Becoming a mainstay in the aquatics world in the 1930s, ORP has been measuring the disinfectant of the water for millions of bodies of water since then. While we have covered this topic briefly many years ago, this article will go a little deeper talking about ORP and how it might tell you how much chlorine is in your water.
Before we can understand ORP sensors, sometimes referred to as Redox sensors, we need to understand a bit more about oxidation. Oxidation is simply a loss of electrons. A classic example is rust: Oxygen takes electrons from iron and so the iron oxidizes (rusts). The opposite of this, gaining electrons, is reduction. In the above example, oxygen gains electrons from the iron and so is reduced. This reaction between the two, both the oxidation and the reduction, is known as “Redox”.
A substance that creates this reaction is known as an oxidizing agent. Chlorine, for example, is an oxidizing agent, or just “oxidizer” for short. An atom of chlorine has a relatively high number of electrons (17) and is a very strong oxidizer. It will rust iron quicker than oxygen, in other words. Therefore, it’s also considered highly corrosive.
ORP is not measuring electrons directly, but rather potential. ORP is simply measuring the potential for a solution or gain or loss electrons. Hence why it is called the Oxidation Reduction Potential, or Redox Potential. Chlorine, being an oxidizer, has a very high potential to oxidize. In water, chlorine will increase ORP. So, the more chlorine in the water, the higher the value.
Meanwhile, there are reducing agents. Decay, plant matter, etc. Organic matter will decrease ORP. Even if chlorine is raising ORP, enough decaying matter in the body of water will still decrease ORP overall.
What does all of this mean for using an ORP probe on a pool?
ORP has its limits and it’s important to know them. There are so many different oxidizing and reducing agents, it can be difficult to manage them. For example, a common product used in pools is a non-chlorine shock which contains the chemical compound potassium monopersulfate, which is an oxidizer. It will not increase the free chlorine in the water, but it will still cause ORP to increase. This can be misleading, and care is required.
There are many factors that can affect ORP, including Total Alkalinity, pH, water temperature, and more. While ORP is useful, it is limiting, and great care is needed when using it. If your ORP has been running higher/lower than normal for a few days, be patient as there could just be some other oxidizing or reducing agent in the water affecting the ORP numbers. Never calibrate ORP based on free chlorine, as that is never the only factor in the water changing the values.