Membrane-based colorimetric flow-injection system for online free chlorine monitoring in drinking water

The majority of water utilities currently employ the in-lab tests of the grabbed samples to determine the free chlorine (FC) residual levels. Under certain circumstances, such approaches are inadequate to timely and accurately reflect the dynamically changed FC residual levels, leading to inappropriate dosages. This work reports a uniquely configured membrane-based colorimetric flow-injection system (MCFIS) capable of accurately and reliably online monitoring drinking water FC residual levels in a rapid (1-5 min per measurement) fashion. The embedded gas-permeable membrane makes MCFIS an interference-free FC monitoring system. The developed pre-calibration strategy avoids the need for on-going calibration. The accuracy of the conventional analytical principles is almost exclusively determined by the absolute analytical signal value of one measurement data point, and any errors from such a single-data point measurement will be directly transferred to the result, which induces uncertainties, hence, poor accuracy and reliability. Differing distinctively from such conventional analytical principles, MCFIS quantifies chlorine concentration based on N, N-diethyl-p-phenylenediamine - Cl-2 colorimetric reaction-controlled membrane transport process that enables the determination of gaseous chlorine concentration according to multiple measurement data points to greatly enhance the accuracy and reliability. The inherent analytical features of this slope-based signal quantification principle, interference-free and on-going calibration-free empower MCFIS with an enormous superiority over other systems for online FC monitoring applications.