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Why do you need three electrodes in ECD flow cells?

The circuitry of the ECD 2.1 detector is designed for operation with electrochemical flow cells with a three-electrode configuration.

The working potential is set between the working electrode (WE) and the auxiliary electrode (AUX). The AUX is kept at a precisely defined reference electrode (REF) potential by means of the so-called voltage clamp. This is an electronic feedback circuit that compensates for polarization effects at the electrodes. At the WE, which is kept at virtual ground, the electrochemical reaction takes place, i.e. electrons are transferred at the WE. This results in an electrical current to the I/E converter, which is a special type of operational amplifier. The output voltage of the I/E converter is digitized in the instrument by means of a 24-bit A/D converter and processed, and the resulting output current Ic can be acquired digitally by PC control software (Elite Dialogue or ClarityChrom®) or analog using the ‘Analog Data output’ on the rear panel connected to a recorder or an external A/D converter.

Essentially, for the oxidation or reduction reaction it would be sufficient to use only two electrodes. However, the three-electrode configuration has several advantages over a two-electrode configuration. If the working potential would be applied only over an AUX versus the WE (without REF), the working potential would continuously change due to polarization effects at the electrodes, resulting in highly unstable working conditions. If the working potential would be applied only over the REF versus the WE (without AUX), the working potential would be very well defined. However, the potential of a REF is only well defined if the current drawn is extremely low (pico-amperes) resulting in a very limited dynamic range. A three-electrode configuration, combines the best of both electrodes. The REF stabilizes the working potential and the AUX can supply high currents. This results in the tremendous dynamic range of a three-electrode system.

 

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