Note: As usual, this is an article that aims to simplify a complex topic. To all the chemists reading this, if something we say needs to be clarified or corrected, please comment below or contact us directly! We strive to publish only the most accurate information possible. Just keep in mind, this article is meant to be in layman's terms. Enjoy!
Have you ever been asked "what is ORP?" For most of us, we kind of know what it represents, but not exactly what it is, or why it is important. Oxidation Reduction Potential (ORP) is a measurement of sanitizer effectiveness in water. ORP is an electronic measurement–in millivolts (mV)–of the ability of a chemical substance to oxidize or reduce another chemical substance. Both oxidation and reduction are chemical processes involving the transfer of electrons between molecules (gaining or losing an electron). So ORP measures the potential for such reactions to occur in your water. Translated into the swimming pool world, the higher the potential for oxidation, the more efficient your sanitizer. More efficient sanitizer means safer, cleaner water. It's a beautiful thing.
ORP is measured by a probe in a small sample of flowing water, usually next to your chemical controller. An ORP sensor consists of an ORP electrode, and a reference electrode. Basically a signal is sent between them which determines your oxidation and reduction potential. The rate of electron transfer is measured in mV and read as ORP. The higher, the better, and most health departments in the United States recognize 650mV as the minimum acceptable ORP, and if you can get over 800mV that's great. Anything over 850mV is exceptional water.
Wait, did you catch that? Oxidation and Reduction are two different things; in fact, they are opposites. So let’s quickly discuss what oxidation and reduction mean. We get the following two definitions from this source.
“Oxidation is the loss of electrons by an atom, molecule, or ion.” Often, the lost electrons are replaced by oxygen.
“Reduction is the net gain of electrons by an atom, molecule, or ion.”
So oxidation is when an oxidizer takes electrons (e-) from an oxidant, and reduction is when that oxidizer's electrical charge is reduced by those stolen electrons. When we say "chlorine gets wiped out", or "used up", we really mean chlorine got reduced. Here's a helpful video explaining more about these reactions, known as Redox reactions.
Why is ORP used?
ORP is widely used because it is practical, usually accurate, and allows us to electronically monitor what is going on in the water. While it does not measure amount of chlorine, ORP is a real-time reading of chlorine's performance. Most operators agree that disinfection is what really matters, from a health perspective; amount of chlorine is not necessarily as important as its effectiveness. Ideally, we would have a minimal amount of chlorine that is super efficient, to accomplish more with less.
We did some research, and we have not yet found data that suggests an ORP that is “too high”, or where it starts becoming less valuable. What we do know is that ORP should be above 650mV; anything over 750 is good; and anything over 800 is excellent. Beyond that, the exact numbers are subject to operator opinions, but we can all agree that higher is better.
How and why ORP is measured
Most automation systems in pools control chemicals in real time, so they rely on accurate metrics at all times. This is because pH fluctuates more often than other variables, and ORP controllers are able to keep up with those changes. The alternative is relying solely on water testing, which usually has several hours in between readings.
If your pool has high chlorine levels, but low ORP…The two most likely culprits are pH and organic contamination (bather load). The more organics you have in the water, the the slower the conductivity between the probes. Most controllers also measure pH, so will be able to see if pH is the problem. If pH is where it should be, chances are, organic waste is the crux of the problem.
Factors that affect ORP
A number of water chemistry factors can affect your ORP. Here are a few that are most common in swimming pools:
The lower the pH, the higher the proportion of Hypochlorous Acid (HOCl) to Hypochlorite Ion (OCl-), and the higher the ORP. Here’s a fancy chart showing the percentage of HOCl (strong chlorine) to OCl- (weak chlorine).
Inverse correlation between pH and percentage of strong chlorine (HOCl) to weak chlorine (OCl-).
Here’s something we find fascinating: recent studies show that this equilibrium spectrum is valid in non-stabilized water only. In other words, water that has no cyanuric acid. Read more about these findings here.
2. Cyanuric Acid
This chart from the CDC shows the relationship of ORP and CYA.
According to the US Centers for Disease Control (CDC), increasing levels of cyanuric acid (also called chlorine stabilizer or conditioner) lower the ORP. This is the main reason why the CDC has put a new limit on CYA levels in the event of a fecal incident. The new limit? Just 15 ppm CYA. 15!
We at Orenda do not make sanitizers, but we absolutely understand and appreciate the importance of them. Residual sanitizers like chlorine are our first line of defense against harmful bacteria and organisms in the water. Since CYA weakens chlorine, in our opinion, keep the stabilizer levels as low as possible.
3. Phosphates (indirectly)
Yes, phosphates can indirectly cause a decrease in ORP. Here's an article and a video to explain more: