What is ORP?

 As we stated earlier, ORP stands for Oxidation-Reduction Potential. In some parts of the world, it is also known as Redox Potential. Sometimes, you'll see the words "oxidation" and "reduction" spelled without the hyphen connecting them. We chose the hyphen because the two chemical reactions are really "joined at the hip" - one cannot occur without the other also occurring.
When chemists first used the term in the late 18th Century, the word "oxidation" meant, "to combine with oxygen." Back then, it was a pretty radical concept. Until about 200 years ago, folks were really confused about the nature of matter. It took some pretty brave chemists to prove, for example, that fire did not involve the release of some unknown, mysterious substance, but rather occurred when oxygen combined rapidly with the stuff being burned.
We can see examples of oxidation all the time in our daily lives. They occur at different speeds. When we see a piece of iron rusting, or a slice of apple turning brown, we are looking at examples of relatively slow oxidation. When we look at a fire, we are witnessing an example of rapid oxidation. We now know that oxidation involves an exchange of electrons between two atoms. The atom that loses an electron in the process is said to be "oxidized." The one that gains an electron is said to be "reduced." In picking up that extra electron, it loses the electrical energy that makes it "hungry" for more electrons.
We also know that matter can be changed, but not destroyed. You can alter its structure, and can increase or decrease the amount of energy it contains - but you can't eliminate the basis building blocks that make things what they are.
Chemicals like chlorine, bromine, and ozone are all oxidizers. It is their ability to oxidize - to "steal" electrons from other substances - that makes them good water sanitizers, because in altering the chemical makeup of unwanted plants and animals, they kill them. Then they "burn up" the remains, leaving a few harmless chemicals as the by-product.
Of course, in the process of oxidizing, all of these oxidizers are reduced - so they lose their ability to further oxidize things. They may combine with other substances in the water, or their electrical charge may simply be "used up." To make sure that the chemical process continues to the very end, you must have a high enough concentration of oxidizer in the water to do the whole job.
But how much is "enough?" That's where the term potential comes into play.
"Potential" is a word that refers to ability rather than action. We hear it all the time in sports. ("That rookie has a lot of potential - he hasn't done anything yet, but we know that he has the ability to produce.)
Potential energy is energy that is stored and ready to be put to work. It's not actually working, but we know that the energy is there if and when we need it. Another word for potential might be pressure. Blow up a balloon, and there is air pressure inside. As long as we keep the end tightly closed, the pressure remains as potential energy. Release the end, and the air inside rushes out, changing from potential (possible) energy to kinetic (in motion) energy.
In electrical terms, potential energy is measured in volts. Actual energy (current flow) is measured in amps. When you put a voltmeter across the leads of a battery, the reading you get is the difference in electrical pressure - the potential - between the two poles. This pressure represents the excess electrons present at one pole of the battery (caused, incidentally, by a chemical reaction within the battery) ready to flow to the opposite pole.
When we use the term potential in describing ORP, we are actually talking about electrical potential or voltage. We are reading the very tiny voltage generated when a metal is placed in water in the presence of oxidizing and reducing agents. These voltages give us an indication of the ability of the oxidizers in the water to keep it free from contaminants.