a galvanic electrode used for measuring electrode potentials. In most cases, a measurement is made of the potential difference between the electrode being tested and a selected reference electrode with a known potential relative to the potential of a normal hydrogen electrode, which by convention is taken as zero (in a stricter sense, zero is accepted as the potential of a standard hydrogen electrode, which differs from a normal hydrogen electrode in that the activity of the H⁺ ions, rather than the concentration, is equal to unity). The measured difference is taken as the potential of the electrode being tested, and the specific reference electrode against which the potential was measured is specified.

Electrodes selected for reference purposes have potentials that are stable and reproducible. They may be classified according to the nature of the electrochemical reactions that take place in them. The reactions must be highly reversible in order to avoid a change of potential when a small current flows through the reference electrode.

The reference electrodes that are most often used include calomel electrodes (HgǀHg₂Cl₂ǀKCl or HCl), silver chloride electrodes (AgǀAgClǀKCl or HCl), mercuric sulphate electrodes (HgǀHgSO₄ǀH₂SO₄), mercury oxide electrodes (HgǀHgOǀKOH), and quinhydrone electrodes (Ptǀhydro-quinone, quinone ǀ HCI).

The potentials of reference electrodes depend on the concentration of potential-determining ions. For example, in calomel reference electrodes the potential depends on the concentration of Cl^– ions. The potentials of 0.1 N type, 1 N type, and saturated calomel reference electrodes at 25°C are equal, respectively, to 333, 280, and 241 millivolts (mV) relative to a normal hydrogen electrode. The variation in the potentials φ of reference electrodes with temperature t in °C is described by temperature coefficients, which are different for the various electrode types. In the case of a 1 N type calomel reference electrode, for example, φ = + 280 – 0.24 (t – 25) mV relative to a normal hydrogen electrode (NHE) at the same temperature (by definition φ_NHE = 0 for all temperatures).

The selection of a reference electrode depends on the measurement conditions. In nonaqueous media it is possible to use an aqueous reference electrode, but in this case the diffusion potentials at the boundary between the aqueous and nonaqueous solutions must be taken into account. Metal electrodes are used in fused electrolytes; they have potentials that do not vary with time in the particular electrolyte.