Reference electrodes are electrodes that reliably produce the same level of voltage regardless of what is going on around them. Reference electrodes have many applications in the physical sciences, especially in the process of measuring the electrical charge of different substances. A reference electrode normally works in conjunction with an indicator electrode, which changes based upon its surroundings. Several types of reference electrodes exist.
Silver/Silver Chloride Reference Electrodes
Silver/silver chloride reference electrodes are the most common type of reference electrode. They are very easy to manufacture and are able to function in a wide variety of ambient temperatures. The maximum temperature at which this type of electrode can function is about 130 degrees Celsius. These reference electrodes are made up of a silver wire coated with silver chloride.
Calomel Reference Electrodes
Calomel reference electrodes, also known as mercury/mercurous chloride reference electrodes, were widely used up until about 1960. However, a big problem with this type of reference electrode presents itself when the electrode is exposed to temperatures greater than 50 degrees Celsius. The mercurous chloride element of the electrode breaks down under this kind of heat, which will cause unreliable readings. However, it is less prone to contamination than standard silver/silver chloride reference electrodes because sections of it are enclosed within a tube. (ref 1)
Hydrogen Reference Electrodes
Hydrogen reference electrodes consist of a glass cell and a platinum electrode. The manufacturer injects hydrogen into the shell and then injects another liquid hydrogen solution. The hydrogen gas reacts with the solution, creating a reaction that the platinum electrode measures. This measurement is expressed as oxidation-reduction potential (ORP). An electrode with an ORP of 1 is a standard hydrogen electrode, and is used as a standard of measurement for all other electrodes. These types of electrodes are not widely used because they are extremely expensive. (ref 1, 3)
Double Junction Reference Electrodes
Double junction reference electrodes have an outer tube that protects the inner electrode elements. Inside of this outer tube is another electrolytic substance which works as a sort of bridge between the outer substance and the inner electrode. (ref 2)
Liquid Junction Reference Electrodes
Liquid junction reference electrodes work from a different logic than the previous types. They are characterized by a porous plug through which the electrolyte, or reactive inner substance, of the reference electrode passes in order to make contact with the substance which is being tested.