Electrochemical detector diffusion current

The wall jet electrode is employed in electrochemical detectors the diffusion-limited current is... 

Within this working range, the presence of a reactive sample will give rise to a cui-rent/potential (// ) curve (Fig. 4.3). This curve is unique for a particular sample/elu-ent/electrode combination. It is characterized by a half-wave potential and by a "diffusion current plateau. The half-wave potential (the potential half way up to the diffusion current plateau) is defined as the potential needed to induce electrolysis of the electroactive species. As the potential is increased, the electrolysis current also increases because more ions migrate to the electrode and become oxidized (or reduced). The electrolysis current eventually forms a plateau in the HE curve because, ultimately, the amount of current is limited by the rate of diffusion of ions to the electrode surface. In normal operation, the electrochemical detector potential is set at the smallest potential possible that is still on the diffusion current plateau. The detector should be on the plateau for consistent performance, but at the lowest potential to lessen the chance of side reactions. 

Electrochemical detectors of the waU-jet type (Fig. 9.6, left) are simple, robust and flexible. Working and counter electrodes (if necessary, the reference electrode as well) are positioned in a large solution reservoir. The disc-shaped working electrode is in plane with one of the walls of this reservoir. In front of it, a fine nozzle is arranged. The carrier solution is ejected as a sharp jet directed towards the electrode area. The jet washes away quickly residues of older solution from the electrode surface and generates a powerful convection. Thus, a relatively high value of the diffusion-limited current is achieved with an amperometric operation, as is a fast response if operated potentiometrically. The virtual dead volume of wall-jet detectors is extraordinarily smalL It cannot be derived from the cell geometry, but must be determined experimentally by calibration. 

This mode is characterized by an incomplete electrochemical conversion of the electroactive analytes that reach the working electrode. In fact, a coulometric efficiency of only 5-10% is usually attained. Consequently, during the residence time of an analyte band flowing through the detector, there is always unreacted analyte in the vicinity of the working electrode. Therefore, the amperometric current measured under these conditions is controlled by the analyte diffusion from the bulk toward the electrode surface which is characterized by a limiting current defined as... 

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