Abstract: Apparatus and process are provided for measuring and monitoring the free active chlorine content of an aqueous solution. Regulation of the addition of chlorinating substances to maintain a desired chlorine level in drinking or bathing water is further provided. The apparatus preferably utilizes a measuring cell having electrodes comprised of a gold-nickel metallic alloy wherein gold is contained in an amount 90-97% by weight and nickel in an amount of 10-3% by weight. The measuring cell is connected to an electrical comparison circuit which controls a regulator or magnetic valve, in order to control the flow of active chlorine into the system.