in the range of 0 to 1000 ppm of a specific ion i e no 2 such equipment is commercially available from dupont the purpose of the on line uv analysis is to measure for residual or unreacted nitrite in the product stream and for this purpose the wavelength of the spectrophotometer is set at 310 nanometers deviations from this set point are transmitted to controller transducer 21 which controls flow control means 14 if values of uv readings are high indicating that the residual nitrite level is above the actual set point then trim ammonia is increased thereby insuring that the desired stoichiometric ratio of no 2 nh 4 is maintained if values of the uv reading are low then trim ammonia is decreased allowing thus the increase of no 2 feed to self adjust to the correct no 2 input at the preset ph and to consume the excess of nh 4 in the system if nh 4 in the feed would exceed for any reason the 2nh 4 1no 2 ratio the trim nh 4 feed would shut off automatically due to the residual nitrite level below the actual set point it will be obvious to those skilled in the art that other instruments available for measuring ion concentrations such as an ion chromatograph may be appropriately utilized in place of the uv analyzer with the control system of this invention there is no need for continuous analysis of the feed for reactant ratios or feed rate adjustments existing and well established instrumentation can be used one for ph and another for residual nitrite each of them tied to the respective flow controllers the signals from the two measuring instruments can be fed to the devices described to make.