By Thomas M. Lachocki, Ph.D., B.Sc., and Rose M. Lyda, MPA
Effective education incorporates the elements of sound data and practical application. While neither is effective in isolation, when combined, data and experience create solutions with dynamic impact.
The advantage of employing education and experience in tandem is illustrated by one of the most commonly taught pool and spa water chemistry topics: combined chlorine and breakpoint chlorination. Field experience has proven industry education on these topics is inadequate and can be improved. Fortunately, new science has helped advance education that more reasonably explains industry experiences. To understand where chemistry education can be improved by the application of experience, this article briefly reviews the two principles of combined chlorine and breakpoint chlorination.
Understanding combined chlorine and breakpoint chlorination
Education and experience have established the importance of testing pool water for free chlorine and total chlorine, and calculating combined chlorine. Free chlorine is commonly defined as the amount of hypochlororous acid (HOCl) and hypochlorite ion (OCl–) present in the water. Measuring the free chlorine level serves to indicate the amount of active disinfectant in the water and satisfies the product label requirement for free chlorine residuals.
When nitrogen-containing molecules like ammonia (NH3) are present in the water, free chlorine will quickly break the hydrogen to nitrogen (H-N) bond and form a chlorine to nitrogen (Cl-N) bond. For example, ammonia (NH3) will react with free chlorine to form monochloramine (Cl-NH2). Chloramine is the term used for chemicals with this nitrogen-chlorine bond.
The characteristics of chloramines are significant: they do not react as well as free chlorine, do not serve as disinfectants, and do not appear in a free-chlorine analysis, which is commonly called the DPD1 test. What raises concern about these characteristics is that trichloramine is irritating in the water and is the primary contributor to the noxious ‘chlorine-like’ odour in pool facilities. Chloramines are detected, however, when another reagent is added in a DPD3 test, which measures total chlorine. Combined chlorine is then calculated by subtracting free chlorine from total chlorine:
Total chlorine – Free chlorine = Combined chlorine
It is commonly taught that chloramines, or combined chlorine, are destroyed through breakpoint chlorination: increasing the free chlorine level to 10 times the combined chlorine level. The chemical reaction associated with breakpoint chlorination is the chlorination of ammonia to form monochloramine, dichloramine, and then trichloramine. Trichloramine breaks down further to form harmless nitrogen gas and chloride ion.
Advancing water chemistry education through experience
Experience and education concur on the practice that total chlorine and free chlorine can be tested and that chloramines are irritants and ineffective disinfectants; and science supports the ability of breakpoint chlorination to destroy ammonia-based chloramines. However, there is a common experience that indicates a flaw in combined chlorine and breakpoint education.
Water, which time after time has received adequate free chlorine to achieve breakpoint chlorination, should be free of combined chlorine, but when it is tested, combined chlorine remains. If education were correct, after breakpoint chlorination, the combined chlorine would be undetectable. Actual experience, on the other hand, indicates users continue to sense and smell nitrogen trichloride in the air.
The scientific method directs, in the presence of contradictory evidence, a hypothesis must be abandoned or modified to support the available evidence. In the example of chloramines, if breakpoint chlorination is performed and combined chlorine is still detected, ammonia cannot be the sole contributor to the formation of combined chlorine. Subsequently, the hypothesis has been modified to address another factor contributing to the presence of combined chlorine and the production of nitrogen trichloride: organic chloramines.