Oxidation Reduction Potential (ORP) is the measurement of a medium’s ability to oxidize or reduce another medium.

Oxidation occurs when an oxidizer accepts electrons from other molecules and reduction is when reducers provide electrons to other molecules. An ORP sensor can determine whether oxidation or reduction is occurring by monitoring a single mV value that can be both positive or negative. When a medium is exhibiting oxidation the mV value is positive. When it is exhibiting reduction the mV value will be negative. In addition, you will often hear an ORP measurement called “Redox”. Redox is a combination of the two words, reduction, and oxidation.

ORP Sensor Construction  

An ORP sensor consists of both an ORP electrode and a reference electrode. The transfer of electrons is important when looking at the construction of an ORP sensor. The metal construction of ORP electrodes, commonly Platinum, has low resistance. The electrode will both give up and accept electrons from oxidizers and reducers. The electrode will continue to accept or give up electrons until a potential is developed. It then outputs in millivolts. The reference electrode in an ORP sensor is typically very similar to a pH reference electrode, made up of Ag/AgCl and immersed in a KCl reference material.

Interpreting the ORP Measurement 

There are many different chemical mediums that could be oxidizers or reducers. When monitoring ORP, you don’t get a specific indication of exactly which chemical you’re measuring but rather an indication of the presence of an oxidizer or reducer.  For example, in Pulp and Paper mills, ORP sensors are often used to control Chlorine injection which is used as a disinfectant at the mill. An ORP measurement is an inferred measurement. However, if used correctly, and in combination with an understanding of the measured process medium, ORP measurements can be an effective way to monitor for specific chemicals in a process environment.

Summary 

We use ORP sensors in process applications throughout many industries where controlling chemical compositions is critical. Understanding the basics of ORP measurements enables you to control the process in an efficient and productive way.