State model and stability analysis of a pH control process

Abstract Basic phenomena of H+and H− ion concentration process are considered, and state apace models are derived for a basic continuous flow system. Inherent non-linearity of the system can he avoided by a change of variables, hut due to the logarithmic measurement via pH the corresponding feedback control system is highly nonlinear. A basic feedback system is proved to be globally asymptotically stable. Through an augmentation of the basic model it is shown that a relatively small non-ideality in a real system is sufficient to make it unstable. On the other hand, a steady state analysis show that the concentration equilibrium is highly sensitive to deviations of parameters related to the controlling chemical reagent, if a concentrated acid or base is being neutralized in a single stage.