Showing papers on "Supporting electrolyte published in 1991"

Electrochemical oxidation of methanol on tin-modified platinum single-crystal surfaces

Abstract: To understand the role of tin as a promoter in the electrochemical oxidation of methanol, the authors have studied the geometric and electronic effect of tin atoms in different chemical states on/in the platinum surface by using single-crystal faces of the ordered alloy Pt{sub 3}Sn and single-crystal faces of pure Pt modified by electrodeposited/adsorbed tin, i.e., the so-called adatom state. They found that none of the alloy surfaces were more effective catalysts than any of the pure platinum surfaces under the conditions of measurement employed here and that alloying platinum with tin to any extent significantly reduced the activity. As reported previously by others, they observed tin to spontaneously adsorb on platinum surfaces from dilute sulfuric acid supporting electrolyte containing Sn(II) in concentrations above ca. 5 {mu}M. At a given concentration, the coverage by tin decreased as the atomic density of the platinum surface increased. However, they did not observe any enhancement of methanol oxidation on any platinum modified by this irreversibly adsorbed tin. They did observe a diffusion-limited enhancement on Pt(111) and on Pt(100) due to Sn(II) in the electrolyte at 1 {mu}M concentration. At this concentration, tin did not appear to be adsorbed to any observable extent, andmore » the catalysis appeared to occur via the direct interaction of a dissolved tin species with the surface. They propose a mechanism of catalysis that is a hybrid homogeneous-heterogeneous sequence based on known homogeneous Pt-Sn catalysts.« less

Ordered phases in concentrated DNA solutions

Abstract: Aqueous solutions of concentrated DNA, a strong polyelectrolyte, in 1:1 electrolyte form liquid-crystalline phases analogous to those observed for other semi-rigid neutral polymers and weak polyelectrolytes. Phase transitions were examined in detail for DNA fragments with a contour length (500 A) approximating the persistence length. These fragments form at least three lyotropic phases. The lowest density or “precholesteric” phase appears to be a nematic with a slight and easily variable twist. The intermediate density phase is a true cholesteric with a pitch of ≈2.1 μm. With increasing concentration the cholesteric phase unwinds prior to formation of the third, high density, columnar phase. Phase transition boundaries were determined as functions of DNA concentration from 10 to 300 mg/ml solvent, supporting electrolyte concentration from 0.01 to 1.0 M, and temperature from 20°C to 60°C. Critical concentrations for formation of anisotropic phase (Ci) and disappearance of isotropic phase (Ca) were only moderately dependent on temperature. Ci, Ca, and the pitch of the cholesteric phase were surprisingly insensitive to the supporting electrolyte concentration. The insensitivity can be most simply related to the high concentrations required for anisotropic phase formation by these rather short, highly charged rod-like DNA fragments. At high concentrations the DNA counterions contribute significantly to the effective ionic strength, hence overall charge screening, and the counterion atmosphere as monitored by 23Na NMR, appears to be perturbed by inter-rod interactions.

Mechanisms for the Retardation of Uranium (VI) Migration

Abstract: This report covers the investigation of mechanisms for the retardation of U(VI) migration. It describes (1) the measurement of the solubility of UO22+as afunction of pH and equilibration time, (2) the calculation of solubility using solution thermochemical data, (3) the measurement of the sorption of UO22+by silica as a function of pH and supporting electrolyte concentration, and (4) attempts to simulate the uranium sorption isotherms in terms of the U solution thermochemical data, measured properties of the silica and proposed sorption reaction mechanisms.

The role of supporting electrolyte during the electrocatalytic hydrogenation of aromatic compounds

Abstract: The electrocatalytic hydrogenation of benzene, aniline, and nitrobenzene was investigated at a Raney nickel powder cathode. The single phase electrolyte consisted of t-butanol, water, and a hydrotropic salt, either sodium or tetraethylammoniump-toluenesulphonate (TEATS). The hydrogenation of benzene was achieved only in the latter case; the only product detected was cyclohexane. The highest current efficiency (73%) was obtained at 50° C, 1.0m benzene, 2.5m TEATS, and at an apparent current density of 4.0 mA cm−2. Aniline was electrocatalytically hydrogenated to cyclohexylamine only in the presence of a quaternary ammonium ion supporting electrolyte (containing either Br− orp-toluenesulphonate anions), with product current efficiencies of ∼40%. When nitrobenzene was hydrogenated with a sodiump-toluenesulphonate supporting electrolyte, only nitro group reduction was observed. When the supporting electrolyte was TEATS, both nitro group reduction and aromatic ring hydrogenation occurred.

Effect of supporting electrolyte on the mediated reduction of [Fe(H2O)6]3+ by an osmium-containing poly(4-vinylpyridine) film

Abstract: The ability to control the mediation kinetics of [Fe(H2O)6]3+ reduction by [Os(bipy)2(PVP)10Cl]Cl films through electrode potential, film thickness and supporting electrolyte is demonstrated; bipy = 2,2′-bipyridyl, PVP = poly(4-vinylpyridine) In H2SO4, the mediated reduction is controlled by film thickness for surface coverages between 7 × 10–10 and 1 × 10–8 mol cm–2(Lk mechanism) For surface coverages > 10–7 mol cm–2 total catalysis is obtained where substrate diffusion in solution limits the reduction rate (LSk mechanism) In HClO4 the mediated reaction is of the surface type and occurs between [Fe(H2O)6]3+ still in solution and the film within a region of molecular dimensions (Sk″ mechanism) These observations are considered in relation to film structure and with regard to potential applications

Electrochemical oxidation of sulphite ions at graphite electrodes

Abstract: The electrochemical oxidation of sodium sulphite has been studied in aqueous sodium sulphate solution at two different graphite electrodes, one being of natural graphite (EC) and the other impregnated with phenol (ECK). The objective of the present work was to obtain some insight into the direct oxidation as well as the indirect oxidation, via produced oxygen radical species, of sulphite on non-metal electrodes. For this reason a study of the oxidation of sulphite in the concentration range between 0–0.10m in aqueous sodium sulphate using a batch electrochemical reactor, operating potentiostatically, was undertaken. The potential range was chosen between 1.0 to 2.5 V/SCE, and the concentration of the supporting electrolyte, sodium sulphate, was kept constant at 0.5m. A kinetic Tafel type law, considering irreversible behaviour for the direct sulphite oxidation and the mass transfer performance in regards to the experimental conditions were applied to predict the time variations of the sulphite conversion.

Electrochemical and in situ electron paramagnetic resonance studies of polypyrrole doped with polystyrene sulphonate

Abstract: The polyanion of polystyrene sulphonate (PSS) combined in polypyrrole (PPY) as the counterion during the electrochemical polymerization of pyrrole is shown to be trapped permanently in the PPY network during subsequent redox switching of the PPY–PSS in a supporting electrolyte, and thus cation (co-ion) exchange is a primary feature of the process. In association with small cations such as K+ and favourable anions such as NO–3, electrochemical charge transport within the PPY–PSS films is reversible and fast with Dct= 6 × 10–8 cm2 s–1. A previously unreported effect of electrolyte anions, however, has also been observed. The effect is profound for the unfavourable SO2–4 anion and is reflected in a significant change in the electrochemical and EPR behaviour of the conducting polymer compared with that of the favourable anions. EPR results indicate that the oxidation level of PPY during film deposition becomes lower as the film increases in thickness. During redox switching in a supporting electrolyte, disproportionation of PPY radical cations is significant at high oxidation levels. PPY–PPS is shown to be very stable electrochemically and capable of scavenging transition-metal ions, which can be used in stripping analysis of these metals.

Simultaneous electrolytic production of xylitol and xylonic acid from xylose

Abstract: A new electrocatalytic process for simultaneous electrolytic production of xylitol and xylonic acid or the corresponding xylonate salts directly from α-d-xylose in alkaline solutions of neutral salt as supporting electrolyte has been developed and presented. An electrocatalytically active cathode coating with essentially hydridic features has been employed to enhance Faradaic yields in xylitol production at low current densities by heterogeneous reaction of hydrogenation with H-adatoms, and the entire mechanism has been proved by experimental evidence. An anionic selective anodic coating for titanium substrate was also used to suppress oxygen evolution and optimize the xylonic acid production by direct oxidation with anodically generated bromine, and to carry out the electrode process at almost equilibrium potential. The kinetic effect of specifically adsorbable anions for decreasing cathodic current yields in xylitol production and the resulting necessity for membrane separation of catholyte from anolyte, with proper optimization of the supporting neutral salt composition, has also been emphasized.

Voltammetric behaviour of salicylic acid at a glassy carbon electrode and its determination in serum using liquid chromatography with amperometric detection

Abstract: Cyclic voltammetry was used to investigate the oxidation of salicylic acid at a planar glassy carbon electrode. The electrode reaction was found to be dependent on the pH and ionic strength of the acetate buffer, which contained 35% methanol. Under these conditions the maximum electrochemical signal was obtained with a supporting electrolyte of 0.06 mol dm–3 acetate buffer in 35% methanol (pH 5.0). The peak current value (ip) increased by approximately 10% when the methanol concentration was decreased to 8%. The substance was found to undergo an irreversible reaction involving one electron and possibly two protons in the initial oxidation step with at least one possible quasi-reversible follow-up reaction. The optimum mobile phase for the liquid chromatography, with amperometric detection, of the serum extracts was found to be 0.06 mol dm–3 acetate buffer in 8% methanol (pH 5.0); the acidified serum was extracted with a mixture of chloroform and acetonitrile (60 + 40), prior to injection onto a reversed-phase column. The peak current was measured at +1.35 V and the calibration graph was found to be linear in the range 4–200 ng of sample injected. The average recovery from serum was found to be 60% with a relative standard deviation of 5.8%. A pharmacokinetic study was carried out and the results obtained were comparable to those found in the literature. It was concluded that the method developed had possible application for the measurement of trace levels of salicylic acid in clinical studies.

The electrochemical synthesis of aminonitriles I. H-cell studies with adiponitrile and azelanitrile

Abstract: Adiponitrile and azelanitrile were electrochemically hydrogenated to their corresponding aminonitriles in a divided H-cell using Raney nickel powder as the cathode material. The effects of current, temperature, and solvent/supporting electrolyte composition on product selectivities were investigated. Syntheses of the fully hydrogenated diamine by-product increased with increasing current and solution temperature. When a 0.8 M adiponitrile/alcohol/water/ammonium actetate electrolyte was hydrogenated at temperatures of 35–45°C, 6-aminocapronitrile selectivities in the range of 79–97% and current efficiencies of 50–60% were obtained. The optimum applied current was 60 mA for each 2.5 g of catalyst (an apparent current density of 4.8 mA cm−2). For the case of azelanitrile, reaction selectivities for the partially hydrogenated 9-aminononanenitrile product ranged from 80–93%.

Investigation of ion and solvent transport accompanying redox reactions of polyvinylferrocene films using an in situ electrochemical quartz crystal microbalance technique

Abstract: The in situ electrochemical quartz crystal microbalance (in situ EQCM) technique has been applied to study ion and solvent transport accompanying the redox reaction of polyvmylferrocene (PVF) film coated on quartz crystal electrodes in aqueous and acetonitrile media. The dependence of apparent formal potentials (Epolyf)app of the Fc°/Fc+ redox couple (Fc° and Fc+ represent the reduced and oxidized sites, respectively, in the PVF film) upon the concentration of supporting electrolyte has been also examined in comparison with those of solution-phase ferrocene derivatives. The PVF film is ideally permselective to ClO4− anion in both aqueous and acetonitrile media. The PVF oxidation (or reduction of polyvinylferricenium ion) in aqueous media is accompanied by the ingress (or egress) of one ClO4− ion per redox site and some solvent (ca. one H2O per redox site). In LiClO4 acetonitrile media, during the oxidation process ClO4− ion with some solvent inserts into the film and the reverse phenomenon occurs during t...

Determination of glutathione at enzyme-modified and unmodified glassy carbon electrodes

Abstract: The oxidation of the reduced form of glutathione (GSH) was found to occur at a decreased overpotential at a glassy carbon electrode when 1.2 mol dm–3 of dipotassium hydrogen phosphate were used as the supporting electrolyte. The resulting current for the oxidation peak of GSH varied linearly over the concentration range 0.16–2.4 mmol dm–3 GSH. The reaction between GSH and hydrogen peroxide catalysed by the enzyme GSH-peroxidase (GSH-PX) could be monitored using a glassy carbon electrode with GSH-PX immobilized under a layer of Nafion film.

Thermodynamics and kinetics of heterogeneous electron transfer at glassy carbon/osmium-containing metallopolymer interfaces

Abstract: The standard rate constant (k0) and transfer coefficient (α), characterising the heterogeneous electron-transfer reaction, have been evaluated for glassy carbon electrodes modified with [Os(bipy)2(PVP)nCl]Cl (bipy = 2,2′-bipyridine, PVP = poly(4-vinylpyridine), 25 n 5) using sampled current voltammetry. The effect of variations in osmium content within the film, electrolyte concentration and temperature on these parameters is examined in hydrochloric and perchloric acid. The relevant enthalpies (ΔH0#)real, (ΔH0#)ideal and entropies (ΔS0#)real, (ΔS0#)ideal, as well as, the reaction entropy of activation (ΔS0rc) for the heterogeneous electron-transfer reactions at the electrode/metallopolymer interface have been evaluated. The influence of the immobilised film on doublelayer formation and heterogeneous kinetics is discussed and the results correlated with homogeneous charge-transport data. The results obtained suggest that the changes in the supporting electrolyte and redox site loading influence the physico-chemical properties of the polymer phase rather than the local microenvironment of the redox centre.

Rotating stabilised cell: a new tool for the investigation of interfacial extraction kinetics between liquid phases

Abstract: New experiments performed with an improved version of the rotating stabilised cell are presented. In this technique the aqueous phase is immobilised: it is found that a gel of polyacrylamide is a good stabilising medium for this purpose. The gel is placed in a cylindrical cell of small dimensions which can be set to rotate. A pure diffusion of the species to be extracted occurs in the aqueous phase. A flat interface is created for the contact with the organic extractant solution. A rotating-disc flow is promoted in this phase, which permits the control of the diffusive layer. The system investigated is the cobalt(II) ion at the interface water (+ buffer solution and supporting electrolyte)/DEHPA [bis(2-ethylhexyl)hydrogenphosphate] 0.2 mol dm–3 in dodecane, for two values of the pH of the aqueous phase: pH 3 and pH 5. A mathematical expression is derived for the amount of matter transferred, and the validity of the formula is examined mathematically. The influence of the rotation speed of the cell and the influence of its length are studied in the case where the pH is 3. The values of the aqueous-toward-organic rate constants found at pH 3 and pH 5 are in reasonable agreement with the results of other workers.