What is ORP and how is it measured?

ORP is a measure of redox equilibrium which is an indication of a solution’s ability to oxidize or reduce another solution. The measured potential is in a sense absolute like temperature. Put another way ORP meters and electrodes measure the absolute electron activity within a solution and displays it as a millivolt reading so ORP/Redox measurements in millivolts are a ratio of the voltage potential of the electrons of oxidized to reduced atoms and molecules in a solution where more than one, and at least two, reacting substances are present. Oxidation, such as rusting Iron, causes a loss of electrons, (to Oxygen), and Reduction is the gain of electrons. In a solution there may be only two species of chemicals present or there may be many species of chemical atoms as well as molecules present.

The ORP meter and ORP electrode measure this aggregate electron exchange in millivolts, mV, and displays it as either a positive + mV reading or a negative - mV reading. Typically negative mV readings are much more common and positive readings may require conditioning the ORP electrode with special chemicals. Since many chemical species of atoms and molecules may be present in any given solution the user should have some idea of what chemicals are in solution and what redox measurements to expect.