How Does TA Affect pH?

[COBrien]

Okay, this is probably a simple question, but I haven't been able to find the answer in a search (admittedly a cursory one...).

How, exactly, does TA affect pH?

I'm keeping my TA right around 100 ppm, and am seeing my pH creep up from 7.5 to 7.8+ within 2-3 days.

I understand TA has a buffering effect on pH, i.e., keeping pH from changing drastically and quickly, but will decreasing my TA keep my pH from rising so quickly? Or is that not the way it works?

 

[woodyp]

Dropping that down to the 60-80 range should help reduce your "creep".

 

[chem geek]

Remember that TA not only buffers pH but is a SOURCE of rising pH itself. The reason is that TA is mostly a measure of bicarbonate and at a given pH this is also a measure of the amount of carbon dioxide in the water. Pools are intentionally over-carbonated. When this carbon dioxide outgases from the pool, it raises the pH with no change in TA. Lowering the TA level reduces this rate of outgassing as does targeting a higher pH level.

 

[woodyp]

So chem geek............if my fill water is naturally at a TA of 30, is there any benefit whatsoever to raising it to the 60-80 range as we recommend here? Or just leave well enough alone and my PH rise will be better off for it?

 

[chem geek]

With a vinyl liner there wouldn't be a need to raise the TA, at least not very much. You want some pH buffering just in case some acid sources are added to the pool, but you could just go to 40 or 50 ppm and be fine. Spas using the Dichlor-then-bleach method are routinely run at 50 ppm and some at 40 ppm with no problems.

 

[duraleigh]

With a vinyl liner there wouldn't be a need to raise the TA, at least not very much.

Does that advice change in a gunite/masonry pool?

 

[Patrick_B]

Does that advice change in a gunite/masonry pool?

Not as a rule, because CSI comes into play, and most will be off balance corrosive to plaster if there isn't enough CH, TA and lower pH exists. This is really important to remember because CSI is a log scale. It isn't linear, and it's easy to get into the corrosive side with low TA.

I can actually adjust my system down to 50 TA and stay in balance with regard to CSI with my very high CH. Provided of course I maintain pH, but it's a fairly tight window I have to operate in.

Its a good subject though, because I think we should take a look at some of our recommended levels VS CSI, particularly regarding Salt pools.

 

[chem geek]

Yes, one can have a low TA in a plaster pool if one compensates with a higher pH and/or CH. In terms of protecting plaster, all that matters is that the water is saturated with calcium carbonate. It doesn't matter whether that saturation is more from calcium or from carbonate because from a chemical standpoint it is the product of these two concentrations that determines whether calcium carbonate in plaster will dissolve (at negative CSI) or will form scale (at positive CSI).

The TDS, so salt level, also affects CSI where salt pools at 3000 ppm have around 0.2 units lower CSI all else equal. Our current Recommended Levels are still inconsistent and low in CSI for SWG pools where they should have an even higher CH for plaster because 1) the higher salt level has the CSI be 0.2 units lower and 2) the higher CYA recommendation has the CSI be 0.1 unit lower and 3) the lower TA has the CSI be 0.07 units lower while the higher pH target only has the CSI be 0.05 units higher.

Basically, at the low end of the Recommended Levels (except using the highest recommended CYA) for SWG pools, the CSI is -0.56 which is quite low. For non-SWG pools the low end of the Recommended Levels is only -0.23. I first brought up this issue 3 years ago though then it was even worse because the ranges were even worse (the pH low end was changed later on from 7.5 to 7.6), but my recommendation of changing the CH range to 350-450 has still not been done. I brought this up over and over again and yet the Recommended Levels have not been changed:

Re: What am I missing? Everything seems good except CSI 1/19/12
Is the Saturation Index always Reliable? 1/19/12
Saltwater Chlorine Generator (SWG) Recommended Levels 3/9/12
Re: How important is it to familiarize w/ CSI? 8/13/13
Re: Help with Pool Math 5/18/14
Re: CSI 11/28/14
Re: What does and what doesn't get discussed on this forum 4/18/15
Re: Micromanaging pool chemistry 5/20/15

 

[Patrick_B]

Thanks Richard, that is the info I was thinking of. We really should make those changes to get them back in the green for those Corrosive CSI parameters.

 

[ITR]

Yes, one can have a low TA in a plaster pool if one compensates with a higher pH and/or CH. In terms of protecting plaster, all that matters is that the water is saturated with calcium carbonate. It doesn't matter whether that saturation is more from calcium or from carbonate because from a chemical standpoint it is the product of these two concentrations that determines whether calcium carbonate in plaster will dissolve (at negative CSI) or will form scale (at positive CSI).

The TDS, so salt level, also affects CSI where salt pools at 3000 ppm have around 0.2 units lower CSI all else equal. Our current Recommended Levels are still inconsistent and low in CSI for SWG pools where they should have an even higher CH for plaster because 1) the higher salt level has the CSI be 0.2 units lower and 2) the higher CYA recommendation has the CSI be 0.1 unit lower and 3) the lower TA has the CSI be 0.07 units lower while the higher pH target only has the CSI be 0.05 units higher.

Basically, at the low end of the Recommended Levels (except using the highest recommended CYA) for SWG pools, the CSI is -0.56 which is quite low. For non-SWG pools the low end of the Recommended Levels is only -0.23. I first brought up this issue 3 years ago though then it was even worse because the ranges were even worse (the pH low end was changed later on from 7.5 to 7.6), but my recommendation of changing the CH range to 350-450 has still not been done. I brought this up over and over again and yet the Recommended Levels have not been changed:

Re: What am I missing? Everything seems good except CSI 1/19/12
Is the Saturation Index always Reliable? 1/19/12
Saltwater Chlorine Generator (SWG) Recommended Levels 3/9/12
Re: How important is it to familiarize w/ CSI? 8/13/13
Re: Help with Pool Math 5/18/14
Re: CSI 11/28/14
Re: What does and what doesn't get discussed on this forum 4/18/15
Re: Micromanaging pool chemistry 5/20/15

Hey, chem geek,

I don't have access to the last three links you posted (CSI, What does and what doesn't get discussed on this forum, and Micromanaging pool chemistry). The last one has an interesting title, but I can't access it. Is that by design?

 

[BuckeyeChris]

The recommended levels may need adjusting, but the CSI calculator in pool math is accurately showing the CSI, taking into account salt/TA/CH/CYA, right?

 

[chem geek]

Hey, chem geek,

I don't have access to the last three links you posted (CSI, What does and what doesn't get discussed on this forum, and Micromanaging pool chemistry). The last one has an interesting title, but I can't access it. Is that by design?

Last three links are in an Answering Questions forum for mods and special contributors who help others on this forum on a regular basis. They have the same basic info/points as in the other threads regarding the CSI so you aren't missing anything.

- - - Updated - - -

The recommended levels may need adjusting, but the CSI calculator in pool math is accurately showing the CSI, taking into account salt/TA/CH/CYA, right?

Yes, the CSI calculator is correct. My point was only that one extreme in the ranges in Recommended Levels for SWG plaster pools gets pretty low in CSI. If you use PoolMath to calculate a CSI, you get a correct result. It is still mostly true that if you are within the Recommended Levels you are OK but you can always calculate your CSI in PoolMath to see. Of course, for vinyl pools that don't need calcium carbonate saturation, the CSI can be very negative with no problem. It's plaster pools that should be saturated with calcium carbonate to protect their plaster surfaces. The effects of a negative CSI would be long-term so certainly it's one of the last things to adjust/tweak in pool water chemistry after you've dealt with everything else of concern.