ORP Measurements In Ultra Pure Water

ORP (Oxidation-Reduction or redox Potential) measurement is used to indicate the presence of oxidizing or reducing conditions in water. In pure water treatment, ORP is most commonly used to assure removal of chlorine or other oxidizing agent ahead of membranes or deionization resins that could be damaged by oxidation. It can warn if the bisulfite feed or carbon bed is not properly removing the chlorine. ORP is also used in other process and wastewater applications.

The correlation between the concentration of chlorine or other oxidizing or reducing material and ORP varies. ORP is logarithmically related to concentration, however, the variables of pH, temperature and other oxidizing or reducing materials in the water can cause ORP response to shift significantly. For this reason there is no reliable conversion from ORP in millivolts to concentration in parts per million or billion.

In moving from an oxidizing to a reducing condition with extremely small (parts per billion) concentration changes, the ORP typically drops several hundred millivolts. It is very sensitive in detecting the change and is therefore quite useful as a go/no-go alarm parameter. In alarm applications, the exact setpoint value is usually not critical. However, where ORP is used to control bisulfite feed, more precision is needed in establishing the control point. A titration curve illustrates ORP response and the considerations in establishing a setpoint.