Controller Concepts: ORP and Oxidation - Part I

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 a 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 trihalomethanes – mentioned elsewhere in Pumproom Press #5. 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 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 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.

A well-known study performed by Dr. Jim Brown of the Oregon State Health Department about fifteen years ago made it crystal clear that ORP is the qualitative measure of choice for sanitarians or operators evaluating the safety of pool water and the efficacy of the sanitizer. In this remarkable study, thirty public spas were examined for all normal pool variables plus plate count (bacteria density) and, finally, ORP. Extremes showed up in pH from 5.7 through 8.3, combined chlorine from 1.4 to 34 ppm, free chlorine from 0 to 30 ppm, cyanuric acid (what’s it doing in a spa??) from 0 to 1,300 ppm, plate counts from 0 through 15,000, and even Pseudomonas up to 12,400! The only correlation that stood up throughout the study was the relationship between ORP and the presence of pathogens. Virtually no plate count existed in the spas where ORP values were found to be above about 630 millivolts, while lower values, no matter the free chlorine residuals present, all had dangerous or near-dangerous levels of pathogenic life (bugs!).

Among the unsafe pools in the study, chlorine residuals bore no resemblance to the plate-count values. Even when the free chlorine was as high as 4 ppm, a significant plate count existed because the ORP in that spa was 537 mV. The pH was 6.9, so why was the ORP so low in that spa’s water? Excessive cyanuric acid was the culprit, as was the case in all but two of the thirteen spas exhibiting ORP levels at or below 630 mV. No matter the reason for low ORP, however ? low sanitizer, CYA, high pH, chloramines ? you could count on unsafe water and eventual turbidity.

At a pH of 7.4, 1 ppm free chlorine results in an ORP of 810 mV.

The chart above, developed by Uniloc and Stranco in the early 1970s, helps you see the relationship between free chlorine at various pH levels and the resultant ORP. Choose one curve, representing a “free chlorine” value you want or may have measured using the DPD method. You will see the effect pH has on that one value of chlorine. HOCl becomes H+ and OCl- as pH rises, and a predictable drop in ORP is the result. One-half part per million of un-stabilized “free chlorine” can produce over 780 mV. at pH 7, dropping to just over 650 mV at pH 8.

Almost all modern pool controllers operate on the principle of ORP measurement and control. It has proven to be reliable, repeatable and affordable. Many states are including ORP thresholds in their new codes, while others will doubtless follow as redox becomes the World standard.

Next issue we will examine ORP further, looking into important issues relating to automation with ppm readouts, measurement in the presence of cyanuric acid, compensations and concerns when measuring and controlling bromine, and so forth. Contributors to this discussion are welcome; give PPOA a call.

~kw