Can't get PH to come down.

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
ptheoph
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Location: Temecula, California

Can't get PH to come down.

Postby ptheoph » Mon 26 May, 2008 13:13

I have a 20K gallon, in-gorund pool, using a salt system. I have added at least 6 gallons of muriattic acid to the pool over the last few weeks but can't get the PH to come down. I have had my water analyzed at my local pool supply store. The readings are as follows: FAC=3.0; TAC=3.0; Water Ph=7.8; TA=160; Cyanuric Acid=60; TDS=2500; Salt=3700. The next week, after adding 2 gallons of acid the readings were as follows (I turned up the dia on my salt system which increased the chlorine to too high levels): FAC=7; TAC=7; Water Ph=7.9; TA=221; CYA=60; Salt=3800.

What am I doing wrong. I want to get Ph down to normal levels.

Thanks


chem geek
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Postby chem geek » Mon 26 May, 2008 16:36

SWG pools tend to rise in pH because the SWG cell produces hydrogen gas bubbles (most easily visible coming out of the returns at night with a pool light on if you run your SWG at night). This aerates the water driving out carbon dioxide and that causes the pH to rise (with no change in TA for technical reasons I won't get into here).

To reduce this problem you want to first reduce other aeration sources such as waterfalls, spillovers, fountains, etc. You also want to minimize the SWG on-time so that it produces the amount of chlorine you need -- specifically a minimum FC level of 7.5% of the CYA level -- and in your case it seems to be set too high as the FC is rising. You might also increase the CYA level a little to 70 or 80 ppm, though 60 ppm is not bad. Finally, you want to lower the TA level to around 80 ppm and can do so following the procedure described in this post where you will do lots of aeration (intentionally) at low pH along with acid addition. It will take a cumulative total of a little over 3 gallons of Muriatic Acid (31.45% Hydrochloric Acid) to lower the TA from 160 to 80 (starting at a pH of 7.8 and ending at a pH of 7.5) but obviously this is not all added at once. It takes this amount of cumulative acid no matter how you try and lower the TA -- the procedure I linked to just accelerates the process so that you won't have to add as much acid as frequently in the future -- it's a pay me now or pay me later sort of situation.

Your test results having the TA rise so much with only a small rise in pH make no sense so I strongly suggest you get your own good test kit, the Taylor K-2006 kit you can get at a good online price here or the TF100 test kit from tftestkits(dot)com here with the latter kit having 36% more volume of reagents so comparable in price "per test".

Richard
ptheoph
Pool Newbie
Pool Newbie
Posts: 2
Joined: Mon 26 May, 2008 12:53
Location: Temecula, California

Can't get PH to come down.

Postby ptheoph » Mon 26 May, 2008 23:12

Thanks for all the info. When you say add 3-gallons of acid, how much acid can I add at one time? Can I add all 3 gallons at once? The guy at the pool supplyn store always advises to add acid a quart at a time. Is that good info and if so what is the reason for that? Thanks again.
chem geek
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Postby chem geek » Tue 27 May, 2008 03:37

As I mentioned in my post, you do not add it all at once. Nevertheless, lowering the pH from 7.9 to 7.2 with your TA of 221 will take 13-1/2 cups of Muriatic Acid in your 20,000 gallon pool. You can, of course, just add a quart at a time, but that's because your pool store doesn't calculate exactly what will happen with larger amounts of acid and higher TA levels.

Once you are at a lower pH and aerate the water, the pH will rise pretty quickly and to lower the pH from 7.4 to 7.2, for example, will take about 5 cups of acid when the TA is around 200. As your TA gets lowered through the procedure, it will take less acid -- at a TA of 100, to go from 7.4 to 7.2 in pH will take about 3 cups of acid.

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