A reference electrode is an electrode

Electrode

An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit...

 which has a stable and well-known electrode potential

Electrode potential

Electrode potential, E, in electrochemistry, according to an IUPAC definition, is the electromotive force of a cell built of two electrodes:* on the left-hand side is the standard hydrogen electrode, and...

. The high stability of the electrode potential is usually reached by employing a redox

Redox

Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....

 system with constant (buffered or saturated) concentration

Concentration

In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...

s 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

Half cell

A half-cell is a structure that contains a conductive electrode and a surrounding conductive electrolyte separated by a naturally occurring Helmholtz double layer. Chemical reactions within this layer momentarily pump electric charges between the electrode and the electrolyte, resulting in a...

 to build an electrochemical cell

Electrochemical cell

An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery"...

. This allows the potential

Reduction potential

Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Reduction potential is measured in volts , or millivolts...

 of the other half cell to be determined.