The Connection Between pH and ORP - What You Don't Know Can Hurt You.

The two most basic measurements in water quality, pH and ORP, are inextricably connected. The relationship is amazingly simple. One goes up and the other goes down. Yet few people understand it and they do so at their own peril.

The beauty of ORP is that it is a bottom line measurement. What I mean is this: The parameter that rules the day in water treatment disinfection is free chlorine. Free chlorine is the sum of HOCl (hypochlorous acid) and OCl- (hypochlorite). The ratio of one to the other is given by the pH. The lower the pH the greater the fraction of HOCl. A simple algorithm combines the pH reading and chlorine reading and calculates both chlorine species. The free chlorine displayed on the analyzer is the sum of both. However HOCl is a stronger disinfectant than OCl- by an order of magnitude. So two solutions at two different pH levels may give the same free chlorine reading but the one with the lower pH reading will be a much more effective disinfectant. In order to make an informed decision on your treatment process you really need to know the total free chlorine and the pH.

if you are a smarty pants you will ask why we can't just take the probe reading corresponding to the HOCl reading and use that raw number as a better measure of disinfection efficacy. To which I can only reply, "Sorry, I didn't write the rules." Or you can measure the ORP which will give you one number that is NOT subject to interpretation. A reading of 665 mV will kill greater than 90% of E. Coli bacteria. No ands, ifs, buts or maybes. There is not even a temperature or pH dependence. Sure, increasing the temperature of an oxidizing solution will increase its ORP but that increase is real. Ditto for pH.

But, just because the ORP reading does not have to be corrected for pH or temperature, it doesn't mean that it is not affected by them. if your ORP reading is not where you want it to be your first impulse might be to add more oxidant. But that might be a terrible mistake. If you don't know the pH you might be missing out on the sure solution that will raise the ORP.

This was the problem faced by a large manufacturer of cleaning agents. If you want to know more read the story in this Water Online white paper just published: