Specificity of the DPD and Amperometric Titration Methods for Free Available Chlorine: A Review

Both the DPD and amperometric titration methods for measuring free available chlorine Analytical methods for FAC (FAC) are subject to interference from monochloramine and organic chloramines. The measured FAC concentration in the presence ofchloramines is often significantly larger than General. Standard analytical methods for the determination of free chlorine in the true FAC residual. Interference in the DPD method can be reduced by rapid titrations. water may be generalized as follows. An Interference in the amperometric titration method can be minimized by maintaining a 200-mV positive potential on the platinum electrode. indicator of strong oxidants is selected. The level of the indicator is measured directly or decreased to a characteristic well as performance data for these endpoint by the addition of a reducing methods in the field. A summary of the agent (titrant). The indicator level or techniques available to minimize inter- titrant volume is correlated with FAC ferences is also presented. standards. Twoof the most common and Chlorine reacts with natural water and wastewaterconstituents toproduce a variety of compounds with a variety of disinfection abilities. Free available chlorine (FAC) is a strong disinfectant. The presence of free chlorine, i.e., a nonzero measured FAC residual, is usually regarded as a sign of adequate disinfection. In fact, US Environmental Protection Agency regulations’ allow for the replacement of up to 75 percent of the required microbiological assays with free chlorine measurements in some finished drinking waters. Proposed regulation9 exempt drinking water utilities from filtration requirements if measured freechlorine residuals (incorporated into concentration X time values) are adequate. These regulations emphasize the need for accurate FAC measurements. If FAC measurements are interfered with by poorer disinfectants, the measured residuals will not ensure adequate disinfection or biofouling control. Inorganic and organic chloramines are much weaker disinfectants than free chlorine.“,“A positive interference occurs if a portion of the free chlorine measurement is due to chloramines. This interference would lead to an overestimation of the true FAC concentration and could result in an incorrect assessment of the microbiological quality of the treated water. The objective of this article is to review the literature concerning interferences from monochloramine and organic chloramines in the measurement of FAC. Two common analytical techniques for the measurement of FAC are considered: DPD (N,N-diethyl-p-phenylenediamine) methods and amperometric titration. This review includes interferences observed in model compound studies as