High chlorine levels can also interfere with this test by bleaching the indicator and preventing red from developing. Diluting the sample with deionized water will help eliminate this interference. Be sure the end result is multiplied by the factor of the dilution.
If potassium monopersulfate is used as an oxidizer, the presence of the non-chlorine oxidizer, which reacts with potassium iodide, will produce a more intense red. This will lead to a false positive reading when testing for total chlorine using the DPD method and false positive reading when testing for combined chlorine using the FAS-DPD method.
Many test kit manufacturers have developed an amine-masking agent, which allows for proper chlorine determination in the presence of non-chlorine oxidizers.
By understanding the principles behind water quality tests, pool and spa professionals can recognize and eliminate interferences normally blamed on the reagents. The theory found in Standard Methods is an invaluable resource allowing the chemistry surrounding testing procedures to be easily understood.
Michael Lowry is an instructor with, and promoter of, the Lowry School of Pool & Spa Chemistry. His 25 years of experience range from servicing pools to selling commercial, industrial, and residential pool chemicals. In 2012, Lowry was recognized by the National Swimming Pool Foundation (NSPF) for exceptional performance at the group’s annual instructor meeting earning two distinguished instructor awards—one for the highest increase in certified pool operator (CPO) certifications in 2011 over 2010, and the second for the highest number of certifications outside the U.S. He can be reached via e-mail at firstname.lastname@example.org.