Question

Why do voltammetric techniques require three electrodes whereas potentiometric techniques require only two electrodes?

Answer 1

The term voltammetry is derived from voltamperometry, and it expresses that the current is measured as a function of voltage, i.e., electrode potential. Since any electrochemical cell needs two electrodes, it would be impossible to extract unambiguous analytical information, if both electrodes would determine the magnitude of the flowing current. Therefore, one electrode is made much smaller than the other, so that the flowing current is limited by this electrode only. This electrode is called the working electrode, and the other (larger) electrode is called the auxiliary electrode.

Although an electrochemical cell needs only two electrodes to operate, it is very beneficial to introduce a third electrode: this third electrode is a reference electrode, i.e., an electrode which has a known and fixed electrode potential. No current should ever pass this electrode, as a current would change its potential, and possibly damage this electrode. The reference electrode is used to control the potential of the working electrode by measuring the voltage between these two electrodes.

Furthermore, To study the behavior of analyte at the electrode/electrolyte interface we require both potential and current to be monitored. Take analyte/electrolyte and put a electrode, and if you want to study the behaviour of analyte at this electrode (WE), we need to perturb the system, i.e. take away the system from its equilibrium. This can be done by polarizing the electrode either cathodically or anodically by simply applying potential or current to the WE. In order to apply potential, we require some standard/reference electrode, whose potential is almost constant. After the perturbation, we have to record the effect of perturbation, to do this we have to record the current that developed across WE and some other electrode (generally two leads require to complete the current circuit). This other electrode can be reference or some other third electrode. But if you use the reference as the current carrying electrode, then its potential will change and finally it no more reference electrode. So we have to avoid using this RE as current carrying electrode. So we require a third electrode called Counter or Auxiallry electrode and its main purpose is to complete the circuit to carry current. So for electrochemical behaviour study of analytes using electrochemical transient techniques, such as LSV(linear sweep voltammetry ), CV(cyclic voltammetry) etc. we should use Three electrode system rather two electrode system.