Dennis C. Johnson

Amperometric detection of simple carbohydrates at platinum electrodes in alkaline solutions by application of a triple-pulse potential waveform

Abstract The response of ten simple carbohydrates was investigated voltammetrically at platinum electrodes in 0.10 M sodium hydroxide by application of a conventional linear sweep waveform and a triple-pulse waveform. Linear-sweep data were interpreted to suggest that electrochemical reactions of the carbohydrates involve oxidation of adsorbed hydrogen atoms produced by surface-catalyzed dehydrogenation of the adsorbed carbohydrate. The triple-pulse measurement technique was evaluated for a flow-injection system by introducing 100-μl samples into a stream of 0.1 M NaOH with a flow rate of 0.375 ml min -1 , and measuring the peak current. Peak currents for ten carbohydrates at 0.5 mM ranged from 17 to 42 μa and a detection limit of 0.01 mM was evaluated for dextrose. Calibration plots of reciprocal peak current ( I -1 p ) vs. reciprocal of concentration (C -1 ) were linear for dextrose and sorbitol concentrations between 0.1 and 1.0 mM.

Concentration dependence of the mechanism of glucose oxidation at gold electrodes in alkaline media

Abstract The catalytic oxidation of glucose in alkaline solution at a gold electrode was studied by cyclic voltammetry at a rotating disc electrode as a function of glucose concentration and rotation velocity. Concentrations M produced mass transport limited currents at a potential of ca. 0.4 V versus SHE with n = ca. 8. Concentrations > 30 m M produced currents limited by a kinetic step involved in the electron transfer reaction with n = ca. 2. From comparison of the voltammetric responses of glucose and glucose derivatives, the mass transport limited reaction was believed to proceed by the pairing of the enediol conformation of glucose, via hydrogen bridges, to the catalytic hydrous gold oxide, followed by oxidative cleavage of the enediol and oxidation of the primary hydroxyl group. The kinetically limited reaction for large glucose concentrations was concluded to effect only the oxidation of the aldehyde or hemi-acetal group producing gluconic acid or gluconolactone. The rate determining step for this process was concluded to be the transfer of an electron from an adsorbed carbohydrate radical species to the electrode. The concentration dependence of the mechanism of glucose oxidation is explained by a decrease in the number of interactions between each glucose molecule and the catalytic hydrous gold oxide as the glucose concentration is increased.

A ring-disk electrode study of the current/potential behaviour of platinum in 1.0 M sulphuric and 0.1 M perchloric acids☆

Abstract The production of small amounts of soluble species during the oxidation and reduction of a platinum electrode in 1 M H 2 SO 4 and 0.1 M HClO 4 has been established using the rotating ring-disk electrode. Pt(II) is produced when an oxidized platinum electrode is reduced. The potential range for the formation of Pt(II) corresponds to that predicted on thermodynamic grounds for the reduction of PtO 2 . An unidentified species is produced during the oxidation of the platinum electrode. This species undergoes a first order homogeneous decomposition with a rate constant of 27 s −1 (mean value for both acids). Repeated potential cycling (5 or more times) of a platinum electrode in 1.0 M H 2 SO 4 between 0.2 and 1.2 V (sce) results in a reproducible current/potential curve, for which 50 μC/cm 2 more charge is required on the anodic cycle than during the cathodic cycle. The soluble products detected at the ring electrode account for 6 μC/cm 2 of the excess anodic charge at the disk electrode.

A Consideration of the Application of Koutecký‐Levich Plots in the Diagnoses of Charge‐Transfer Mechanisms at Rotated Disk Electrodes

It has become common practice to estimate the numbers of electrons (n, eq mol−1) transferred in faradaic reactions at rotated disk electrodes (RDEs) from the slope of plots of reciprocal current (1/I) vs. reciprocal square root of rotational velocity (1/ω1/2). This practice is based on the Koutecký-Levich equation derived for a one-step electron-transfer mechanism. Furthermore, the intercept of the Koutecký-Levich plot is assumed to be a reciprocal function of the heterogeneous rate constant (kh, cm s−1) for the electron-transfer process. In this review, we examine the validity of the practice of estimating values of n and kh for various multi-step mechanisms at RDEs.

Direct electrochemical degradation of organic wastes in aqueous media

The goals of research reported here include the discovery of optimal electrocatalytic materials and electrolysis conditions for rapid degradation of organic compounds to CO2(g). Concepts of the designated procedure are summarized with emphasis on the carboxylic acids generated as intermediate products during anodic degradation of 4-chlorophenol and benzoquinone. Data are summarized for various electrode materials including oxide-covered Pt, as well as PbO2 and SnO2 films that are heavily doped with altervalent metallic cations. The anodic degradation reactions have in common the fact that they involve transfer of O-atoms from H2O in the solvent phase to the oxidation products. Therefore, variations in reactivity of electrode materials are ascribed to differences in the extent of electrocatalytic participation of these electrode surfaces within the anodic O-transfer mechanisms. Results are explained on the basis of the following mechanistic speculations: (1) a prerequisite of the required anodic O-transfer reactions is the discharge of H2O to generate adsorbed hydroxyl radicals; (2) a co-requisite is preadsorption of the reactant species; (3) the O-transfer step occurs from (OH)ads to (R)ads; and (4) an inevitable but undesirable concomitant reaction is the anodic evolution of O2(g). The expected benefit of elevated anode temperature is verified for degradation of acetic acid at a Pt anode. Also, cyanuric acid generated as a stable product of photocatalytic degradation processes is demonstrated to be degraded by anodic reaction at a Pt electrode. Recovery of nitrate is approximately 70% during degradation of cyanuric acid and it is speculated that some volatile N-species is lost during the procedure.

Electroanalytical voltammetry in flowing solutions

Abstract A review is given of the principles, key developments and representative applications of small electrodes in flow-through electrochemical (ec) detectors having very low effective dead volume (

Pulsed amperometric detection of carbohydrates, amines and sulfur species in ion chromatography —the current state of research

A review is given of so-called pulsed amperometric detection at Au and Pt electrodes. Of greatest interest is the application of pulsed amperometric detection for polar aliphatic compounds not easily detected by conventional photometric or fluorometric techniques. The anodic detection mechanisms are electrocatalytic in nature under the control of potential-dependent surface states. Oxidations of carbohydrates at Au electrodes in alkaline media occur in a potential region where a submonolayer of adsorbed hydroxyl radicals (·OHads) is formed and speculation is offered on the role of this species in the anodic mechanisms. Very little anodic signal is obtained at Au electrodes for low-molecular-mass n-alcohols; however, a large response is obtained from oxidation of the alcohol moiety of n-alkanolamines. This is attributed to the beneficial effect of adsorption via the amine moiety with the result that the residence time of the molecules at the electrode surface is increased to give a high probability for ultimate oxidation. Amines and sulfur compounds with non-bonded electrons on the N and S atom, respectively, are adsorbed at Au electrodes and are oxidatively desorbed concomitantly with formation of inert surface oxide (AuO). The simultaneous formation of surface oxide produces a large background signal in pulsed amperometric detection. Hence, a modification of the pulsed waveform is described whose application is called integrated pulsed amperometric detection. Applications are shown for pulsed amperometric detection and integrated pulsed amperometric detection in ion chromatography to illustrate strengths of these combined technologies.

Amperometric detection of simple alcohols in aqueous solutions by application of a triple-pulse potential waveform at platinum electrodes

Abstract Application of a triple-pulse waveform is described for the anodic detection of methanol, ethanol, and ethylene glycol. The execution of the waveform incorporates the cleaning and reactivation of the platinum electrode, by alternate anodic and cathodic polarization, with measurement of the faradaic signal for the analyte at the reduced electrode surface. Some results for formic acid are also presented. The waveform is completed within approximately 2 s and the faradaic signal exhibits no decay with time as would be the case for amperometric detection at a constant applied potential. Calibration curves made by plotting —1/ I vs. 1/ C are linear. This is consistent with a reaction mechanism in which the analyte is adsorbed prior to anodic detection. The technique is applicable for detection in chromatographic and flow-injection systems.

Triple-pulse amperometric detection of carbohydrates after chromatographic separation

Abstract Triple pulse amperometry at a platinum wire electrode is demonstrated for chromatographic separations of carbohydrate mixtures on two commercially available “carbohydrate columns” and the performances of the columns are compared. The effect of detector temperature is described; an increase from 35 to 85°C resulted in an increase of sensitivity by a factor of 1.5–2.1. A consideration of errors is presented and dilution is recommended for samples of high concentration to minimize relative error. Results are given for determination of dextrose, fructose, glycerol, and ethanol in three wines with relative uncertainties of approximately 10% or better, calculated at the 90% confidence level.

Electrocatalysis of Anodic Oxygen‐Transfer Reactions: Titanium Substrates for Pure and Doped Lead Dioxide Films

In this paper, voltammetric and kinetic results are described for the electrocatalytic oxidations of various compounds at doped PbO{sub 2}-film electrodes on Ti substrates. Compounds tested include dimethyl sulfoxide and several thiophene derivatives. Results are emphasized for PbO{sub 2} films doped with Fe{sup III} (Fe-PbO{sub 2}), and with Ag{sup I} and Bi{sup V} (Ag--Bi-PbO{sub 2}), that were electrolytically deposited on Ti substrates with the presence of an interlayer of PbO{sub 2} deposited from acidic solutions of Pb{sup 2+} and NaF. These laminar film electrodes are quite stable and offer significantly improved catalytic activity for various anodic oxygen-transfer reactions in comparison to pure PbO{sub 2}-film electrodes.