Why pH Matters in 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 on above. Above pH 8, there is a very small amount of HOCl present in solution, making amperometric detection of free chlorine a more challenging process.

At pH = 7, the mixture is about 77% HOCl, and ideal for the Kuntze measurement. At pH = 8, the mixture is about 25% HOCl. At pH = 9, the mixture is about 3% HOCl. Measurement at this pH will be challenging. Contact a Kuntze representative to discuss possible changes to your measurement system.