Reference electrode
An electrode which has a constant and perfectly reproducible potential whatever the medium in which it is placed.
The standard reference electrode is the hydrogen electrode, which is based on the H3O+ / H2 couple.
As the hydrogen electrode is difficult to handle reproducibly, other reference electrodes are used. Most commonly these employ the calomel/mercury, mercurous sulphate/mercury, and silver chloride/silver couples.
Such electrodes are used together with an indicating or measuring electrode for potentiometric titrations.