Systems and methods for controlling a chloramine synthesis reaction. The system includes at least one water pump configured to deliver a first water stream to a first junction where the first water stream is mixed with a first solution including an ammonium compound, and a second water stream to a second junction where the second water stream is mixed with a second solution including hypochlorite, a reaction chamber downstream of the first and second junctions which is configured to mix the first and second water streams in order to react the ammonium compound and the hypochlorite to form chloramine, an optical measurement device disposed upstream of the second junction, the optical measurement device configured to measure an absorbance of the hypochlorite in the second solution, and at least one controller programmed to determine a concentration of the hypochlorite in the second solution based on the measured absorbance.
