Understanding ORP

The Heart of Chemical Automation

Oxidation-reduction potential - or ORP, as it is better known - is the measurement of a body of water's ability to oxidize contaminants.

Great. What does that mean?

Most simply, oxidation involved an exchange of electrons between two atoms.  The atom that looses an electron has been oxidized.  The one that gains an electron has been reduced.

While we can't see this process occurring atom-by-atom, there is ample evidence of oxidation taking place every day.

Take a rusting car body or a piece of fruit rotting on the counter.  These are both examples of items being oxidized relatively slowly.

By comparison, a fire or explosion are examples of rather rapid oxidation.

In the pool and spa industry, chemicals like chlorine, bromine and ozone are all oxidizers.  It is their ability to "steal" electrons from other substances that makes them good water sanitizers.

This occurs because as they alter the chemical make-up of unwanted organic matter - like algae or bacteria and they kill it.  Along the way they also "burn up" the remains, leaving a few harmless chemicals as the by-product.

Unfortunately, in this process the oxidizers in the sanitizing chemicals are reduced, lessening their ability to further oxidize organics.  They may combine with other substances in the water, or their electrical charge may simply be used up.  To make sure that the job of oxidizing continues, we need to continue to add a more concentrated amount of chemical.

So you now know the "O" and the "R" in our equation.

Knowing how much to add is where the final letter in the O-R-P alphabet terminology fits into things.  The "P" stands for Potential, a word that refers to the ability rather than the action.  Potential energy is energy that is stored and ready to be put to work.

In electrical terms, this potential energy can be measured in volts.  When you put a voltmeter across the leads of a batter, the reading you receive is the difference in electrical pressure - in this case, the potential - between the two poles.  It represents excess electrons present at one pole of the battery that is ready to flow to the opposite pole.

So, when we use the term potential in describing ORP, we are actually talking about the electrical potential or voltage generated when a metal is placed into water in the presence of oxidizing and reducing agents.  This voltage is, however, very small and measured in millivolts.

Once ORP instruments were developed in the 1960s, researchers began working to determine at what voltage reading could the water being measured be considered of good quality.

In other words, at what measure of voltage is the sanitizer in the water active enough to destroy all of the harmful organisms?

In 1972, the World Health Organization adopted a standard for ORP of 650 millivolts (about 2/3 of a volt), a standard that has since been accepted as a recommended level.

In practical uses, ORP does not specifically tell you the chlorine (or other sanitizing agent) concentration in parts per million, but rather it measures its effectiveness.  This is because of the effect that pH has on sanitizer effectiveness.

As the pH reading goes up, the millivolt reading goes up, the millivolt reading on an ORP meter will go down.  Bring the pH down or adding more sanitizer will raise the millivolt reading.

This is why most ORP instruments work in connection with a pH meter - which measures the difference in electrical potential between the pool water and a sample of known pH that is contained in the meter.

ORP technology is now receiving widespread nationwide use in pool and spa applications as the basis of automated chemical control equipment.  The reasoning is clear.  Only an ORP sensor can deliver the kind of electronic feedback needed to control generating devices or feeders for sanitizer and pH - adjusting chemicals.

Unlike constant-feed or timer-controlled devices, ORP-based chemical controllers are designed to dispense pool and spa chemicals as they are needed.  Combined with a pH sensor, these controllers can be used to activate liquid feed pumps, gas chlorinators, chlorine/bromine generators and ozone-generating units.

Article Credits:
Copyright © 2010 Service Industry Publications, Inc.
Pool & Spa Service Industry News
Volume 24, Number 22
November 2010

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