Determination of free chlorine based on ion chromatography — application of glycine as a selective scavenger

Free available chlorine (FAC) is the most widely used chemical for disinfection and in secondary disinfection; a minimum chlorine residual must be present in the distribution system. FAC can also be formed as an impurity in ClO₂ production as well as a secondary oxidant in the ClO₂ application, which has to be monitored. In this study, a new method is developed based on the reaction of FAC with glycine in which the amine group selectively scavenges FAC and the N-chloroglycine formed can be measured by ion chromatography with conductivity detector (IC-CD). Utilizing IC for N-chloroglycine measurement allows this method to be incorporated into routine monitoring of drinking water anions. For improving the sensitivity, IC was coupled with post-column reaction and UV detection (IC-PCR-UV), which was based on iodide oxidation by N-chloroglycine resulting in triiodide. The method performance was quantified by comparison of the results with the N,N-diethyl-p-phenylenediamine (DPD) method due to the unavailability of an N-chloroglycine standard. The N-chloroglycine method showed limits of quantification (LOQ) of 24 μg L⁻¹ Cl₂ and 13 μg L⁻¹ Cl₂ for IC-CD and IC-PCR-UV, respectively. These values were lower than those of DPD achieved in this research and in ultrapure water. Measurement of FAC in the drinking water matrix showed comparable robustness and sensitivity with statistically equivalent concentration that translated to recoveries of 102% for IC-CD and 105% for IC-PCR-UV. Repeatability and reproducibility performance were enhanced in the order of DPD, IC-CD, and IC-PCR-UV. Measurement of intrinsic FAC in the ClO₂ application revealed that the N-chloroglycine method performed considerably better in such a system where different oxidant species (ClO₂, FAC, chlorite, etc.) were present.