In 1982 the German Standards Agency adopted the ORP (750 mV) for public pools and in 1988 the National Swimming Pool Institute adopted ORP (650 mV) for public spas. In 1971 ORP (700 mV) was adopted by the World Health Organization (WHO) as a standard for drinking water. These tests were confirmed by studies on drinking water and swimming pools in other areas of the world. These studies showed a strong correlation of ORP and bacterial activity. ORP electrodes were first studied at Harvard University in 1936. That charge is measured relative to the solution, so the solution “ground” voltage comes from the reference junction – the same type used by a pH sensor. As with pH, a very dilute solution will take time to accumulate a measurable charge.Īn ORP sensor uses a small platinum surface to accumulate charge without reacting chemically. In a solution of only one active component, ORP indicates concentration. Like pH, it is not a measurement of concentration directly, but of activity level. ORP is measured in millivolts (mV), with no correction for solution temperature. For instance, chlorinated water will show a positive ORP value whereas sodium sulfite (a reducing agent) loses electrons and will show a negative ORP value. Like acidity and alkalinity, the increase of one is at the expense of the other.Ī single voltage is called the Oxidation-Reduction Potential, where a positive voltage shows a solution attracting electrons (oxidizing agent). Oxidizers accept electrons, reducers lose electrons.Įxamples of oxidizers are: chlorine, hydrogen peroxide, bromine, ozone, and chlorine dioxide.Įxamples of reducers are: sodium sulfite, sodium bisulfate and hydrogen sulfide. Oxidation Reduction Potential or Redox is the activity or strength of oxidizers and reducers in relation to their concentration.

Sodium sulfide charge site