In waste water where toxic hexavalent chromium ions are present, it is essential that the hexavalent chromium ions are reduced to the less harmful trivalent chromium ions by dosing with reducing agent such as ferrous sulphate. The problem posed in controlling such a reduction process is that chromium ions cannot be easily measured on-line. In such a process, oxidation-reduction potential (ORP) is widely used. The ORP value, apart from being dependent on the concentration of chromium ions, is also dependent on other redox processes due the presence of other ion species, concentration of ions, temperature, etc. For this reason, the reduction process cannot be effectively controlled using conventional set-point control. The profiles of the ORP during the reduction process shows certain characteristic features which can be used as an indication of the completion of the process. In this paper we have successfully demonstrated a robust switching algorithm using a Kalman filter to control the dosing pump for this reduction process.