Re: Help
First, it is impossible to lower TA without lowering pH.
If you understand what TA is then it becomes easy to understand why this is true and also how to lower it.
TA is just a measure of the bicarbonates (and other alkaline substances, but for the most part bicarbonates so it's safe to ignore the rest for our purposes) in the water. Bicarbonates are a part of the buffer system in pools. The other parts are carbonates (of which there are hardly any at normal pool pH) and carbonic acid, essentially carbon dioxode dissolved in water--think seltzer.
When you add acid to baking soda it fizzes and gives off carbon dioxide and water. Same thing happens in your pool when you add acid, the bicarbonates are converted into carbon dioxide and water. It does not matter HOW the acid is added, whether it is drizzled around the perimeter or 'slugged' into the deep end with the pump off, the bottom line is that a given amount of acid will convert a given amount of bicarbonate into a given amount of carbonic acid (CO2 in water).
The very act of adding acid to the water and covert bicarbonate into carbonic acid IS lowering the TA, period! However, our pH is now lower so we need to get rid of this excess acid somehow to bring the pH back up. If we add an alkaline chemical to raise the pH we will just convert the carbonic acid back into bicarbonates, defeating our purpose (and if we use pH up--soda ash aka sodium carbonate--we are actually adding MORE carbonation than we started with and will end up with a higher TA than before so this is counterproductive).
We need to figure out another way to get rid of the carbon dioxide in the water (carbonic acid). Remember I said to think seltzer. If you let a bottle of seltzer sit on the kitchen counter it eventually goes flat (loses carbon dioxide). If you shake the bottle this happens much faster. As the carbon dioxide leaves into the atmosphere something else happens, besides going flat the pH rises! As the seltzer goes flat the amount of carbonic ACID present becomes less. Operative word here is acid. As the amount of acid present decreases our pH increases! Pretty cool!
(This is the infamous "pH rise from outgassing of CO2" that is often mentioned on this and other forums and that most people in the industry are clueless about.)
Now back to your pool...you can put in a lot of acid at once (slugging an acid ball--wasn't that something from the '60s?
) which often drops the pH low enough in some areas to cause damage to pool surfaces) and then wait for the pH to climb on it's own as carbon dioxide outgasses (waiting, in effect, for your pool to go flat like our bottle of seltzer sitting on the counter) OR you can speed up the process by "shaking up the bottle" or aerating your water.
So the SAFE way to lower TA is to add enough acid to drop the pH to about 6.8-70 and not lower (since lower pH can cause damage to equipment, vinyl liners, and plaster), testing the TA to see how low it is, and speed up the pH rise from outgassing of CO2 (the water going "flat") by aerating the water, to bring the pH back up to above 7.4 without having ANY effect on the TA. This process is repeated until the TA tests at the desired level and the pH is above 7.4. Sometimes it can take a while to complete, depending on how high your initial TA is and how well you are able to aerate your water. Patience and perseverance is the key to success here! It will be well worth it!
You didn't say how big your pool is so I will use 10000 gallons as an example. In 10k gal of water with a TA of 230 and pH of 7.5 1 gal of muriatic acid (or 11 lbs of dry acid) will drop the TA approx 50 ppm and drop the pH to about 4.4-4.5!
To drop your TA to around 100 you would need 2 and a half gallons of muriatic acid (or about 28 lbs of dry acid!!!) and that would drop your pH DANGEROUSLY LOW if added all at once in an "acid ball"!
Now that you have a better understanding of what TA really is and realize that it's easy to lower it (the act of adding acid lowers TA) you realize the problem is how to lower it without dropping the pH in the pool dangeously low and how to bring the pH back up quickly without bringing the TA back up again. This really leave only adding acid to bring the ph to around 6.9-7.0 and then aerating to bring the pH back up and repeating the process until your are where you want to be.
Hope this is helpful.
EDIT: at far as the CYA goes...I saw in another thread you stated that your pool is 25000 gallons. 5 lbs of CYA will not raise your level enough to even test! You need to add another 2-4 lbs to get your CYA up to around 30-40 ppm IF you are accurate about the 25k gallons!
As far as lowering the TA you would need (in the ballpark) OVER 6 gallons of acid or OVER 60 lbs of dry acid to get your TA to around 100 ppm BUT NOT ALL AT ONCE! It's hard to say how much acid is safe to add at once without an acid demand test since the higher the TA the more acid you can add without causing the pH to drop fast. As the TA comes down you will need less acid for the same pH drop. However, the amount of acid needed to covert a certain amount of bicarbonate into carbon dioxide is not really affected by pH so the total amounts of acid I gave above should be in the ballpark of what you can expect to use.
BTW, it would be very helpful if you could include info about your pool when you post such as size, surface (vinyl in your case so you do NOT want to slug acid to lower TA!) pump size and filter type, etc. along with current test results. It makes it much easier to advise you.
Last edited by waterbear; 07-24-2010 at 11:48 AM.
Retired pool store and commercial pool maintenance guy.
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Originally Posted by CarlD
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