This article summarizes the importance of considering pH when making a free chlorine measurement.
When chlorine (Cl2) is added to water, hypochlorous acid (HOCl) and the hypochlorite ion (OCl-) are formed. The term "free chlorine" refers to the combination of Cl2, HOCl, and OCl- that is present in solution. HOCl is the predominant biocidal agent, or what "kills" the pathogens that may be present.
All amperometric measurements of free chlorine, including the Kuntze measurement, measure only the presence of HOCl. HOCl is reduced on the measuring electrode, which yields a current that gets translated by the instrument to a free chlorine concentration.
The pH of the system determines what species are present, and in what ratio, shown in the scheme below. Above pH 8, there is a very small amount of HOCl present in solution, making amperometric detection of free chlorine a more challenging process. You can learn more about how Kuntze's Krypton Multi® system compensates for pH here.
If your pH is around 7, then your system is ideal for the Kuntze free chlorine measurement. If your pH is closer to 8, be sure to keep a close eye on it. Large pH changes (greater than ± 0.1 pH unit) may influence the quality of the measurement. If your pH is above 8.5, be aware that measurement in this range can be a challenge. Contact a Kuntze representative to discuss possible changes that can be made to your system to best optimize the measurement conditions.