Theory of amperometry

Classically amperometry has been concerned with the maintenance of a fixed potential between two electrodes. More recently, pulsed techniques have come to the fore some details of these are included in Table 8.3. The applied potential at which the current measurement is made is usually selected to correspond to the mass transport limited portion of the corresponding voltammetric scan. Theoretically, in a quiescent solution, this is electrochemical suicide. The current ii obtained at conventional electrodes under such circumstances gradually decays to zero according to the Cottrell equation (4)  

This decrease in current is due to the slow spread of the diffusion layer out into the bulk solution, with a concomitant decrease in the concentration gradient. In practice, this process continues for around 100 s or so, after which time random convection processes in the solution take over, and put an end to the further movement of the diffusion layer. Waiting for nearly two minutes, and relying on chance solution convection for a steady-state current, is not particularly attractive. Accordingly, in amperometric measurements for sensing applications, the spread of this diffusion layer is limited by the use of one or more of the following mechanisms convective diffusion the 

The electrode potential is pulsed to a region in which the analyte is electroactive. The decaying current reflects the growth of the diffusion layer. 

Electrode conditioning, analyte sorption and catalytic electrooxidation are all promoted by the use of this waveform. The current is only measured in the last part of the cycle (point A). 

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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. 

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