Application Note: Chlorine Measurement By Amperometric Sensor

Chlorine in aqueous solutions is used for a number of purposes such as disinfection, taste and odor control, bleaching, and as a powerful oxidizing agent in various manufacturing processes. Chlorine is a costly chemical and requires large amounts of energy to produce. About 3000 KWH are required to produce a ton of chlorine from brine, and the delivered cost ranges from $160 to $300 per ton. As energy becomes more expensive, so inevitably will chlorine.

If for no other reason than cost, it is important to measure and control chlorine concentration. Perhaps more important is the fact that the bleaching and disinfecting ability of chlorine depends on concentration. Too little chlorine is ineffective. Too much chlorine is wasteful and may create other problems in the process. A good illustration is the chlorination of drinking water. Underfeeding chlorine results in incomplete disinfection with consequent danger to the public health. Overfeeding chlorine may produce water with an objectionable odor. Overfeeding may also increase the levels of trihalomethanes (THMs) in the finished water. THMs are suspected carcinogens. In certain applications, such as the chlorination of drinking water, it is important to know the chemical form of the chlorine. Certain forms of chlorine are 80 to 100 times more effective in disinfecting water than other forms.

The reactivity of chlorine and its complex chemistry in aqueous solution make the measurement of chlorine difficult. Even under ideal laboratory conditions, results vary with the method used. With process instruments the situation is even more complicated. Calibration, drift, reliability and maintenance become important issues.