PH stable only when TA very low.

[rob.ert]

Hey, everyone. Maybe one of the chemistry wizards here can help me out.

I have a 450gal spa with an ozonator. I sanitize it with 6% bleach. I've also added about 30ppm CYA and 50ppm borates when I first filled up.

I originally balanced the water to a TA of 80 per the booklet that came with the Taylor test kit. But my PH would drift up like crazy, requiring 1-2 oz of dry acid every week to bring down to normal.

Then I tried a TA of 40 based on info on forums, but PH would still drift sky high by the end of the week.

Then I decided to not add baking soda anymore and let the dry acid bring TA down. TA eventually dropped to about15ppm and my PH has been stable for the past two weeks.

Does this indicate a problem? Is it bad to keep the TA so low? AFAIK, TA's sole purpose is to keep PH steady. Is there any other reason why I should try to balance it higher? (scaling, etc?). I'd love to be able to just keep it at 15 and be done with having to dump dry acid in there every weekend.

Thanks!!

 

[JamesW]

As long as you can maintain a good ph, I would leave the TA where it is.

It is unusual to need such a low TA, especially with borates and cya. You might be trying to keep the pH too low. Try maintaining the pH in the 7.6 to 7.8 range.

Also, be aware that the pH can read a false high when the chlorine or bromine is high.

 

[chem geek]

Carbon dioxide outgassing should stop at the TA listed for the following pH and given your other parameters (30 ppm CYA, 50 ppm Borates):

pH .. TA
7.0 . 13
7.1 . 15
7.2 . 18
7.3 . 20
7.4 . 23
7.5 . 27
7.6 . 32
7.7 . 38
7.8 . 45
7.9 . 55
8.0 . 65

So as James wrote, you are likely trying to keep your pH too low and should instead target a higher pH that should let you have a somewhat higher TA level. Also note that the above levels are extreme and would usually require a lot of aeration to notice a pH rise. Usually one gets to a stable level above the TA values listed in the table.

 

[JamesW]

You should be able to maintain a stable pH with a TA of at least 30 or 40. Here are a few suggestions:

1) Make sure that you have fresh pH reagent to ensure good readings
2) Test when the FC is lower than 10. Not after adding bleach.
3) Don't lower the pH unless it goes over 7.8 and then only lower it to 7.8, and not lower.

 

[rob.ert]

Thanks guys!
My PH target so far was 7.6. I test it once a weeek and it would always max out the taylor k-2006 scale (8.0 is the max) and require 2-4 drops of "acid demand reagent" to bring back to 7.6. Now with this low TA, PH is still at 7.6 where I left it last week.

I bought the test kit in November, so it can't be very old. And I make sure to test when FC is lower than 5.

Based on your advice, I'll raise the TA up to 35 or so and see if that keeps the PH stable at 7.8.

Thanks again.

 

[JamesW]

If you're able to maintain a good pH, then I would not change the TA.

A TA of 15 ppm is below what we normally recommend, but if that works for you, then there is no reason to change it. It sounds like you have a pretty good handle on the chemistry, and would not allow the pH to get too low.

 

[rob.ert]

Thanks James!