects

Abstract Estimates of the conduction electron mobility, lifetime and trap density of anodic bismuth oxide films, obtained by measuring the transient response of a polarized bismuth anode to an intense flash of white light, showed that the films had a highly defective structure. for thicker films (∼240 nm) the photo-response arose from the production of electron-hole pairs in the bulk of the film, but for very thin films (∼5 nm) the response arose from photo-emission of electrons from the metal into the film, and effects attributed to the decay of space charges of both electrons and mobile ions were observed. The photo-response was used to investigate the possible formation of films of bismuth oxo-halides under conditions in which these compounds were thermodynamically stable. Incorporation of iodide and bromide into the very thin films at pH 5 was detected. No effect of chloride was observed. Comparison with cyclic voltammetric results indicated that an anodic layer of bismuth oxoiodide might be formed by direct nucleation on to the metal surface.