Abstract: In a chemical reactor or analogous apparatus, a tank or similar system, pH and other concentration variables are controlled by a method wherein -- when at least one desired outlet concentration or a function dependent on outlet concentrations is given -- initially, the apparent output concentration of at least one feed component is calculated with the aid of the inlet concentration and the residence-time distributioned either the requisite feed concentration or flow of each control component with the aid of a control algorithm based on the residence-time distribution of said control component between its feed point and the outlet point of the reactor, or the control of each actuating variable that substantially affects a physical control, with the aid of an appropriate control algorithm; whereby other corresponding mathematical representations, e.g.