TA and PH

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
fcw
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TA and PH

Postby fcw » Tue 15 Apr, 2008 10:37

new to this site,lots of great info. just refilled my pool after an acid wash, PH 7.6 TA 370 My question is can i continue to run my pool filter while adding acid using Chem Geek method or should i turn filter off and use the acid ball method as described by others. i understand that the areation can continue. also, what would be the max. amount of acid that could safely be added to a 10,000 gal. plaster pool at one time.


chem geek
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Postby chem geek » Tue 15 Apr, 2008 11:51

The acid ball, aka slug method or acid column method, does not work well and is fully debunked here . The correct procedure for lowering the TA is described here.

In your 10,000 gallon pool assuming your pH of 7.6 and TA of 370 are correct (and assuming a CYA of 30), it would take 65 ounces (about 8 cups) of Muriatic Acid (31.45% Hydrochoric acid) to lower the pH to 7.2

After that, going from a pH of 7.4 (which would rise from aerating the water) to 7.2 would take 36 ounces or about 4-1/2 cups. As your TA gets lower, it will take less acid to go from 7.4 to 7.2. For example, when the TA gets to 120, it only takes 14 ounces (less than 2 cups) to go from 7.4 to 7.2. If you are able to aerate enough to have the pH rise to 7.8 relatively quickly, then going from 7.8 to 7.4 at a TA of 370 requires about 10-1/2 cups of acid while at a TA of 120 it will require 3-3/4 cups.

No matter how you do it -- that is, what range of pH you alternate between when allowing pH to rise from aeration and then lowering both pH and TA with acid -- it will take a cumulative total of around 741 ounces or about 92-1/2 cups or about 5-3/4 gallons of acid total. To speed this process up, you should use whatever means are at your disposal for aerating the water. You can see that it's a pain to lower the TA as it takes multiple cycles of aeration to get the pH to rise followed by acid addition to lower both pH and TA. You should check the TA of your fill water as perhaps doing a partial drain/refill will lower your TA more easily.

Why is your TA level so high in the first place? Are you sure the measurement is accurate?

Richard
fcw
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Joined: Tue 15 Apr, 2008 10:08

TA

Postby fcw » Tue 15 Apr, 2008 17:01

Thanks for the reply Richard. Yes the TA readings are correct at 370 out of the tap. im on well water here and i suspect that the CH levels are high also although i havent received that test kit yet to confirm. At the moment i am areating with a submersable pump in the deep end,a pipe running up from that to the surface then shooting back down to just above the waterline 24/7 and when noise permits, an air compressor in the deep end as well. its interesting that you mention 5 to 6 gal. of acid total to bring the TA to 120. I have added approx. 4 gal. of muriatic already and seen very little drop in TA, which went back up very quickly. Could adding other chemicals such as chlorine, stain control or cya account for this.
chem geek
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Postby chem geek » Tue 15 Apr, 2008 17:38

An air compressor is excellent, but you should have the open end of the hose connected to a pipe with small holes in it and then submerge that into the deep end of the pool. The idea is to get lots of small bubbles flowing through the water for as long as possible as that maximizes aeration.

Yes, definitely check for CH as well. The best test kit for all the water chemistry parameters is the Taylor K-2006 which you can get at a good price here or the TF100 from tftestkits(dot)com here where the TF100 has 36% more volume of reagents so is comparably priced. These kits contain a FAS-DPD chlorine test kit which won't bleach out so can measure up to 50 ppm FC and CC accurately (very useful if you ever need to shock the pool).

As for other chemicals adding to TA, the CYA will add to it at about 1/3rd its amount (at a pH of 7.5), but that's not very much. If you added 30 ppm CYA, then that's only 10 ppm added to TA. Chlorine shouldn't add very much nor should the metal stain control (metal sequestrant).

Adding acid will make the pH and TA go down, but the aeration should only make the pH go up with no change in TA. So something is very strange about your situation if you've added 4 gallons of acid. The only things that should shoot up the TA a lot would be adding either pH Up (sodium carbonate) or Alkalinity Up (sodium bicarbonate).

So are you saying that the acid lowers the pH and TA, but that the aeration (or time) is having both the pH and TA move up? Perhaps after the acid wash there was plaster dust, which is calcium carbonate, and that has been causing the pH and TA to rise together. That's my best guess at this point and hopefully if that's the cause it should stop or slow down when it's fully dissolved. If after the acid wash any new plaster was applied, then the curing of that plaster would add to calcium hydroxide which would also cause the pH and TA to rise together, though without as much rise in TA as with plaster dust.

Remember that when adding acid you should do so very slowly over a return flow at the deep end. It's VERY strong and should get diluted in the pool water -- you NEVER add it to the skimmer.

Richard
fcw
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TA

Postby fcw » Tue 15 Apr, 2008 19:41

Thanks again Richard. I suspect you are on to something about the plaster dust from the acid wash. I also gave the pool a light sanding. I noticed this evening that the TA was down about 30ppm. will keep trying and keep you posted.

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