AbstractThe size distribution of precipitates formed in a double decomposition reaction has been found previously to be highly sensitive to pH. Consequently, automatic control of pH is necessary to maintain a constant product size distribution. In the present study, steady‐state, dynamic, and pH control tests were performed on a continuous precipitator in which calcium carbonate was produced. Agreement between the experimental dynamic response and that predicted by a model based on equilibrium and stoichiometry was poor. However, satisfactory feedback control of the pH was achieved using proportional plus integral action. Simultaneous measurements of the precipitate size distribution using a Coulter Counter demonstrated that this resulted in good conrol of the mean paricle size.