Hydrodynamic amperometry

Mass transport in amperometric systems in which the reagent stream is forced to flow along the surface of the electrode may be described in terms of convective diffusion. Effectively this means that at sufficiently high values of Pg, the Peclet number, the liquid above an electrode may be divided into two distinct zones. In one zone, far away from the electrode surface, convection is important, and the concentration profile is substantially flat. In the other zone, adjacent to the electroactive surface, there is a sharp concentration gradient here diffusion is the predominant mass transport process. The Peclet number is given by v l/D, where is the main stream fluid velocity, and / is the length of the electrode (measured in the direction of fluid flow). Under these conditions, the mass transport limited current z l for a reversible electrode couple (i.e. the concentration of the electroactive form is zero at the electrode surface) is given by 

This current is time-dependent too, but in the convective diffusion case, the current rapidly (within a few ms or so) reaches a stationary value. The speed with which this current plateau is reached arises from the establishment of a well-defined steady-state diffusion layer. The thickness d of this diffusion layer is given approximately by 

the limiting current obtained after the electrode potential is pulsed from a region of electroinactivity to a region of electroactivity is a function of time, decaying from an initial high value to the steady state within a few hundred ms. Further details on the RDE can be obtained from (5). 

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LCEC is a special case of hydrodynamic chronoamperometry (measuring current as a function of time at a fixed electrode potential in a flowing or stirred solution). In order to fully understand the operation of electrochemical detectors, it is necessary to also appreciate hydrodynamic voltammetry. Hydrodynamic voltammetry, from which amperometry is derived, is a steady-state technique in which the electrode potential is scanned while the solution is stirred and the current is plotted as a function of the potential. Idealized hydrodynamic voltammograms (HDVs) for the case of electrolyte solution (mobile phase) alone and with an oxidizable species added are shown in Fig. 9. The HDV of a compound begins at a potential where the compound is not electroactive and therefore no faradaic current occurs, goes through a region... 

Information about suitable working potentials for the amperometric detection of electroactive species are obtained in voltammetric experiments. The term voltammetry refers to the investigation of current-voltage curves in dependence of the electrode reactions, the concentrations and its exploitation for analytical chemistry. Of the different types of voltammetry, information from the hydrodynamic and pulsed voltammetry can best be applied to amperometry. In both cases, the analyte ions are dissolved in a supporting electrolyte which has several functions ... 

This kind of amperometry is the most widely used electrochemical detection method in liquid chromatography. A constant DC potential is continuously applied to the electrodes of the detector cell. The theory of amperometry with constant working potential does not differ from the theory of hydrodynamic voltammetry, even though the applied potential remains constant. 

The instrumentation of HPCE is uncomplicated (see the schematic drawing in Figure 1). Briefly, both ends of the narrow-bore fused silica capillary are immersed into reservoirs containing a buffer solution that also fills the capillary. The reservoirs also contain electrodes that provide electrical contact between the high-voltage power supply and the capillary. The sample is loaded onto the capillary by replacing one of the buffer reservoirs by a sample reservoir and applying external pressure (hydrodynamic injection) or an electric field (electrokinetic injection). After the injection, the reservoir is replaced, the electrical field is applied, and the separation starts. The detection is usually performed at the opposite end of the capillary (normal polarity mode). UV/vis detection is by far the most common detection technique in HPCE. Other techniques include fluorescence, amperometry, conductivity, and mass spectrometry. Modem HPCE instruments are fully automated and thereby allow easy operations and precise quantitative analyses. 

Miller, B. and Bruckenstein, S. (1970) Hydrodynamic potentiometry and amperometry at ring-disk electrodes. 

The determination of the antioxidant capacity is based on the current generated by the electrochemical reduction of DPPH during analysis. Since the current is proportional to DPPH concentration, it is possible to evaluate the percentage of DPPH consumed by the antioxidant. Therefore, the analysis of antioxidant capacity is achieved by the decrease in the DPPH current measured at a constant potential selected by the cyclic voltammetry study (or by the hydrodynamic voltammogram). The current analysis can be performed by various electrochemical techniques, such as cyclic voltammetry, differential pulse voltammetry, and amperometry. 

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