TA not coming down with Acid.

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watson524

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Nov 13, 2014
182
Lake Ariel, PA
Trying to switch over to bleach and add borates.

Started with these results:
pH: 7.4
TA: 60

500 gallon hot tub. Says it'd be 0.9oz to lower the pH to 7.0 and get TA to 50 unless I interpreted that incorrectly. Added half of the 0.9oz, aerated for a cycled an retested. TA still shows 60 (tho in reality may be between 50 and 60) but the pH went up to 8.0. You'd think with any acid in there it would have least dropped a bit even if not all the way down. Added the other half. Tested again after a cycle with aerating. TA still 60'ish per Taylor kit and pH MAYBE went to 7.8 (tho by color it's really between 7.8 and 8.0)

Are we doing something wrong or do we just keep adding little bits of MA and repeating until we get a true 50 on the TA and then see what the pH is. Should I even test pH until I get a 50 reading on the TA?

thanks!
 
Under Effects of Adding Chemicals in poolmath (near the bottom) it says .5 oz 31.45% acid will lower pH by .24 and lower TA by 3.9. It's unlikely you'd be able to see that small reduction in TA on the tester.

Now that pH has risen again, add acid to lower pH again. The acid will also lower TA a little bit. Repeat.

It's sort of a ratcheting thing. The ratchet itself is pH. The bolt it's on is the TA. Acid pushes down on the ratchet and both pH and TA drop. Aeration lifts the ratchet and pH rises. The TA stays where it was. Then you push it down again with acid. pH goes down, TA goes down. Aeration lifts the ratchet again and TA stays at that new lower setting. Back-and-forth, just like tightening a bolt with a ratchet.
 
Ok so we added another .45oz of MA and aerated. pH back up to 8.0 but this time TA did hit 50 on the test (didn't change color on the 6th drop). So if I want to target 7.6 pH, it says that if I add 0.7oz MA, it'll drop the pH by .34 (which is close enough for me) and the TA by 5.5. Is it ok that the TA goes into the 45 range? I thought I was targeting 50. And when it says it'll drop the pH by .34 is that accounting for aeration? Or in static water and then when we aerate it'll go back up and I need to add more and aerate again (thus dropping TA Again).

- - - Updated - - -

Something else I just noticed. In the top of pool math, if I put in all the numbers as they stand now, it says to get from 8.0 to 7.6 pH add 0.3oz of MA. But in effects of adding chemicals, it says adding 0.7oz of MA will lower the pH .34. How can they be so different?
 
watson524 said:
Funny I was just looking at that and the TA is clearly not in the range we're targeting :)

So is it ok if the TA goes below 50 a bit to get the pH in range?
Click to expand...
Yes. The TA acts as a buffer to prevent wild pH swings. As you've observed, as the TA goes down, it takes less and less acid to effect a pH change. The borates also have that buffering effect.
 
When doing the TA lowering procedure in a spa where you have lots of aeration sources with jets, it's OK to add more acid that has the pH drop lower than 7.0 since it will be for a rather brief period of time. If you figure out the total amount of acid needed to get the TA down, which you can figure from the Effects of Adding Chemicals that IS accurate for the acid to TA relationship (it's just not accurate for the acid to pH relationship outside a TA range), you can split the total acid dosage into fourths. Then just add the acid, aerate until the pH is up to 7.2 or more, then add the next one-fourth dose, etc.
 
watson524 said:
Ok we'll keep playing with it adding a bit at a time until we get pH at around 7.4 - 7.6 after aeration. Boric acid is all measured out and ready to go in once we get this situated.

Thanks for the help!
Click to expand...
When you reach your TA goal, you can stop with the aeration and the acid. 42, 46, or 49 will all read as 50 when you test. Are you going to keep see-sawing over and under until you go too far and have to add baking soda? Adding borates will change things. After its in there and mixed, then worry about fine-tuning the rest.
 
Ok so if I put everything into pool math as what I started with as goal and current, effects of adding chemicals told me 1.3oz of MA would drop the pH by 0.62 (from my original 7.4) and lower TA by 10 which would have been perfect.

So I put in 0.45oz of MA each time 3 times. TA did get down to 50 but the pH went up 0.6 after aeration. Maybe that's what I'm missing is that the effects part doesn't take into account the raise in pH after aeration... Or I'm just being dumb about the whole process, entirely possible.

We have 0.5oz MA in there now aerating around so we'll see what that is for readings in about 10 minutes when the cycle is over.
 

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Think we were typing at the same time. I guess that means I should stop after what we just put in, see if TA is still at 50 on a test, not worry about pH and dump the borates in? But then I may still need to take the pH down after that anyway, no? So why does order matter? Not questioning you, just trying to learn.
 
You obviously don't understand.

Adding the acid lowers the pH and the TA. Stop right there. The TA would have gone down whether or not you aerated afterwards. Aeration only affects pH. You're trapped in an endless loop. You lower TA so pH drops and then you aerate so pH gets too high so you lower pH but then you aerate further which makes it rise again so you add acid to reduce pH but that pushes TA too low and on and on and on. Is the TA where you want? Then stop adding acid. Is the pH okay? Stop aerating. If the pH is between 7.2 and 7.8 just leave it alone. Stop micromanaging. Pool chemistry does not have to be that exact.
 
Ok so after a total of 1.85oz MA, we have a TA of 50 and a pH of 7.8 so I'm calling it a day and adding the 19oz of Boric Acid. I guess where I was getting hung up is maybe I was aerating too long thinking I had to aerate a full cycle (like if we were sitting in the tub) vs checking it mid cycle. So more aeration = higher pH.

So for now I'll add the Boric acid and test things as normal next Saturday.

And cut over to bleach for the FC with dichlor once / month to make up for the lost CYA.
 
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