Analytical solution channel electrode

Qian and Bau [144] have analyzed such electroosmotic flow cells with embedded electrodes on the basis of the Stokes equation with Helmholtz-Smoluchowski boimdary conditions on the channel walls. They considered electrode arrays with a certain periodicity, i.e. after k electrodes the imposed pattern of electric potentials repeats itself An analytic solution of the Stokes equation was obtained in the form of a Eourier series. Specifically, they analyzed the electroosmotic flow patterns with regard to mixing applications. A simple recirculating flow pattern such as the one... 

Of considerable interest is the use of small isolated electrodes, in the form of strips or disks embedded in the wall, to measure local mass-transfer rates or rate fluctuations. Mass-transfer to spot electrodes on a rotating disk is represented by Eqs. (lOg-i) of Table VII. Analytical solutions in this case have to take account of curved streamlines. Despic et al. (Dlld) have proposed twin spot electrodes as a tool for kinetic studies, similar to the ring-disk electrode applications of disk and ring-disk electrodes for kinetic studies are discussed in several monographs (A3b, P4b). In fully developed channel or pipe flow, mass transfer to such electrodes is given by the following equation based on the Leveque model ... 

Convection-based systems fall into two fundamental classes, namely those using a moving electrode in a fixed bulk solution (such as the rotated disc electrode (RDE)) and fixed electrodes with a moving solution (such as flow cells and channel electrodes, and the wall-jet electrode). These convective systems can only be usefully employed if the movement of the analyte solution is reproducible over the face of the electrode. In practice, we define reproducible by ensuring that the flow is laminar. Turbulent flow leads to irreproducible conditions such as the production of eddy currents and vortices and should be avoided whenever possible. 

Channel cell An electrochemical cell in which analyte solution flows at a velocity V over flat stationary electrode(s). 

Convection That form of mass transport in which the solution containing electroanalyte is moved natural convection occurs predominantly by heating of solution, while forced convection occurs by careful and deliberate movement of the solution, e.g. at a rotated disc electrode or by the controlled flow of analyte solution over a channel electrode. 

EC reactions at tubular and channel electrodes have been considered [208]. An analytical solution is not possible due to the non-uniformly accessible nature of the electrode. However, an approximate equation for the half-wave potential can be written, for a reduction, as... 

The first problem has been overcome through the use of an electrically insulating interface in the cathodic buffer reservoir. This interface is porous, and connects the separation capillary to a detection capillary. The porous contact allows the application of the ground potential in the cathodic reservoir, and electrically insulates the detection capillary from the applied separation voltage. The detection capillary channels the analyte solution to the two-electrode detector. A diagram of the instrumental setup is shown in Figure 12.9. 

One of the advantages of hydrodynamic electrodes over those employed in stationary solution is that both steady-state and time-dependent measurements can be made. Whilst, for the majority of applications, channel electrodes are operated under steady-state conditions, switching to the "time-dependent mode is often useful. Firstly, the sensitivity is enhanced (this is of particular benefit in analytical applications) and secondly, the concentration of species adsorbed or deposited on the electrode can be controlled. In systems in which the electrode is prone to fouling, for example, the time... 

Zero-order kinetics attracts special attention, due to its analytical simplicity and particular characteristics, especially when annulment of concentration at the solid surface is involved. Sellars et al. [61] and Siegel et al. [76] gave Graetz-type solutions for uniform axial heat flux, using the eigenfunction expansion method. Compton and Unwin [77] presented the Laplace s domain analytical solution of the mass transfer problem in a channel cell-crystal-electrode system under Leveque s assumptions. Rosner [78] wrote the solutions for the wall concentration profile as c att 1 -z/zb (z < Zo)> for several classes of boundary layer problems. 

EC detection is a promising alternative for capillary electrophoresis microchips due to its inherent characteristics, allowing a proper miniaturisation of the devices and compatibility with the fabrication processes, in case of an integrated detection. Moreover, the low cost associated permit the employment of disposable elements. As the EC event occurs on the surface of electrodes and the decrease in size usually results in new advantages (see Chapter 32), the possibilities of incorporating EC detectors are broad. The simplicity of the required instrumentation, portable in many cases, suit well with the scaling-down trend. Moreover, as the sample volume in conventional micro-channel devices is less than 1 nL, a very highly sensitive detector should be constructed to analyse even modest concentrations of sample solutions. Since sensitivity is one of the accepted characteristics of EC detection EC-CE microchips approach to the ideal analytical devices. 

Yao et al. reported a flow injection analytical system for the simultaneous determination of acetylcholine and choline that made use of immobilized enzyme reactors and enzyme electrodes [25]. Acetylcholineesterase-choline oxidase and choline oxidase were separately immobilized by reaction with glutaraldehyde onto alkylamino-bonded silica, and incorporated in parallel as the enzyme reactors in a flow injection system. The sample containing acetylcholine and choline in 0.1 M phosphate buffer (pH 8.3) carrier solution was injected into the system. The flow was split to pass through the two reactors, recombined, and mixed with 0.3 mM K4Fe(CN)6 reagent solution before reaching a peroxidase immobilized electrode. Because each channel had a different residence time, two peaks were obtained for choline and total acetylcholine and choline. Response was linear for 5 pM-0.5 mM choline, and for 5 pM 1 mM acetylcholine plus choline. The detection limits were 0.4 pM for choline and 2 pM for acetylcholine. 

---