ORP and Oxidation:
By Kent Williams, Professional Pool Operators of America
Oxidation/reduction potential… We’ve all heard the term, but what does it really mean? We’ll see that it is, essentially, a relative measure of the desired “work value” of our pool sanitizer and a pretty darn’ good one at that. But first, let’s dissect the term itself.
“Oxidation” is pretty clear, so is “potential”, but what’s that other word, reduction? It’s not very important to us, actually — having to do with electron exchange and implying a chemical process pretty much the opposite of oxidation. The name of the technology wouldn’t be complete without the R, so we’ll use it to and remain correct. (The phrase “redox potential” is also used, still embodying both reactions in the name.)
One can simply imagine the ORP of a solution to be the measurable “oxidation potential” or the potential within an active solution to do chemical “work”.
We talk a lot about oxidation in the pool business. ‘Seems that, while sanitation comes along for the ride, it’s oxidation that we really have to work for. We frequently relate oxidation to burning, however biological/organic oxidation is actually not combustion at all, with the source of oxygen being water, not the molecular gas. Biological oxidation, found in animal digestion, is a stepwise reaction which releases energy in manageable amounts, consuming and/or converting organic material into proteins and sugars and other useful stuff. In a pool, the goal is to render organic contaminants invisible and non-offensive, with the primary resultant being carbon dioxide. One of the most complicated areas of chemistry, oxidation in water produced by a halogen (chlorine, bromine and others) results in the clear and sanitary water we want; however, conditions can exist which encourage the formation of ineffective, sometimes offensive halide compounds as well as other, even less desirable, “products of incomplete oxidation” such as trihalo-methanes – mentioned elsewhere in this article. The object, then, is to optimize the use and the control of the chlorine or bromine, the ozone, peroxide, chlorine dioxide, monopersulfate or whatever it is we manage to add to the water for those important oxidation duties. And this is done by measuring the quality of the process, not the quantity. Nothing beats ORP for this job!
ORP is measured in pool water using a sensitive voltmeter and a platinum electrode. The voltage across the pure platinum tip and a potassium chloride/silver chloride reference cell is measured in millivolts (thousandths of a volt) and, miraculously, can be directly related to efficacy, or “work value” of any sanitizing/oxidizing product present in the water. Unstressed, un-treated water reads a background potential of a few hundred millivolts (mV.), while the generally accepted (German DIN-Standard originated) range for effective sanitation (with chlorine or bromine, typically) is 650 to about 850 mV. Values much below 650 mV. become unsafe, whether in pools or in drinking water preparation. Oxidation suffers proportionately as ORP drops below that magical 650, and turbid water can show up right on cue. Read more »