Free chlorine sensing using an interferometric sensor

A waveguide interferometer based free-chlorine sensing technique has been developed. A polymer film for a specific free chlorine binding was designed, synthesized and applied on the surface of a waveguide. The material is based on cyanuric acid moieties along each repeating unit covalently tethered to poly(norbornene)s. Chlorine sensing was accomplished by measuring the refractive index change of the polymer, as a result of the reaction between cyanuric acid and free chlorine, interferometrically by the evanescent field extended above the waveguide surface. The free chlorine binding to cyanuric acid is reversible and a linear calibration curve from 0.1 to 10 mg L−1 of HOCl concentration was obtained with the level of detection (LOD) and level of quantification (LOQ) of 0.047 and 0.328 mg L−1 of HOCl, respectively. A free chlorine measurement with less interference from combined chlorine than DPD based colorimetric method was developed as a result of the different sensing responses of free and combined chlorine. Free chlorine residual was measured in samples collected from tap water and poultry processing waters by both optical sensor and DPD-based colorimetric method. Good agreement between both methods was observed although the levels for free chlorine measured by the optical sensor are systematically lower than the readings obtained from the DPD method. The difference might be the result of the interference from combined chlorine during the DPD measurement.