The above answers are right, but for more clarity: 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 I 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, CV etc. we should use Three electrode system rather two electrode system.

Large currents passing through an electrode can change its potential. Therefore, if you want careful control and measurement of both potential and current through a cell, you want to use three electrodes. The working electrode is your sample you are studying, the reference electrode sets the potential of the solution, and the counter electrode is a current source.

The easiest way I have found to understand this concept is to look at the basic circuit of a potentiostat (the instrument used for most electrochemical measurements). The basic circuit of a potentiostat is an op-amp. There is a very good description of this on wikipedia and on Gamry's website.

Wiki Potentiostat: http://en.wikipedia.org/wiki/Potentiostat

Wiki Op-Amp: http://en.wikipedia.org/wiki/Opamp

Gamry Instruments - 2, 3, or 4 electrode cells:

http://www.gamry.com/assets/Application-Notes/2-3-4-Electrodes.pdf

Gamry Instruments - Reference Electrode:

http://www.gamry.com/assets/Application-Notes/ReferenceElectrodes.pdf

A side note I thought of, if you are doing anodization or electrodeposition, then you generally just want two electrodes. For most other electrochemical processes / measurements my understanding is that three electrodes is preferred / more accurate.

I would add to the correct answer by Nathan that the reason because the potential can change in a two electrodes system is the iR drop, where the R is the resistance of the solution, and i the current passing for the circuit. Thus, minimizing the iR drop could not be necessary to use a three electrode system. It can be achieved for several ways, for example, lowering the distance between the electrodes or using ultramicroelectrodes, in which the current measured is very small.

But, like Nathan said for most electrochemical measurements, especially in analysis, is necessary to use a three electrode system.

The answers are right, just to add: in a two electrode system you never know where the interfacial potential difference occures if you change the call voltage; it might be on both electrodes and not at the one you want to study: in bad conducting organic solvents the ixR drop can be considerable. Best is to use a Luggin-capillary for the reference electrode and go with the tip as close to the working electrode surface as possible.

The above answers are right, but for more clarity: 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 I 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, CV etc. we should use Three electrode system rather two electrode system.

Thanks to all for their valuable explanations.....

Three electrode system characterize the electrode material.  It is a basic analysis that gives the information of oxidation/reduction, resistance values and reversibility details of the material.  To optimize and select suitable material via comparison of electrode performance of many electrodes, 3 electrode system helps. 

Then we can study two electrode system for final studies.

Hi,

3-electrode cell just gives you the electrochemical signature of the material by itself. A 2-electrode cell just gives you the current flowing between the 2 électrodes and none of the electrode potential is fixed so that you cannot know at whci potential (vs a reference) a reaction occurs.

Best,

Patrice

The three electrode system remedies many of the issues of the two electrode configuration. The three electrode system consists of a working electrode, counter electrode, and reference electrode. The reference electrode’s role is to act as a reference in measuring and controlling the working electrode potential, without passing any current. The reference electrode should have a constant electrochemical potential at low current density. Additionally, since the reference electrode passes negligible current, the iR drop between the reference and working electrode (iRU) is often very small. Thus with the three electrode system, the reference potential is much more stable, and there is compensation for iR drop across the solution. This translates into superior control over working electrode potential. The most common lab reference electrodes are the Saturated Calomel Electrode and the Ag/AgCl electrode.

In the three electrode configuration, the only role of the counter electrode is to pass all the current needed to balance the current observed at the working electrode.

why it is necessary to use auxiliary electrode in voltametery? why current measure between auxiliary electrode & working electrode , not between , indicator electrode & reference electrode.

Hi Aiman Shamim ,

The purpose of reference electrode is to act as a fixed voltage source, with respect to which, we measure the remaining voltages in the electrochemical cell.

It is designed to not to allow any leakage currents. In any electrochemical experiment, we are interested to know the redox reactions that happen on the indicator electrode and as the result of the reactions, any current generated has no where to go except through the auxiliary electrode.

Therefore, measuring current via the auxiliary electrode gives the direct measure of the current generated from the cell. Ideally, there is no current flow between working electrode and the reference electrode. Hence, no point in measuring current between them.

Hope this helps,

Sujeevan.

but why do not use only in voltametry technique, in potentiometry technique?is it because, constant current maintained during potential measure.

Hi

because in three electrode unit the potential will be controlled and much more stable