Showing papers on "Cyclic voltammetry published in 1972"

Electrochemical Oxidation of Uric Acid and Xanthine at the Pyrolytic Graphite Electrode Mechanistic Interpretation of Electrochemistry

Abstract: The quantitative course of electrochemical oxidation of uric acid and xanthine at the pyrolytic graphite electrode has been examined between pH 1–7, the most commonly encountered pH range in biological systems. Basically, uric acid is oxidized in a primary oxidation of the bond to give a readily reducible and highly reactive bis‐imine. The bis‐imine can be detected as a cathodic peak by fast sweep cyclic voltammetry. Complete hydration of the bis‐imine gives rise to uric acid‐4,5‐diol that breaks down to alloxan and urea at pH 1, allantoin and a small amount of urea at pH 7, and at intermediate pH values mixtures of alloxan, allantoin, urea, and occasionally traces of parabanic acid are formed. Xanthine undergoes a oxidation that proceeds by two stages. The first involves oxidation of the bond to give uric acid which, being more easily oxidized than xanthine, is immediately further oxidized to the uric acid bisimine. This then hydrates and fragments in essentially the same way as described for uric acid, the same products being observed. Mechanisms are developed which explain the observed electrochemistry and over‐all formation of products from this reaction. The nature and yields of products formed electrochemically and the effect of pH on the reaction parallels the biological (enzymatic) oxidation of these compounds.

Electrochemical Studies of Zinc Tetraphenylporphin

Abstract: The reduction of zinc (II) tetraphenylporphin (ZnTPP) in dimethylformamide (DMF) was investigated. The products of electron transfer and associated chemical reactions were studied by polarography, cyclic voltammetry, thin‐layer controlled potential coulometry and spectroscopy, and epr. Reduction proceeds initially as a series of two reversible one‐electron steps. After the second and subsequent steps, however, proton abstraction from the solvent medium occurs which, in turn, determines the reaction path. Characteristics of the porphyrin ring reactions are studied and compared with those of porphyrin free bases.

A Study of Gold Reduction and Oxidation in Aqueous Solutions

Abstract: The electrochemical behavior of gold in alkaline cyanide, citrate, and phosphate buffered solutions has been studied using cyclic voltammetry and galvanostatic transients. Two reaction paths were observed. At low overvoltage the reaction goes through an adsorbed intermediate. (rate determining) At larger overvoltages a direct transfer between the gold complex in solution and the metal atom was found on reduction. The reduction reaction was the same in all the solutions but in the phosphate and citrate baths the gold did not oxidize to a soluble species.

Electrohydrodimerization Reactions II . Rotating Ring‐Disk Electrode, Voltammetric and Coulometric Studies of Dimethyl Fumarate, Cinnamonitrile, and Fumaronitrile

Abstract: The reduction of the activated olefins dimethyl fumarate, cinnamonitrile, and fumaronitrile in tetra‐n‐butylammonium iodide‐dimethylformamide solutions at a platinum electrode has been studied by rotating ring‐disk electrode (RRDE) voltammetry, cyclic voltammetry, and coulometry. The results indicate that each compound undergoes a one‐electron reduction to the anion radical which then undergoes a dimerization reaction. Rate constants for this dimerization reaction were found to be 110 (dimethyl fumarate), 880 (cinnamonitrile), and . Evidence of some bulk polymerization reaction was obtained from the coulometric results.

Anodic displacement of adsorbed H in electrochemisorption of organic molecules at platinum

Abstract: A new electrochemical adsorption effect is described in which chemisorption of organic substances such as thiourea, nitriles, benzene and dimethyl sulphide at Pt electrodes occurs by anodic displacement of previously adsorbed atomic H, if the adsorption is initiated at potentials less than 0.35 V against H2 electrode. Anodic adsorption transients are measured potentiostatically and the charges corresponding to quantities of H displaced can be quantitatively related to the blocking of the surface for H adsorption as determined in subsequent cyclic voltammetry experiments. The effects are shown to be quite different from previously described dissociative chemisorption effects at electrodes where anodic transients arise from electrochemical ionization of H atoms resulting from dissociation of the organic molecule itself. Applications of the effect to electrochemical hydrogenation processes are discussed for cases where cathodic component currents complicate the interpretation of the H displacement transients, e.g., with nitriles.

The Electrochemistry of the Liquid Crystal N‐(p‐Methoxybenzylidene)‐p‐n‐butylaniline (MBBA) The Role of Electrode Reactions in Dynamic Scattering

Abstract: The electrochemical reduction and oxidation of MBBA were studied by cyclic voltammetry and by controlled potential coulometry. The reduction in DMF leads to the formation of the radical anion which decomposes with a half‐life of about 4 sec, apparently by both ec and ece pathways. The oxidation product was unstable on the time scale of this study. The possible importance of electrode reactions in interpreting the dynamic scattering phenomenon in nematic liquid crystals is suggested.

Evaluation of organic battery electrodes: Voltammetric study of the redox behaviour of solid quinones

Abstract: The redox behaviour of solid quinones has been examined by potential sweep and cyclic voltammetry. A number of quinones are insoluble in aqueous electrolytes and can be completely discharged and recharged without appreciable polarization, as found, for example, with solid chloranil (tetrachloro-p-benzoquinone) and solid duroquinone (tetramethyl-p-benzoquinone). Hence quinones are reversible redox systems not only in solutions but also in the solid state. During discharge and recharge in dilute sulphuric acid only slight polarization occurs, which is attributed to a pure diffusion overvoltage. Polarization is not much greater in salt solutions. In this case, however, the peaks of the voltammetric curves are broader. In ammonium chloride solutions a double peak corresponding to the two electron transfers is obtained for chloranil both anodically and cathodically; this shows that the semiquinone is stable in ammonium chloride solution. Duroquinone also gives two peaks, but these are observed only in non-stationary measurements so that they do not correspond to the state of equilibrium. The pH dependence of the redox potential is 59 mV per pH unit. Only chloranil in ammonium chloride solution shows two redox potentials, depending on the discharge depth, which are both lower than the potential corresponding to the pH. The possible application of quinones in secondary battery cathodes is discussed.

Anodic oxidation of Schiff bases. Part I. Oxidation of N-benzylidene-p-anisidines in acetonitrile

Abstract: The anodic oxidations of several N-benzylidene-p-anisidines have been investigated by cyclic voltammetry and controlled potential electrolysis at a glassy-carbon electrode in acetonitrile. All the compounds tested were oxidized irreversibly. N-Benzylidene-p-anisidines showed two anodic waves. The relationship between the peak potentials of their first wave and the σ+ values of the substituents on the benzylidene group was linear, whereas that for the second wave was not. On controlled potential electrolysis at the first wave, protonated benzylidene-p-anisidines were identified, and on electrolysis at the potential of the second wave, p-benzoquinone imine and the corresponding benzaldehyde were identified as the main products. When water was added to the solution, the second wave shifted to a lower potential, whereas the potential of the first wave remained unchanged. Hence a combined wave was observed in the presence of sufficient water. On controlled potential electrolysis in the presence of 5% water, p-benzoquinone and the corresponding benzaldehyde were formed as the main products. Analysis of the minor product suggested that the p-anisidine radical cation was formed as an intermediate. A possible reaction mechanism is suggested.

Anodic oxidation of organic nitrogen compounds. Part I. Cyclization of 1-arylmethylenesemicarbazides

Abstract: The anodic oxidation of a number of 1-arylmethylenesemicarbazides (I) has been examined in acetonitrile–acetic acid containing sulphuric acid. The reactions were very sensitive to the concentration of water; under ordinary conditions the oxadiazole being formed in high yield, while stringent exclusion of water by introducing acetic anhydride into the solvent system resulted in the formation of the corresponding triazolinones. Rapid-sweep cyclic voltammetry showed that the cyclizations are extremely rapid. The ease of oxidation of the compounds (I) was sensitive to the para-substituent in the aryl group, electron-donating groups facilitating oxidation in the expected manner.

Electrochemical reactions. Part X. 4-Styrylpyridine: polarography and cyclic voltammetry in dimethylformamide and reduction in protic solvents

Abstract: Polarography of 4-styrylpyridine in anhydrous dimethylformamide shows two separate one-electron waves with E½–1·88 and –2·22 V (versus s.c.e.). Cyclic voltammetry shows the first wave to be reversible and the 4-styrylpyridine anion radical produced has a half-life of ca. 10 s at 25 °C and longer at lower temperatures. In aqueous methanol 4-styrylpyridine shows one polarographic wave. The products from electroreduction in aqueous methanol are the dihydro-derivative and meso- and (±)-2,3-diphenyl-1,4-di-(4-pyridyl)butane. The (±)-isomer was resolved. Other substituted 4-styrylpyridines give analogus products on reduction.

Electrochemical studies of copper and thallium

Abstract: The anodic growth of a Cu2O film on polycrystalline copper surfaces was studied by a potentiostatic method. Plots of anodic current versus time allowed the growth law to be determined and the Cu2O film thickness was found to vary linearly with the cube root of time. The same growth law applied when Cu2O films were formed by aqueous oxygen attack at the copper surfaces. A mechanism has been proposed explaining this law, based on assumptions of Cabrera and Mott for gaseous oxidation of copper. The cyclic voltammetry technique was used to study the growth, dissolution and reduction of Cu2O and the higher copper oxides over the pH range 5.20 to 13.83. Cu2O was the first oxide formed, and depending on the pH, either CuO or Cu(OH)2 was formed over the underlying Cu2O at higher potentials. In NaOH solutions, of concentration 0.10M and greater, presence of the oxide, Cu2O3 was noted at potentials corresponding to oxygen evolution. During cathodic reduction of the films formed anodically, the Cu(II) product was initially reduced to Cu2O, followed by Cu2O reduction to Cu metal. Solid state reactions for the growth, dissolution and reduction processes are included. No explanation for peak formation during anodic film growth by the cyclic voltammetry technique can be found but plateau formation is predicted. The inclusion of chloride in the anodically formed films increased the conductivity of the oxide films greatly. A 0.100M Na2B407 solution containing 9.6 x 10-4M benzotriazole was found to inhibit the formation of films at the copper surface probably by the formation of a Cu(BTA) polymer. The presence of sodium hexametaphosphate had no effect on oxide growth kinetics at pH 9.18. Potentials of specimens of copper piping corroding in Auckland's supply water were measured against a saturated calomel electrode. The values depended largely on the conditions of water flow and stagnation. Illumination and surface preparation of the samples had little effect. The corrosion potentials reached a maximum after approximately 30 days and the high potentials observed were attributed to the resistance of the films of corrosion products covering the metal surface. A mechanism for the corrosion process has been proposed with the electrophoretic deposition of a layer of silica gel over the underlying Cu2O film causing local cell action and the formation of anodic and cathodic areas over the copper surface. The gel allows diffusion of Cu2+ into the solution from the anodic regions and OH-from the cathodic regions, forming a loosely adherent flocc of Cu(OH)2 the metal surface. This flocc is readily scoured from the surface leading to Auckland's green water problem.

The reduction mechanisms of water soluble porphyrins

Abstract: ix INTRODUCTION 1 Electrochemistry 5 Photochemical and Chemical Reduction „ 9 Solution Chemistry „ 9 Crystallography 10 Theoretical Considerations 11 METHODS 12 Apparatus 12 Materials 18 Procedures 19 RESULTS AND DISCUSSION 22 Thin-layer Cell Design 22 Reduction Mechanism of (TpPyCH3P)l4 29 Structure of (TpPyCH3P)l4 29 Polarography and Cyclic Voltammetry 31 Thin-layer Coulometry 42 Thin-layer Spectrometry 46 Rapid-scan Spectrophotometry 51 Proposed Mechanism 54 Reduction Mechanism of TmPyP Diacid , 59 Structure of TmPyP 59 Cyclic Voltammetry 61 Thin-layer Coulometry 63 Thin-layer Spectrometry 63 Proposed Mechanism 69 Reduction Mechanism of T0PyP Diacid 69 Structure of T0PyP 69 Cyclic Voltammetry 71 Thin-layer Coulometry 71 Thin-layer Spectrometry 74

Chemical and electrochemical investigations of cobalt cyanide and ruthenium ammine complexes

Abstract: Part I . The stoichiometry and kinetics of the reaction between Co(CN5H3- and HgX2 (X = CN, OH) have been investigated. The products of the reaction are two new complexes, [(NC)5Co-HgX]3- and [(NC)5Co-Hg-Co(CN)5]6-, whose spectra are reported. The kinetic measurements produced a value for the forward rate constant of the reaction Co(CN)5H3- + OH- k1/k-1 Co(CN)54- +H2O, k1 = (9.7 ± 0.8) x 10-2 M -1 sec-1 at 24°C, and an equilibrium constant for the reaction K = 10-6 M -1. Part II . Unusually large and sharp "adsorption waves" appear in cyclic voltammograms of Co(CN)53- and several cobalt(III) pentacyano complexes at stationary mercury electrodes. The nature of the adsorbed species and the reasons for the absence of the adsorption waves in polarograms taken with a d.m.e. have been examined. The data are compatible with the adsorption, in all cases, of a coordinatively unsaturated cobalt(II) complex, Co(CN)42-, by means of a cobalt-mercury bond. When the resulting adsorbed complex is reduced, a series of subsequent chemical and electrode reactions is initiated in which three faradays of charge are consumed for each mole of adsorbed complex. The adsorption of the anionic complex strongly retards the reduction of other negatively charged complexes. Part III . A number of formal redox potentials for RuIII (NH3)5L + e = RuII (NH3)5L and RuIII(NH3)4L2 + e = RuII (NH3)4L2 (where L is various ligands) has been measured by cyclic voltammetry, potentiometry, and polarography and are discussed in terms of the properties of the ligands, such as π-accepting capability. Reduction of coordinated pyrazine in the complexes, Ru(NH3)5 Pz2+, cis- and trans-Ru(NH3)4Pz22+, on a mercury electrode has been observed. The behavior of this reduction in various acidity of the solution as well as the reoxidation of the reduction products are discussed.