THE SILVER/SILVER CHLORIDE ELECTRODE
The silver/silver chloride reference electrode is an electrode where the metal (silver) is in contact with a slightly soluble salt (silver chloride), which in turn is in contact with a solution containing the common anion (chloride ion). The ions in the solid phase and those in the liquid phase are in equilibrium.
Silver/silver chloride electrodes have been used extensively in laboratories as internal reference electrodes, for example pH devices, because of their stability and accuracy. They have been used for many years for cathodic protection particularly monitoring purposes, particularly marine environments where the conventional copper/copper sulphate electrodes are contaminated by the salt (NaCl) content. Recent developments in production techniques now enable robust silver/silver chloride electrodes to also be used as portable or permanently installed electrodes to measure the potential of steel in concrete and soil environments. Silver/silver chloride electrodes utilise the reversible reaction shown below:
As with any electrode of this type the potential difference between the silver/silver chloride electrode in a constant chloride ion solution and the Standard Hydrogen Electrode (SHE) have been measured under standard conditions of temperature and pressure under laboratory conditions. The change in potential due to variations in the temperature and anion concentration, which would be the chloride ions in the case of the silver/silver chloride can be theoretically calculated using the Nernst Equation.
Silvion reference electrodes are developed in-house and are manufactured using a "unique” and advanced technique which results in a porous silver matrix. This porous matrix increases the active surface area of the monitoring electrode and enables the deposition of a larger quantity of silver chloride. The deposition is precisely controlled by electrolytic coating and provides high reliability, high stability, greater accuracy and increased life performance. Silvion electrodes consist of an acetal housing body, and a porous ceramic disc at one end of the assembly which allows contact between the field environment and the silver chloride electrode via the electrolyte. These discs are specially designed to reduce drying out problems and to ensure low contact resistance to earth, providing reliable readings. An insulated lead wire connects the central silver wire which can be connected to a high input impedance voltmeter (10MOhm) when measuring the potential of structures to be protected. For embedding in concrete or for use in soil or potable water, the reference electrode contains a known concentration of potassium chloride (KCl) to ensure a constant chloride ion concentration around the silver chloride electrode.