Do I need to adjust PH immediately after shocking my pool?

by Brad
(Sydney, Australia)

I shocked my pool with chlorine and noticed my acid dosing machine measured the pH at 8.8, though before the chlorine addition the pH was only 7.6.

Do I need to add acid immediately?

Thanks for the question Brad

If you used calcium hypochlorite, the granular form of chlorine, the pH will rise. Cal Hypo has a pH of 12 in and of itself.

If you're going to shock the pool it's best to use liquid chlorine. This won't raise the pH.

Be sure you're shocking the pool for the right reasons and not because you "should do it" once per week. Test the combined chlorine (CC). If it's above 0.6ppm then you should shock the pool but if not and all of your other readings are in line, there's no reason the shock on a weekly basis.

The chlorine, when kept between 1.5 - 3.5ppm and the CYA at 30 - 50ppm, will do its job through the week in killing off bacteria and organic matter.

You should try to reduce the pH as soon as possible, but at minimum after 2 - 3 hours after the shock. You don't want to be mixing chemicals. Allow some filtering before adding acid.

pH that is 8.0 - 8.2 is only allowing about 25% of the active chlorine so it's best to get the pH down. You can even shoot for 7.2. That's very acceptable, but I've always run my pools on my route and the the YMCA commercial pool at 7.6 - 7.8 with no problems at all.

At a pH of 7.6 - 7.8 you're still allowing about 45 - 50% active chlorination. Above 8.0 and it takes a nose-dive very quickly.

To post a reply, or if you have a similar question, you can see your post on the Q&A page in the "Shocking A Pool" category.

Check back to this post for updates or answers.

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Hope this helps.


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Feb 23, 2013
Spot on Advice
by: Brad

I did use the chlorine you stated. I will remember to use liquid next time. I shocked the pool due to an algae problem which started when my LM3-24 chlorinator's board malfunctioned and only ran for about 1 hour per cyle instead of 4.

I lowered the pH with the acid feeder over 2 hours and problem fixed.

How effective is chlorine at pH 7.2 to 7.5? The local pool shop tells me 7.5 is ideal, do you agree?

Keep up the great work, you have a great web-site and information, thank you for your help :)

Thanks for the follow Brad

A pH of 7.5 is fine. Pretty much anything between 7.2 - 7.8 is good. It's when the pH gets 8.0 and above is when you're using about 25% of the chlorine.

I've always run the pools on my route and the YMCA at 7.6 - 7.8. The splashing around causes air bubbles. These bubbles burst which cause the CO2 (carbon dioxide) to be released. This reaction causes the pH to rise naturally. By keeping the pH at 7.2 you're using alot of acid which isn't good. You just need enough to keep the pH below 8.0.

You're using about 50% chlorine at a pH of 7.2 - 7.4 and 45% chlorine at a pH of 7.6 - 7.8. That's only about an extra 2 - 3 tablespoons of granular chlorine per one pound or 0.45 kg.

The YMCA was using 25 - 30 gallons of acid per month to keep the pH at 7.2. I allowed the pH to rise to 7.6 - 7.8 and locked in the TA at 100 ppm. Now they only use about 1 gallon per month. Huge savings and much better for the swimmers and pool.


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