Lowering the pH of pool water using HCl

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Hi guys,

I need help with a question. I have a pool with a volume of 120,000L. Its pH is currently at 8.2 and I need to lower it to 7.5 using muriatic (hydrochloric) acid with a concentration of 9M. How many litres of hydrochloric acid do I need to add? I've been given a table that lists the different amounts of HCl needed for different volumes of water but I'd like to know the steps behind calculating it. (Table recommends ~2.3L for a 120,000L pool)

Matt

Edited by Matt333
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• 1 month later...

Matt - you're making it way to complicated. To get the pH just right, you'd have to know the alkalinity also (buffering capacity). I have a pool also (~18,000 gal - 68,130 L) and when my pH gets to 8.2, I only have to add about 1/3 gal (~1.26 L) of muriatic acid (my HCL is about 10.2 M) to get it back down to about 7.5. I would start by adding about 1.5 L of acid, let pump run at least 24 hr, then check pH. Then add more HCl accordingly to sneak up (or down) on it. Key is to mix (pump) for at least 12-24 hrs before adding more HCl. If you don't mix well enough in between additions of HCl, you could get too acidic and cause other problems. Be patient.

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• 3 months later...

I think HCl is not the best option to lower the pH. We should use other weak acids like acetic acid (Vinegar)

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Dilute some of the acid 1000 fold with clean (rain/ deionised?) water.

Take 1 litre of water from the pool.

Slowly add the diluted acid and measure the amount you need to get the right pH.

For the rest of the pool you need 120 times as much acid. (Probably best to start with, say, 3/4 of the calculated amount initially and see how that goes).

Incidentally, it might not be possible to get the pH to 7.5. The cement etc might buffer the pH rather higher than that and you don't want to dissolve the concrete.

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