Donald T. Sawyer

Electrochemistry of oxygen and superoxide ion in dimethylsulfoxide at platinum, gold and mercury electrodes

Abstract The electrochemistry of dissolved oxygen and of superoxide ion in dimethyl-sulfoxide solutions has been studied at platinum, gold and mercury electrodes using voltammetric and chronopotentiometric techniques as -well as controlled-potential coulonietry. In the absence of protic electrolytes and solvents, oxygen is reduced in two one-electron steps, first to superoxide ion at a potential of −0.75 V vs. S.C.E. and then to peroxide ion at a potential of −2.02 V vs. S.C.E. The second step is not observed with a platinum electrode. Superoxide ion is oxidized to oxygen at −0.73 V and is reduced to peroxide ion at −2.02 V. Peroxide ion is oxidized directly to oxygen at +0.75 V by a two-electron process. In the presence of a protic electrolyte (0.1 F NH4ClO4) oxygen is reduced in one step to hydrogen peroxide at −0.28 V. The kinetic parameters for the oxygen-superoxide ion electrode reaction are essentially the same for all three electrodes; the average for all determinations is: αna = 0.55; ks,h0 = 10−3. The diffusion coefficients for oxygen and superoxide ion in 0.1 F (Et4)NClO4-DMSO solutions have been evaluated; Do2 = 3.23·10−5 cm2 sec and Do2 = 1.08·10−5 cm2 sec. Mechanisms are proposed for the electrode reactions and for disproportionation reactions that are consistent with the observed experimental data.

Voltammetric determination of carbon dioxide using dimethylsulfoxide as a solvent

Abstract A voltammetric method has been developed for the determination of dissolved carbon dioxide using dimethylsulfoxide as a solvent. Analytically useful reduction waves are obtained with either a gold electrode or an amalgamated platinum electrode, but the latter gives lower residual currents. The peak current is diffusion controlled and is approximately linearly proportional to the percentage by volume of carbon dioxide in gas mixtures used to saturate the electrolysis solution. In the concentration range 1–30% by volume, the method gives errors in accuracy of less than ± 2%.

Electrochemistry of dissolved gases: II. Reduction of oxygen at platinum, palladium, nickel and other metal electrodes

Abstract The reduction of dissolved oxygen has been studied at Pt, Pd, Ni, Ag, Au, Ta, W, Cu and Pb electrodes. Voltammetric and chronopotentiometric studies have established the effect of supporting electrolyte. solution-pH and electrode preconditioning upon the electrode reactions. Oxygen reduction at pre-oxidized metal electrodes is pH-dependcnt ; for the Pt, Pd and Ni electrodes the electrode reaction for oxygen reduction is M(OH) 2 + 2 e − → M + 2 OH-. The oxygen in solution re-oxidizes the electrode which is then re-reduced. The reduction of oxygen at all pre-oxidizcd metal electrodes except Ag and Au appears to occur by the same mechanism. The half-wave potential for oxygen reduction at pre-reduccd metal electrodes is −0.18 to −0.24 V vs . S.C.E, and is independent of pH ; hydrogen peroxide is the primary reduction product. A sequence of reactions is proposed to account for the formation of H 2 O 2 in acidic solutions by a pH-independent mechanism. Potassium iodide causes the reduction of oxygen to become pH-indcpendent at Pt, Pd and Ni electrodes; the potential for reduction becomes dependent, however, upon iodide concentration. The formal reduction potentials for the Pt(OH) 2 . Pd(OH) 2 and Ni(OH) 2 electrodes in 0.1 F K 2 SO 4 have been evaluated; the average values are −0.14, −0.19 and −0.74 V vs . N.H.E., respectively.

Electrochemistry of nitric oxide and of nitrous acid at a mercury electrode

Summary The electrochemistry of nitric oxide and nitrous acid have been studied at a mercury electrode in buffered aqueous solutions using chronopotentiometry and controlled-potential coulometry. The reduction of NO is an irreversible one-electron process with α n a =0.40 and k s,h =10 −6 cm sec −1 . Gas chromatographic analyses establish that the only product is N 2 O. The electroactive species appears to be a slowly formed intermediate, HNO + , with a formation constant of 10 7 . Nitrous acid is irreversibly reduced at a mercury electrode by a one-electron process, the kinetic parameters for the rate-controlling electron transfer step having the values α n a =0.49 and k s,h =10 −7 cm sec −1 . The electroactive species appears to be a rapidly formed intermediate, H 2 NO 2 + . Controlled-potential coulometry for the reduction of nitrous acid indicates an overall three-electron process (at pH 3.00). Gas chromatography establishes that the major products are equivalent amounts of NH 2 OH and N 2 O. The actual electrochemical reduction of HNO 2 is an overall four-electron process to give NH 2 OH which reacts with HNO 2 to give N 2 O. Mechanisms consistent with the experimental data are proposed for the reduction of NO and of HNO 2 .

The electrochemical reduction of nitrous oxide in alkaline solution

Summary The reduction of N 2 O has been studied by cyclic voltammetry for a variety of solution conditions. At a platinum electrode with a freshly formed oxide film N 2 O is reduced by an adsorption process. The reduction process is blocked by the presence of NO, NO 2 − , PbO 2 − or I − in the sample solution. At more negative potentials the N 2 O appears to be repelled from the surface by double layer effects.

Electrochemistry of dissolved gases: III. Oxidation of hydrogen at platinum electrodes

Abstract Voltammetric and chronopotentiomatic studies have been made to determine the effect of supporting electrolyte, pH, and electrode pre-conditioning upon the oxidation of dissolved hydrogen at platinum electrodes. The results of these investigations lead to the conclusion that hydrogen can be electrolytically oxidized by four different mechanisms; (a) atomic hydrogen adsorbed on the platinum surface (observed with pre-cathodized electrodes), (b) absorbed hydrogen in the platinum electrode (observed after the electrode is exposed for long periods of time to dissolved hydrogen), (c) molecular hydrogen at an activated platinum electrode (accomplished by first forming platinum oxide electrolytically, which, subsequently is reduced by hydrogen to give the activated surface), and (d) cyclic reaction of molecular hydrogen with platinum oxide and the electrolytic re-formation of the platinum oxide (observed with unconditioned electrodes). The third mechanism is the most important, as well as the most reversible, reaction. Only the second mechanism is independent of pH. In acidic solutions a platinum electrode, which has been aged in dissolved hydrogen, exhibits an oscillating potential for the electrolytic oxidation of hydrogen. This behavior appears to be due to a combination of mechanisms (c) and (d). Hydrogen is not oxidized under similar conditions at gold or boron carbide electrodes, which supports the conclusion that platinum is a somewhat unique electrode for the electrolytic oxidation of hydrogen.

Electrochemical oxidation of hydrazine and of the dimethylhydrazines in dimethylsulfoxide at a platinum electrode

Summary The electrochemical oxidation of hydrazine, 1,1-dimethylhydrazine and 1,2-dimethylhydrazine in DMSO has been studied by chronopotentiometry, controlled-potential coulometry, and cyclic voltammetry at platinum electrodes. For these compounds the overall reaction is a one-electron oxidation in DMSO. The heterogeneous kinetic parameters for the rate-controlling electrode reaction for the three hydrazines have been evaluated. The values of (1−α) and log ( k s,h /cm s −1 ) for hydrazine, 1,1-dimethylhydrazine, and 1,2-dimethylhydrazine, respectively, are: 0.33, −6.5; 0.46, −9.0; and 0.44, −7.5. The oxidation products have been identified, and oxidation mechanisms consistent with the data are proposed.

Electrochemistry of phenazine at a platinum electrode in aprotic solvents.

The nonaqueous electrochemistry of phenazine has been studied as function of solvent and solution acidity in dimethylsulfoxide, dimethylformamide, and acetonitrile. Cyclic voltammetry, chronopotentiometry, and controlled potential electrolysis have been used to determine the stoichiometry, thermodynamics, and kinetics of the electron transfer processes. The results indicate that phenazine is reduced in neutral solutions by two one-electron steps with a stable radical anion produced by the first of these. Under acidic conditions, phenazine is reduced by a single two-electron process in DMSO and DMF, and by two reversible one-electron steps in acetonitrile. The interactions of oxygen and of hydrogen peroxide with phenazine and its reduction products have been determined. Reduction mechanisms are proposed which are consistent with the electrochemical data and the products.

Determination of sulfur dioxide in solution by anodic voltammetry and by u.v. spectrophotometry

Abstract A voltammetric method, has been developed for the determination of total sulfite in sulfuric acid solutions using activated platinum electrodes. The diffusion coefficient for sulfur dioxide in 0.1 F H 2 SO 4 is 2.1 · 10 −5 cm 2 /sec on the basis of the voltammetric data. The proper conditions for the spectrophotometric determination of total sulfite have been established. The molar absorptivity in 0.1 F H 2 SO 4 is 388 l/moles cm at the absorption maximum, 276 mμ.

Electrochemical oxidation of cyanide ion at platinum electrodes

Abstract Voltammetric, chronopotentiometric and galvanostatic studies have been used to investigate the electrochemical oxidation of cyanide ion at platinum electrodes. The reaction is a diffusion controlled process involving a one-electron oxidation of free cyanide ion to cyanogen. The anodic transfer coefficient, (I-α), for a pre-oxidized electrode in a pH 9.5 solution varies from 0.35 for 10 −3 F KCN to 0.18 for 0.25 F KCN. The average value for the forward, heterogeneous rate constant, k o f , h , is 2.1·10 −8 cm sec −1 (using potentials relative to the normal hydrogen electrode). The proposed mechanism for the oxidation reaction involves a platinum oxide film. Increasing cyanide ion concentrations are postulated to cause metathesis of the oxide to platinum cyanide, which brings about a decrease in (1-α) and inhibition of the oxidation reaction.