A method for monitoring and adjusting the level of free chlorine in an aqueous fluid is described. In the described method, first aqueous fluid (50) substantially free of free chlorine is introduced into a suitable mixing vessel (26) wherein it contacts a solid chemical material that is a source of free chlorine and chloride ion. Second aqueous fluid (58) containing free chlorine and chloride ion is removed from the mixing vessel and forwarded to a holding tank (10). The chloride ion concentration of second aqueous fluid in the holding tank is measured by a chloride ion specific sensor in probe housing (22), which sensor is in electrical contact with microprocessor (30). Microprocessor (30), in response to the chloride ion measurement, sends an output signal to valve control means 44, which if necessary increases or decreases the flow of first aqueous fluid into mixing vessel (26). Second aqueous fluid in holding tank (10) is forwarded by pump means (20) to the point of application.