Abstract: In a hemodialysis system, a microprocessor periodically controls the injection of a quantity of a halogen solution, preferably an iodine solution, to trigger an event alarm. At a predetermined time, the solution is circulated into a chlorine-specific sensor probe to the point of exceeding a threshold on the sensor system. This event is then recorded and a chlorine alarm event light is illuminated. After the slug of solution is flushed from the system, the alarm clears but the event light remains lit. Then, the next time an operator arrives to operate the hemodialysis system, she can verify that the monitor recorded a chlorine event since the previous day. She then resets the event light.