Abstract: A flow cell with a rotating glassy carbon disk electrode has been exploited for voltammetric and amperometric determinations. The high analytical currents due to the efficient mass-transport are coupled with a new hydrodynamic modulation technique that discriminates against the major background currents, resulting in high sensitivity and low detection limits. For a rotated disk electrode immersed in a stream at a constant flow rate, the current difference is measured with the rotation switched on and off. Well defined current-potential curves are obtainable. Mass-transport properties, sensitivity, precision and linearity of response are reported. Modulation rates range from 0.1 to 1 Hz. Cell performance is demonstrated by the anodic oxidations of micromolar concentration levels of ascorbic acid, NADH, and hexacyanoferrate(II).

Abstract: The design and properties of a porous tubular electrode, in which effective mass transport is ensured by a stirrer rotating inside the tube, are described. High currents are possible owing to the high surface area and mass transport rates. The dependence of the limiting current and the degree of conversion on stirring and flow rate and on electrode length are described. A hydrodynamic modulation mode, based on measuring the current difference while switching the stirrer on and off, is used to correct for background. The cell was used for the determination of caffeic acid, paracetamol, dopamine and Fe(CN)64- in flow injection and continuous-flow analyzes, with detection limits at the nM level.

Abstract: Conductive carbon cement (CCC) was evaluated as matrix material for the preparation of electrodes bulk-modified with electrocatalysts. For pure CCC electrodes the background current characteristics were examined. In acidic or neutral phosphate buffers the useful electrode potential range was from -0.3 to + 1.0 V vs. SCE, while in 0.1 mol 1-1 NaOH it was from -0.3 to + 0.7 V. The electrochemical reversibility of CCC electrodes was examined by measuring the standard rate constants for the reduction of hexacyanoferrate (III) and the oxidation of hydroquinone, using cyclic voltammetry (CV) and rotating disk experiments. The reversibility of a CCC electrode was comparable with that of a freshly polished glassy carbon electrode and better than that of carbon paste electrodes. CCC was used as matrix for the preparation of electrodes bulk-modified with cuprous oxide and cobalt phthalocyanine (CoPC). With a Cu2O-CCC electrode the oxidation potential of glucose, which shows sluggish kinetics at unmodified carbon electrodes, was strongly reduced. The kinetics of the mediated glucose oxidation has been studied with a rotating disk electrode. It was shown that at glucose concentrations higher than approximately 1 mmol L-1 the electrochemical regeneration of the catalyst becomes rate-determining. The Cu2O-CCC modified electrode has been applied with a constant potential in flow-injection analysis for the determination of glucose. The long-term stability of the electrode was studied; repeated injections of a glucose solution during a period of 6 h yielded a relative standard deviation of the peak height of 1.8% (n = 57). In CV experiments the electrocatalytic activity of CoPC was shown for the oxidation of various compounds such as penicillamine, hydrazine and bile acids. Application of the CoPC-CCC electrode for the detection of bile acids in flow-through detection with a constant or pulsed potential failed, due to a rapid deactivation of the electrode.