The ME4-H2S-E4 sensor is a constant-potential electrolytic-type hydrogen sulfide (H2S) sensor. H2S and O2 undergo corresponding redox reactions at the working and counter electrodes, releasing electrical charge to generate a current. The magnitude of this current is directly proportional to the H2S concentration; therefore, the H2S concentration can 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.