Need help lowering ALK

meck60

New member
Jun 8, 2007
1
I filled my pool (13400gal) with well water and the ALK is at 400, PH 7.6. I know that I have to lower the PH to 7.0 and then aerate the water to bring the PH back up. Is there a formula I can use to calculate how much acid I need to lower the PH to 7? I am using the bleachcalc app but I am assuming that I will need a lot more acid since the ALK is so high. I could just add small amounts until I get to 7 but since I'll have to repeat this process many times it would be nice to add the correct amount the first time. Can someone help me out?
 

mbar

LifeTime Supporter
Apr 24, 2007
257
Ashland PA
You have to add small amounts to gas off the alkalinity - the muriatic acid will not lower the alkalinty itself. So you have to keep bringing your ph down to 7 - 7.2, aerate the water till the ph goes up, bring it back down etc. The more you keep on top of it, the faster it will go :)
 

Buggsw

LifeTime Supporter
Apr 22, 2007
925
Arizona
For your pool volume 11 ounces of acid will bring down the pH .2

I personally don't add more than 3 or 4 cups at a time in that size of pool over a period of 24 hours.

How are you applying the acid? Drizzling it around the pool or in one spot?

I have found that pouring about 1.5 cups in one spot without the pump running (known as the acid ball method) works faster at lowering TA without lowering the pH too much. So I pour 1.5 cups in one spot and another 1.5 cups in another spot - at least a foot away from the wall and in the deepest part of the pool. Wait 20 minutes and then turn the pump on and aerate.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
I calculate it would take 161 fluid ounces (1.26 gallons) of Muriatic Acid (31.45% Hydrochloric Acid) to lower your pH from 7.6 to 7.0 in your 13,400 gallon pool, assuming the CYA is at 30 ppm. The "acid ball" method sounds similar to the "slug" method of pouring acid in one location to lower TA and this does not work very well. Lowering pH will lower TA anyway and though adding acid in once place can make the local pH low to drive off carbon dioxide, in practice the volume is too low to have any large effect. For example, just lowering the pH from 7.6 to 7.0 will lower the TA from 400 to 353 with no aeration whatsoever. You can, of course, try that method using less than the amount of acid I say it will take and see what happens. You should generally add less acid than you think and remeasure after it mixes in the pool (at least 30 minutes with good circulation -- a couple of hours to be safe).

I also calculate that you will need a LOT of aeration and that during the aeration you will add another 935 fluid ounces (7.3 gallons) of Muriatic Acid, though obviously not all at once. It takes a heck of a lot of acid and aeration to drive off 79% of the total carbonate which is pretty much what you will be doing. It's obviously a lot easier to do this a bit at a time with fill water than it is with an entire pool, but if you could only fill with well water you didn't have much choice.

Richard
 

Buggsw

LifeTime Supporter
Apr 22, 2007
925
Arizona
I don't understand why you say the acid ball method doesn't work very well. Could you explain? Because, in my experience it works faster than drizzling the acid around the pool and doesn't affect your pH very much.
 

sevver

Well-known member
Mar 27, 2007
477
Morris, IL
You are adding an amount of acid to an amount of water, I don't see how the method of application could alter the outcome. The acid will combine with the water, equally, and will lower the Ph. If the acid combines with the water equally, then how does the method of introducing the acid effect anything. Sorry, I just don't see how this would help. I am far from a Chemistry major though, I just see that a solution is the eventual outcome of this.
 

ric

Well-known member
Apr 27, 2007
217
Ohio
Hi all
I have always understood to drop the total alkalinity using muriatic acid was to use the slug method with no splashing and to raise the ph you would walk the acid around the pool. Is this a wise tale??
 

JasonLion

TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
37,879
Silver Spring, MD
The idea behind the slug method is to temporarily lower the PH of some small volume of water dramatically enough to induce CO2 out gassing without aeration. The PH then rises, because of the CO2 out gassing, partially canceling out the acid, and the remainder just spreads out in the pool having little effect on the large volume. By comparison, if you pour the acid slowly around the edge of the pool you will speed up the dilution process and hopefully avoid taking the PH down far enough in any one spot to cause out gassing.

The slug approach to ALK reduction clearly has the potential to work but it is unclear if the overall effect is large enough to be very useful. Some people report getting it to work, others are skeptical. For large changes in ALK the aeration method (lower PH to 7, aerate to bring PH up, repeat) appears to be more efficient.
 

MikeInTN

TFP Expert
LifeTime Supporter
May 27, 2007
1,335
Middle Tennessee
ric said:
Hi all
I have always understood to drop the total alkalinity using muriatic acid was to use the slug method with no splashing and to raise the ph you would walk the acid around the pool. Is this a wise tale??
It's partially a wise tale. Adding muriatic acid to the water will lower BOTH the TA and ph, regardless of using the slug method or walking the acid around the pool, and will never raise the ph.
 

MikeInTN

TFP Expert
LifeTime Supporter
May 27, 2007
1,335
Middle Tennessee
meck60 said:
I filled my pool (13400gal) with well water and the ALK is at 400, PH 7.6. I know that I have to lower the PH to 7.0 and then aerate the water to bring the PH back up. Is there a formula I can use to calculate how much acid I need to lower the PH to 7? I am using the bleachcalc app but I am assuming that I will need a lot more acid since the ALK is so high. I could just add small amounts until I get to 7 but since I'll have to repeat this process many times it would be nice to add the correct amount the first time. Can someone help me out?
I just finished lowering the TA on my pool that's the same volume as yours. 11 ounces of 31% muriatic acid should lower the ph by .2 each time you add it. Personally, I wouldn't go lower than 7.2 before I started aerating to get the ph back up, so slowly add 11 ounces in front of one of your returns, then add another 11 ounces, wait about 20 minutes, and check your ph. If it's at 7.2, go ahead and start aerating until the PH gets up to 7.4, then add another 11 ounces, and keep on aerating, get the ph back up, add more acid, etc.

Unfortunately, you have to lower the TA in small steps like this, instead of dumping a large amount of acid in. If you get the ph too low, you can damage your pool.

Since your TA is so high, it's going to take you a good long while, and quite a bit of acid, as chem geek pointed out. I lowered my TA by 60 ppm, and it took a little over a gallon of acid, and about a week of acid/aerating. I used the top return of my pool to churn the surface of the water. I could have used a fountain to aerate the pool, but I didn't want the water cooling that occurs with using a fountain.
 

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
12,083
San Rafael, CA USA
The acid addition is non-linear with pH. To lower the pH by 0.2 from 7.6 to 7.4 when the TA is at 400 (and for 13,400 gallons) takes 37 fluid ounces of Muriatic Acid. The 11 ounces is what it would be IF the TA were at or near 100. With the much higher TA it takes more acid to lower the pH by the same amount. It is true that you can do the aeration method by bouncing between 7.2 and 7.4 pH and that this is safer, but the TA will get lowered faster (the aeration will be more effective) if you bounce between 7.0 and 7.2. Of course, that assumes you have a pH test kit that measures to at least 6.8 since you have to know for sure that you are at 7.0 and not below that.

Though it takes 89 ounces of acid to go from 7.6 to 7.2 and the TA will go from 400 to 374, it takes another 72 ounces of acid to go from 7.2 to 7.0 and the TA will go from 374 to 353. This is what I mean by the amount of acid being non-linear with respect to pH. It is harder to make the pH go lower at lower pH -- sort of like pushing or pulling on a spring. This is due to the carbonate buffer that becomes "stronger" in resistance to pH change at the lower pH.

For MikeInTN, I calculate that going from a TA of 100 ppm to 60 ppm (assuming a CYA of 30 ppm) in the same volume of water takes a total of 137 fluid ounces or 1.1 gallons -- about what he said it took. Mike, did you start at a TA of around 100? I'd like to validate the calculations with more real-world results.

As for the acid addition method, I believe JasonLion summed it up best. There IS some effect by concentrated acid addition, but when I calculate even the best case of completely removing the carbonate from a volume that gets to high acidity, it just doesn't contribute very much. By the time the acid disperses into enough volume to make a difference in terms of the amount of potential carbonate that can be removed, the acid is then diluted enough to not have as much effect on pH (so not as much effect on driving out carbon dioxide). Also, I wouldn't do this in a vinyl pool since concentrated acidity (low pH) isn't good for the vinyl liner though it is for a short time. Generally speaking, adding concentrated pool chemicals to the pool very slowly over a return (at the deep end) so that it gets readily dispersed is safer.

Of course, experiment trumps theory so if you find the acid ball method to be measurably better, then at least for plaster/gunite pools (where the method would be safer) it might be worth it. If you can quantify the difference in the two methods (next time you or anyone needs to lower the TA in their pools) that would be helpful info. Basically what is needed is a measurement of the TA drop after adding acid quickly with the pump off (of course, the measurement would have to be made after the pump was turned back on so as to mix the pool water for the measurement) vs. after adding acid slowly with the pump running (waiting 30 minutes or so for mixing).

[EDIT]
As a simple example starting with standard starting numbers of pH 7.5, TA 100, CYA 30 in a 10,000 gallon pool, then adding 2 cups (16 fluid ounces) of Muriatic Acid if fully mixed would result in a pH of 7.19, TA 93.7 while if the acid mixed with only 10% of the pool water than that 10% would have a pH of 6.0 and a TA of 37.4 with an outgassing rate about 13 times higher than normal. Even if this resulted in a complete loss of all carbonate in that 10% water volume, this would result after dilution of the acid in the rest of the pool water in a pH of 7.72 and a TA 93.7 or with further acid addition is equivalent to a pH 7.5, TA 91. So it is theoretically possible for the "slug" or "acid ball" method to have an effect, but it is highly unlikely that very much of the carbonate is actually driven out of the water into the air (as carbon dioxide) since it only does so at the surface (unless there is aeration) and the concentration of acid in a part of the pool water is not static (i.e. will not last very long, even with the pump not running).
[END-EDIT]