How Does pH Effect Chlorine?

The importance of maintaining proper pH balance cannot be overstated. pH levels in swimming pools effect everything from swimmer comfort, longevity of pool surfaces and equipment, water clarity and more. One of the most overlooked problems associated with pH that is not properly balanced is the effect it has on your sanitizers performance. 

The most common sanitizer for swimming pool water is chlorine. For this post we will focus just on chlorine but pH is a factor in almost all pool water sanitizers effectiveness.

For most pool owners, chlorine is added using chlorine tablets, chlorine granules, salt chlorine generators and liquid chlorine. Chlorine is effective at killing or inactivating pathogens and algae. The active ingredient in chlorine is Hypochlorous acid.

The chart below shows the level of active Hypochloris Acid is in the water with a pH level of 6.0 all the way to 8.5.

% Active HOCI (Hypochlorous Acid)

 pH
97 % 6.0
91 % 6.5
76 % 7.0
66 % 7.2
50 % 7.5
33 % 7.8
24 % 8.0
9 % 8.5

*Chart is from the National Swimming Pool Foundation's Pool and Spa Operator Handbook.

With a pH level of 6.0 your chlorine will be highly effective with 97% of your chlorine available to kill bacteria and algae. However, at 6.0 your pool water would be extremely acidic and unsafe to swim in. On the flip side, if you pH is at 8.5 only 9% of your chlorine will be available to kill bacteria and algae.

If pH is in the ideal range your chlorine will be about 50% effective. If you maintain an ideal chlorine level of 2 to 4 ppm you can effectively kill or inactivate most pathogens in the water within seconds to minutes. Some like Giardia and Cryptosporidium can take much much longer but are less common. What we commonly see when customers bring their water into our store to get tested is that the pH is often at 7.8 or higher. If the pH is this high it cuts the effectiveness of the chlorine by half or more these pathogens can take much, much longer to kill or inactivate thereby making the pool less safe to swim in.

To learn more about pH please see our blog post What is pH.

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