The Caliper
A Publication for Users of Vernier Products

Vernier ORP Sensor

What is an ORP Sensor? "ORP" stands for "oxidation-reduction potential." Simply put, the ORP sensor measures the ability of a solution to act as an oxidizing agent or reducing agent. For example, ORP electrodes are often used to measure the oxidizing ability of chlorine in swimming pools, or to determine when the equivalence point has been reached in an oxidation-reduction reaction.

The electrode has two components: a measuring half cell comprised of platinum metal immersed in the solution in which the redox reaction is taking place, and a reference half cell (sealed gel-filled Ag/AgCl) to which the platinum half cell is referenced.

The Vernier ORP can measure redox potential in the range of -450 to +1100 mV. Readings toward the positive region of this range indicate a strong oxidizing agent, while readings toward the negative region indicate a strong reducing agent. Resolution (with LabPro) is 0.5 mV.

The ORP Sensor can be used to perform a potentiometric titration. This is a fairly common experiment in AP chemistry or college general chemistry classes. When a redox titration just exceeds its equivalence point volume, the potential measured by an ORP electrode will increase rapidly (if there is an excess of oxidizing agent) or decrease rapidly (with excess reducing agent), as seen in the graph below.

ORP mV vs. volume data for the titration of + Fe²⁺ solution with Ce⁴⁺

In the reaction for the titration curve shown above

Second derivative vs. volume plot for the titration of + Fe²⁺ solution with Ce⁴⁺