pH is measured to control caustic addition to the stripper to maintain a pH level that balances efficient stripping of hydrogen sulfide with efficient stripping of ammonia. Measuring pH in sour water can be challenging due to the high temperatures and the tendency of hydrogen sulfide to poison and plug a sensor's reference junction. Sensors measuring sour water often have a short life. The Rosemount™ 3500P pH Sensor features high-temperature glass, poison-resistant electrolyte and a double reference junction to provide maximum sensor life in sour water.