Most water analysts are using DPD kits and colorimeters to measure free chlorine residual, but there is a better way.
Links:
[1] http://admin.indiaenvironmentportal.org.in/feature-article/measuring-true-effectiveness-sanitizers
[2] http://admin.indiaenvironmentportal.org.in/category/author/richard-james-spahl
[3] http://admin.indiaenvironmentportal.org.in/category/journal/everything-about-water
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/water-treatment-technology
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/water-quality