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The purpose of a reference electrode is to provide a constant and defined potential. This potential is determined by the electrolyte inside the electrode, and the reference element used. Most electrodes are combination electrodes, meaning they combine a stable reference and a working cell (half-cell) in one probe. Using a combination pH electrode has its advantages, but it is not practical for every application. The most common reason for using a separate sensing (half-cell) and reference electrodes is if the different parts of the electrode are expected to have different lifespans. There are also certain applications in which it is more practical or even necessary to use a separate reference electrode.
A range of separate reference electrodes are available that can be used in conjunction with any half-cell. Some of the most common reference systems include:
Ag/AgCl is the most common type of reference system, however if your sample is incompatible with Ag or Cl then the second most common choice is a saturated calomel electrode (Hg/HgCl). Calomel electrodes are very stable; however, they contain mercury which makes them unsuitable for use in certain applications such as food, beverage or environmental studies. Their disposal must also be carefully controlled due to the environmental implications.
Double junction electrodes have lower chamber that contains an electrolyte that differs to the electrolyte in the top reference chamber. The chemical composition of the lower chamber electrolyte can be customised to match (or be more compatible with) the sample. This is important because the lower chamber electrolyte comes into contact with the sample via the junction, and if there is an interaction between the electrolyte and sample, it can cause the junction to block and give erratic readings. Our team can help with application compatibility advice.
We will focus here on the Ag/AgCl reference, since it is the most commonly used reference in electrochemical analysis. The structure of this reference electrode consists of a silver wire with a silver chloride dip, surrounded by a salt electrolyte (this can be solution or gel). The salt is usually potassium chloride (KCl) which has been saturated in silver chloride to ensure the silver chloride dip does not become stripped. In order for a reference electrode to work, a small amount of the internal fill solution will leak from the sensor to the sample, which provides electrical contact and a stable, unchanging potential. This leak path is through the electrode junction, which can be made from a range of materials including ceramic, cotton, teflon and more. The fill solutions should always be selected according to the requirements of the application, so that it does not interact with the sample or cause an error in measurement (such as if it acts as an interfering ion in ISE analysis).
This electrode is based on the following half-cell reaction:
AgCl + e- <->Ag+ + Cl-
An Ag/AgCl reference electrode provides 199mV (±5mV) versus a normal hydrogen electrode if surrounded by saturated KCl/AgCl solution measured at 25°C. Hydrogen electrodes are used as more of a theoretical standard than used practically across laboratories, therefore reference electrodes are often checked against other electrodes with the same reference system, or against other references with known potentials for which data is available.
Author: Amy Measor
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