Pool Chlorine and pH Relationships

We’ve always mentioned that to maximize your pool chlorine’s effectivity, your pool’s pH levels must be on point. In this blog post, we’ll be going over the relationship of pool chlorine with the pH levels to find out WHY it’s needed to maintain proper pH levels for your pool.

How does the pH level affect pool chlorine?

First off, the majority of home pools in Australia apply pool chlorine through four methods namely: Liquid Chlorine, Granular Chlorine, Chlorine Pucks, or through Salt water Generators. These methods provide an active chlorine component that is known as hypochlorous acid. We’ve taken a chart from the National Swimming Pool Foundation's Pool and Spa Operator Handbook to see the direct relationship between the effectiveness of pool chlorine (hypochlorous acid) and the pH level.

Active HOCl (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

If we take a look at the chart above, we can see that the lower the pH levels of our pool is, the more effective our pool chlorine is. You might be thinking, well, I should just lower the pH level of my pool then, right? Wrong! The lower your pH level, the more acidic your water is. Approaching pH level six, your pool water will be unsafe to swim in and it can get irritating very fast. On the other end of the spectrum, once your pH level reaches the high scale, your pool chlorine is rendered basically ineffective and you’ll end up with a lot of chloramines in your pool.

Finally, we have the ideal level of pH 7.4, which places our pool chlorine effectivity at slightly above 50%. While this may not sound like much if you’re a stickler for efficiency, it’s more than enough to effectively sanitise our pool and kill off bacteria, algae, and oxidize any organic contaminant that may enter your pool.

Why do I get algae blooms even if my chlorine levels are at 2-4ppm?

As we’ve seen in the chart above, even if you maintain free chlorine levels of 2-4 ppm, if your pH level is too high, then there’s still a chance for you to get algae blooms or green pool water simply because the effectivity of your pool chlorine has been lowered by the high pH.

How do I stabilize my pH levels?

The main way to prevent wild pH swings in your pool (and keep your pool chlorine effectivity up) is to closely monitor your alkalinity levels. The recommended pool alkalinity level is anywhere between 80ppm to 120ppm. If you have the proper alkalinity levels in your pool then you really don’t have anything to worry about.

Take note that alkalinity levels in pools are not the same as your pH levels. It’s one of the things that many pool owners, even veterans, get confused about. Total alkalinity is the number of dissolved alkalines in your pool and is measured in parts per million. Alkaline water (high pH) is a totally different thing in itself and it is measured on a scale level (pH level).

Alkalinity and pH, what to do in different situations

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There are many possible pH and alkalinity issues that we may come across in our pools and here are some of the most common ones and what the possible causes are.

  • pH levels are always high - This is common in pools that use salt water chlorinators as the chlorine generated is high in pH. There’s nothing to be done here except to monitor your TA levels and reduce your pH as needed.
  • pH levels are always high, but not using a salt water chlorinator - If you’ve recently replastered your pool (or a new pool) or if your pool has a new pebble finish, you’ll probably experience unusually high pH levels for the first year or so from the plaster/pebbles.
  • pH levels are always low - the two causes for pH levels being constantly low is the usage of stabilised chlorine and chlorine pucks (which are also stabilised) since these products have a low pH level of around 3. Another reason is if you live in an area with a lot of leaves and organic debris tend to find their way to your pool.
  • High Alkalinity levels but low pH levels - while it may sound confusing, like, how can a pool have high alkalinity and be acidic at the same time right? Well, the reason for this is your TA levels are above 120ppm, your pool will have a resistance to changes in pH. So to fix this, lower your TA levels and adjust pH levels accordingly.
  • Low Alkalinity levels but high pH levels - This is just the reverse of the previous problem. Check your TA level and if it’s below 80ppm, increase it to hit at least 80ppm and adjust your pH levels accordingly.
  • Alkalinity is high and pH is high - While it may sound alarming, this is actually the easiest problem to fix. Dose it with the proper amounts of pH down and you should be good to go.
  • Alkalinity is low and pH is low - This is another situation that’s pretty easy to resolve. Raise your total alkalinity and your pH level should follow suit.
  • Pool pH will not change - This is usually caused by having a high total alkalinity level making your pool water resistant to pH changes. Reduce your total alkalinity levels to fix this problem.

Alkalinity and pH tips

Here are some important points to remember when dealing with your pool alkalinity and pH.

  • Whenever adding any type of pool chemical, always allow the pool water to fully circulate before re-testing the pool water. This is to ensure that whatever chemical is properly dissipated and you’re getting the correct readings.
  • Always ensure that your total alkalinity (TA) levels are around 80ppm to 120ppm.
  • Never mix different chemicals when adding to a pool. Add them one at a time, dissolve fully before adding another type of chemical.
  • Before adjusting TA and pH, make sure to check and balance your calcium hardness levels.
  • Always adjust total alkalinity before adjusting your pH levels. Proper total alkalinity levels will buffer your pH levels so that you won’t get wild swings.
  • Always use fresh test strips and testing reagents. (i.e. take note of the expiration dates, and note when you first opened the package as constant air exposure can degrade the quality of test strips)

Conclusion

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While it may sound all so confusing, in a nutshell, as long as you keep your TA levels within the recommended range then your pH level should follow, and in turn, your pool chlorine levels should be functioning at optimal levels and you won't have to worry about algae blooms or inactive chlorine!

Tip: Always keep a good supply of pH adjusting chemicals to make sure that your pH levels are where they are supposed to be to ensure that your pool chlorine will work as intended!

Do you have any questions about this topic or the featured products? No worries, we're here to help! Drop us a question down below and we'll get back to you ASAP.

Happy swimming :)

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Post author:

Tom Hintze

Head of eCommerce & Operations at Mr Pool Man,
Co-Founder at Water TechniX

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