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Is There an “Ideal” Measurement Approach for Free Chlorine?

 If you’re faced with measuring free chlorine in drinking water or other applications, you know it can seem like there’s no ideal measurement technology solution. The reagent-based method requires continuous resupply of costly chemicals, while the reagent-free method is inherently pH measurement dependent, requiring integration of an additional pH system. And yet the measurement of chlorine is critical. That’s why this recent white paper – Improved Chlorine Analysis for Effective Disinfection Reduces Water Treatment Costs – will be of interest to anyone dealing with the challenges of these measurement methods.

Since chlorine is a costly chemical and requires large amounts of energy to produce, it’s important to control expenses by measuring and controlling chlorine concentration. Even 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 or may create other problems in the process. In the chlorination of drinking water, underfeeding chlorine results in incomplete disinfection with consequent risk to public health. Overfeeding chlorine produces water with an objectionable odor. Overfeeding also increases the levels of trihalomethanes (THMs) in the finished water. THMs are suspected carcinogens.

The reactivity of chlorine and its complex chemistry in an aqueous solution make the measurement of chlorine difficult. Free chlorine exists as an equilibrium between hypochlorous acid and hypochlorite ion. The relative amount of hypochlorous acid or hypochlorite present depends on pH values, resulting in the need for pH measurement.

There are two main approaches to chlorine measurement. In the colorimetric method, chemicals added to the sample react with chlorine to produce a color. The darkness of the color is proportional to the amount of chlorine. The process instrument measures the color and converts the result into a ppm chlorine reading. Reagents for this system typically last for 30-60 days. On estimate, reagents for a single system can cost between $750 and $1,000 per year. Since most water authorities have hundreds of supply systems, annual reagent costs can be many thousands of dollars.

Most users agree that the second main method of measuring free chlorine with amperometric technology is preferred. The amperometric method has many inherent benefits. The sensors are relatively free from interference. Manganese, iron, nitrate, and chromate – substances that interfere with other methods – have little influence on amperometric sensors. The sensors are low maintenance, although they do require periodic cleaning to wash away solids that slowly accumulate on the membrane. Because the technology does not need reagents, amperometric systems do not require the purchase of costly consumables or ongoing maintenance of the reagents, and these systems reduce errors due to depletion of reagents. The one remaining significant issue is the need for pH measurement and compensation.

In this white paper on improving chlorine analysis, you can explore a discussion of the two main measurement technologies and how new changes in design are making it possible for water plants and other users to find the ideal measurement approach they’ve been missing. Download your copy of the white paper today.

How are you currently measuring free chlorine in your application?