Swimming Pool Chemicals: Getting to Know Chlorine

In our last post about pool chemicals we talked about how chlorine is the most common pool disinfectant. It’s also a common household product (read: bleach, although it’s more diluted than what you would be adding to your Utah swimming pool). So what should you know about chlorine?

Chlorine is typically prepared in liquid, powder or tablet form (though some professionals use gaseous chlorine), and it can be added to the water anywhere in the cycle. Chlorine comes in different strengths, and all chlorines are definitely not created equal. Pool experts generally recommend adding it just after the filtering process, and specifically after the heater here in Utah (since for the most part, all pools in Utah tend to have heaters, specifically up here along the colder part of the Salt Lake Valley). You don’t want to be flowing highly chlorinated water through the pool equipment, especially your heater. One way some pool owners add chlorine to their pools is through the skimmer boxes, which isn’t very safe because the chlorine tends to be too concentrated in those areas.

One problem with hypochlorous acid is that it’s not particularly stable. It can degrade when exposed to ultraviolet light from the sun, which of course all pools are subject to, and chlorine may combine with other chemicals to form new compounds. Pool chlorine tablets often include a stabilizing agent, such as cyanuric acid, that reacts with the chlorine to form a more stable compound that does not degrade as easily when exposed to ultraviolet light. You should be testing your pool, at least at the beginning of the summer, to make sure your cyanuric acid level is the right level (less than 100 parts per million and greater than 30 parts per a million according to the Health Department).

Even with a stabilizing agent, hypochlorous acid may combine with other chemicals, forming compounds that are not very effective sanitizers. For example, hypochlorous acid may combine with ammonia, found in urine, among other things, to produce various chloramines. Not only are chloramines poor sanitizers, but they can actually irritate the skin and eyes and have an unpleasant odor. The distinctive smell and eye irritation associated with swimming pools are actually due to chloramines, not ordinary hypochlorous acid — a strong smell usually means there is too little free chlorine (hypochlorous acid), rather than too much. To get rid of chloramines, you have to shock treat the pool, which is to say, to add an unusually strong dose of chemicals to clear out organic matter and unhelpful chemical compounds.

Chlorine also affects the overall pH balance of the pool, and also is effected by the overall pH balance of the pool. As overall pH rises, chlorine is slowed down and is slower to kill bacteria.  And as pH lowers beyond 7.5, the chlorine effectiveness speeds up killing bacteria faster, but also becoming unstable where it is “spent” more quickly.

And that, my friends, is how chlorine works in your swimming pool.

*Thanks to HowStuffWorks “Pool Chemicals” for help in writing this article.

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