Total Chlorine Monitor
Many processes discharge chlorinated water to rivers, estuaries and other environmentally sensitive water courses. Environmental legislation is now tightening and the discharge limits are in the low ppb range.The A15/79 provides ppb resolution even in application with contamination from e.g. algae
Chlorination of potable water, wastewater effluent, and 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 once it has passed through the distribution system. 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. In addition to these common uses, there are many other applications where chlorine addition helps to reduce biological contamination.
Residual chlorine is found in many chemical forms in water systems. Residuals in clean water are often predominantly free chlorine while wastewater, cooling water, and chloraminated water can contain mixtures of free chlorine, combined chlorine, and organochlorine species. Measurement of residual chlorine can be relatively difficult where a variety of chlorine forms exist, and a measurement of "Total Residual Chlorine is often required in these applications.
This total chlorine measurement is normally done by reacting the chlorine in a water sample, buffered to pH 4, with potassium iodide. All of the various chlorine compounds react with the iodide to release an equivalent amount of iodine, and the released iodine is measured using various methods. Many on-line monitors for total residual chlorine use the iodometric method for measurement, often measuring current between two exposed electrodes to determine the iodine concentration.
ATI's A15/79 Total Chlorine Monitor uses this same standard iodometric method, but with a unique sensing technique for measuring the released iodine. The system takes the reacted sample containing iodine and uses a membraned gas sensor rather than exposed electrodes. The gas permeable membrane allows only iodine to permeate into the sensor. Sample contaminants are eliminated from coming in contact with measuring electrodes, providing better stability. And measurement can be made either with a direct contact gas sensor, or with a special gas stripping system for highly contaminated samples.
The A15/79 monitor is an on-line automated chemistry system that provides the components necessary to perform total residual chlorine measurement. In operation, a small amount of sample is pumped into the system and mixed with pH 4 buffer, and then with potassium iodide. At pH 4, chlorine compounds in solution react as follows (equation shown for one chlorine compound only):
HOCl + 2KI + HCl 2KCl + I2 + H2O
Two types of chemistry modules are used to measure the iodine concentration released in the chemical reaction. The simplest version provides flow of the treated sample directly into a chamber containing the iodine gas sensor. A membrane across the face of the sensor allows iodine to diffuse to the active electrode surface. Iodine is reduced to iodide, generating a current proportional to the iodine to diffuse to the active electrode surface.
A second system, used for highly contaminated samples, brings the reacted sample to an air-stripping chamber where the released iodine is stripped from the sample, with the resulting air stream directed to the iodine gas sensor located in a separate chamber. This method is ideal for very low range measurement of de-chlorinated wastewater, where biological growth from sulfur reducing bacteria can cause severe fouling problems in contact systems.
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