Chlorine keeps our pools clean thanks to a chemical reaction which is detrimental to germ cells. When chlorine is added to water (usually in the form of a salt or gas), a weak acid— hypochlorous acid—is formed. The acid is able to breach the cell wall of bacteria and, once inside, destroy enzymes and structures in the cell, rendering it harmless. Chlorine is also effective against viruses, most likely by also acting on the enzymes and structural components. The parasites Giardia and Cryptosporidium have protective outer shells, which makes them better able to withstand an onslaught of hypochlorous acid, and therefore more resistant to chlorine.
If you’ve ever had a backyard pool, you’ll know that it’s important to get the water’s pH level right (that is, how acidic or alkaline it is). One reason for this is that the pH level of the water affects this chemical reaction (and, in turn, determines how much chlorine is available to kill germs).
How? Well, in water, hypochlorous acid (HOCI) splits up into hydrogen ions (H+) and hypochlorite ions (OCl–). If you’ve studied chemistry, the process looks like this:
HOCl ↔ H+ + OCl–
When pH levels are high (when the water is more alkaline), there is more dissociation. And, because, hypochlorites are not as fast or strong as hypochlorous acid when it comes to killing germs, more dissociation means less effective disinfection.
Very low pH levels aren’t a good idea either. Low pH can lead to corrosion of your swimming pool’s pipes, and can (as can high pH) irritate eyes and skin. A water pH between 7.2 and 7.8 is just perfect.