An apparatus comprises a first controller configured to control at least the flow rate of a polymerization inhibitor or acrylonitrile product to a first location where the two are mixed. The apparatus comprises a first on-line analyzer configured to measure polymerization inhibitor concentration in the acrylonitrile product at a second location, the second location downstream of the first location. The first on-line analyzer is configured to provide information relating to the measured inhibitor concentration at the second location to the first controller, the first controller configured to process the information from the first on-line analyzer and to adjust at least a first operating parameter of a first addition device if the measured inhibitor concentration at the second location is outside a first predetermined condition. A second controller and a second on-line analyzer are provided to provide similar control of water addition to acrylonitrile product.