ORP And pH — What You Don't Know Can Hurt You

By Mark Spencer, PhD, President of Water Analytics

When I look back at 2016 one of the most significant trends I observed was the growing acceptance of ORP measurements, especially for monitoring disinfection. With one caveat, the ORP value measured in a solution provides an unambiguous measure of the disinfection efficacy. (That one caveat is that there are no chemicals in appreciable concentration that oxidize or reduce but do not disinfect—a typically safe assumption.) Probe-based free chlorine measurements, on the other hand, must be corrected for pH so as to yield the correct free chlorine value. Even then a free chlorine measurement, which is actually the sum of two species of differing disinfecting power (HOCl and OCl^-) has a variable disinfecting efficacy depending on pH.