Measurement of Electrode potential

The measurement is generally conducted using a three-electrode setup (see the drawing):

1. Working electrode
2. Counter electrode
3. Reference electrode (standard hydrogen electrode or an equivalent)

In case of non-zero net current on the electrode, it is essential to minimize the ohmic IR-drop in the electrolyte, e.g., by positioning the reference electrode near the surface of the working electrode (e.g., see Luggin capillary), or by using a supporting electrolyte of sufficiently high conductivity. The potential measurements are performed with the positive terminal of the electrometer connected to working electrode and the negative terminal to the reference electrode.

 Working elctrode

                                        

The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring.The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode  in a three electrode system . Depending on whether the reaction on the electrode is a reduction or an oxidation, the working electrode can be referred to as either cathodic or anodic  Common working electrodes can consist of inert metals such as gold silver  or platinum, to inert carbon such as glassy carbon  or pyrolytic carbon, and mercury drop and film electrodes. Chemically modified electrodes are employed for the analysis of both organic molecules as well as metal ions.

Auxiliary electrode

The Auxiliary electrode, often also called the counter electrode, is an electrode used in a three                                                             electrode electrochemical cell for voltammetric analysis or other reactions in which an electrical                                                                              current is expected to flow. The auxiliary electrode is distinct from the reference electrode, which                                                                                          establishes the electrical potential against which other potentials may be measured, and the                                                                  working electrode, at which the cell reaction takes place.

In a two-electrode system, either a known current or potential is applied between the working                                                                          and auxiliary electrodes and the other variable may be measured. The auxiliary electrode                                                                           functions as a cathode whenever the working electrode is operating as an anode and vice versa.                                                                     The auxiliary electrode often has a surface area much larger than that of the working electrode                                                                           to ensure that the half-reaction occurring at the auxiliary electrode can occur fast enough                                                                             so as not to limit the process at the working electrode.

When a three electrode cell is used to perform electroanalytical chemistry, the auxiliary electrode, along                                                                             with the working electrode, provides circuit over which current is either applied or measured.                                                                           Here, the potential of the auxiliary electrode is usually not measured and is adjusted to so                                                                                                      as to balance the reaction occurring at the working electrode. This configuration allows the                                                                               potential of the working electrode to be measured against a known reference electrode                                                                                   without compromising the stability of that reference electrode by passing current over it.

The auxiliary electrode may be isolated from the working electrode using a glass frit.                                                                                    Such isolation prevents any byproducts generated at the auxiliary electrode from contaminating                                                                     the main test solution: for example, if a reduction is being performed at the working electrode                                                                        in aqueous solution, oxygen may be evolved from the auxiliary electrode. Such isolation is                                                                        crucial during the bulk electrolysis of a species which exhibits reversible redox behavior.

Auxiliary electrodes are often fabricated from electrochemically inert materials such                                                                                    as gold, platinum, or carbon.

Reference electrode

A reference electrode is an electrode which has a stable and well-known electrode potential.                                                                                                      The high stability of the electrode potential is usually reached by employing a redox system                                                                                                          with constant (buffered or saturated) concentrations of each participants of the redox reaction.
There are many ways reference electrodes are used. The simplest is when the                                                                                                                           reference electrode is used as a half cell to build anelectrochemical cell.                                                                                                                                             This allows the potential of the other half cell to be determined.                                                                                                                                                                 An accurate and practical method to measure an electrode's potential in isolation                                                                                                                                                                                (absolute electrode potential) has yet to be developed.