Chlorination of potable water, wastewater effluent, and industrial cooling water is widely used throughout the world to control biological activity in the water. Disinfection of potable water with chlorine ensures that tap water is safe to drink after passing through the distribution system piping. Wastewater disinfection helps to ensure that receiving streams are safe for recreational use, and cooling water chlorination reduces biofouling that can degrade heat transfer efficiency. This wide use of chlorination for disinfection purposes results in the need for a reliable “residual chlorine” analyser that can operate in a variety of applications.

Residual chlorine is found in many chemical forms in water systems. Residuals in clean water are often predominantly free chlorine while wastewater and cooling water can contain mixtures of free chlorine, combined chlorine, and organochlorine species. Measurement of residual chlorine in applications where only free chlorine (potable water) or only combined chlorine (chloraminated water) exist can often be monitored with direct sensor measurement. However, applications where a variety of chlorine forms can exist (wastewater effluent and some cooling water) require a more complicated measurement method. These applications generally require a “Total Chlorine” measurement and involve chemically converting all chlorine species into a single chemical form. This is normally done by reacting the sample with pH 4 buffer and potassium iodide, where the various chlorine compounds convert iodine ion into molecular iodine.

Many on-line monitors for total chlorine use this iodometric method, often measuring the current between two exposed electrodes to determine iodine concentration. ATI’s Model Q46H/79S uses this same standard iodometric chemistry, but with a unique sensing technique for measuring the released iodine. The system takes the reacted sample containing iodine and uses an air-stripping system to remove molecular iodine from solution. The gas-phase iodine from the water sample is channelled through a conditioning module and then directly to an iodine gas sensor. The result is that the iodine measurement is made without any contact between the water sample and the sensor. Contaminants in the sample that cause fouling and contamination of standard electrodes do not affect the Q46 system, providing greater operational reliability.

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