The ME4-Cl2-E4 sensor is a constant-potential electrolytic-type chlorine (Cl2) sensor. Cl2 and O2 undergo corresponding reduction and oxidation reactions at the working and counter electrodes, releasing charges that generate an electric current. The magnitude of this current is directly proportional to the gas concentration—in accordance with Faraday’s Law—allowing the Cl2 concentration to be determined by measuring the current. The sensor features a fourth electrode—an auxiliary electrode—used to compensate for the zero-point current, resulting in a device characterized by a strong signal level and a low zero-point current.