reducing total alkalinity with muratic acid

[menace50]

We have a vinal liner pool. Some places we've read that you shouldn't use muriatic acid some say its okay. My question is about the addition of it and how long you should keep the pump off after it has been added.

our pool dimensions are:
16X36 rectangular pool. We have about a 10' shallow end that is 3' deep and about a 10' section of deep end at about 9.5' deep. the rest is the taper between. I estimate we have about 27000 gallons of water.

Currently, using a cheap test kit, our TA = 220ppm.

I see from the basics of pool maintenance from these forums that we should have a better test kit and we've been actually increasing the TA along with the PH when adding baking soda to increase PH...

Basically things are all okay --- again from our CHEAP test kits perspective --- except for the TA...

 

[Buggsw]

20 minutes should be long enough for the method you may be describing - pouring the acid in one spot.

However, there are some here who feel it doesn't make a difference if the pump is running when trying to lower TA quickly.

It will lower your TA whether the pump is running or not.

 

[JasonLion]

I don't belive in the "acid slug" approach to lowering TA. It has been fairly throughly discredited and poses a risk to the liner. With the pump off the acid will collect on the bottom of the pool and cause a small amount of damage to the liner and any decrease in TA will go away when the PH is brought back up.

The correct way to lower TA is to reduce the PH to between 7.0 and 7.2 with acid and then aerate the pool (with spa jets, waterfall, fountain, rain, kids playing, return aimed up to break the surface, air compressor, etc). The acid will lower the PH and the TA and then the aeration will raise the PH without affecting TA. Repeat as needed till you reach the desired TA.

Muriatic acid should be poured slowly in front of a return jet while the pump is running and you should leave the pump running for at least 30 minutes to help it mix in throughly.

 

[duraleigh]

I've never thought much of the "slug" method, either. In fact, it's logic is lost on me.

Aeration makes sense and it works.

 

[Ohm_Boy]

You do need to use acid to lower the TA. It also lowers the pH, and you need to use the pH as a lower limit for how much reduction you do at one time. Don't let the pH drop below 7.0 if you can, because you cannot reliably measure 6.8, if that is the lower limit of your tester.
Once the pH and TA are reduced by the acid, you should aerate to release carbon dioxide from the water, which will allow the pH to rise - the TA will not go up from this. When the pH is back up to 7.6/7.8 or so, use more acid to bring it and the TA down, and aerate to raise the pH back up. Repeat the process until the TA is where you want it.

You do not need to leave the pump off.

 

[chem geek]

To make the process go even faster, don't wait for the pH to rise to 7.6 -- add acid when it gets to 7.2 to bring it back down to 7.0. The outgassing is much faster at lower pH (see this chart for relative rates) and there is also quite a lot of acid that needs to be added to get from 7.2 to 7.0 in buffered pool water.

 

[engteach43]

I have a question along this same line. How long should you wait between adding the correct amount of acid to try to lower the TA?

 

[JasonLion]

Acid mixes fairly quickly with the water, perhaps half an hour to an hour with the pump running before you can test the water to see what effect it has had.

Adding acid lowers both the PH and TA. If you then raise the PH back up with borax the TA will come back up as well. Using soda ash will raise the TA above where it started. Aeration will raise the PH without raising the TA.

 

[engteach43]

Thanks Jason. Good info. How long of an interval should you wait between adding acid, or in other words, how long after adding acid, say, this afternoon, should I wait until I add acid again? Thanks again.

 

[JasonLion]

You want to aerate until the PH has gone up detectably before ading more acid. Depending on the TA and the amount of aeration that might be anything from a few minutes to days. If you are in a hurry you can add more acid when the PH gets up to say 7.3 or 7.4, but waiting for a PH of 7.6 or 7.8 is fine. The more aeration you can manage the faster it will go.