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. Oxidation is a chemical process involving transfer of electrons between molecules, and ORP is an electronic measurement—in millivolts—of the water's potential for oxidation. The higher the millivolts, the more oxidation potential, which translates to a higher effectiveness of the sanitizer (like chlorine). In other words, it measures how fast chlorine can kill.
Understanding ORP, and why it is used
Oxidation reduction potential is widely used because it is practical, accurate, and allows us to electronically monitor what is going on in the water. While it does not measure amount of chlorine, it is a real-time reading of its 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
ORP is measured with a two-probed sensor submerged in a flowing sample of water. The sensor measures the electrical difference between the two probes. One probe is typically platinum or gold, and the other is silver. These readings can be affected by how clean the probes are, which is another reason to consider using enzymes in your water. This also applies to other types of probes in the system, such as chlorine probes.
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.
If your pool has low chlorine levels but high ORP…Either your probes are malfunctioning, or congratulations, you’ve got great water quality. It's rare, but totally possible. For example, a big commercial pool in Arizona has been able to maintain 750 ORP, despite less than 1 ppm chlorine! Think about that. Even with a slightly high pH, the pool was effectively oxidizing. (Spoiler alert: they use our enzymes.)
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.
Just how much weaker does CYA make chlorine? According to renown Richard Falk, there is a 7.5% reduction factor. Here’s a video that explains it.
Yes, phosphates. We have an entire article devoted to how phosphates can weaken chlorine. It boils down to the chemistry of dissociation, and how phosphates pull hydrogen from HOCl to leave behind a higher proportion of OCl-. Here’s a video explaining more.