Problem Getting Ph Right

[chk57]

I'm trying to follow NITRO'S approach for water chemistry. Here is my problem.

It doesn't seem to want to balance?

I open up everything to aerate...get it around 8.0...throw Dry Acid in there...bring it back down. Aerate it goes back up...Dry Acid again...

it just doesn't seem like I'm getting any closer though? I've got up and down probably four or five times now....how many of these ups/downs does it normally take?

It IS my first time trying to do this stuff CORRECTLY...maybe its just my inexperience?

[PaulR]

The acid/aeration technique is for reducing your Total Alkalinity (TA) which is a measure of alkalines dissolved in the water. (chem geek might have a more technical explanation but this is good enough for mere owners.) The technique doesn't balance your pH, in fact it is deliberately pushing the pH around, because that's how you reduce the dissolved alkalines.

Have you tested your TA and decided it's too high or too low?

--paulr

[chk57]

The acid/aeration technique is for reducing your Total Alkalinity (TA) which is a measure of alkalines dissolved in the water. (chem geek might have a more technical explanation but this is good enough for mere owners.) The technique doesn't balance your pH, in fact it is deliberately pushing the pH around, because that's how you reduce the dissolved alkalines.

Have you tested your TA and decided it's too high or too low?

--paulr

Yes. I started and it was too low...40ppm. So I raised it to 100 using baking soda. Thats when I began the process of aerating, etc. thinking that was how I would get to the "best" level of TA.

It took A LOT of acid to bring it down to roughly 7.0 pH. Then it took a LONG time (>60m) to get it to roughly 8.

Should I be checking TA after every pass?

Here is another question that I think I know the answer to....when adding acid to lower pH...should I be running the pumps? I would think it needs to "mix" in someway.

Thanks for your help...if nothing else...I feel like a little kid again playing with chemicals!

[DonR6702]

I'm trying to follow NITRO'S approach for water chemistry. Here is my problem.

It doesn't seem to want to balance?

I open up everything to aerate...get it around 8.0...throw Dry Acid in there...bring it back down. Aerate it goes back up...Dry Acid again...

it just doesn't seem like I'm getting any closer though? I've got up and down probably four or five times now....how many of these ups/downs does it normally take?

It IS my first time trying to do this stuff CORRECTLY...maybe its just my inexperience?

You shouldn't be checking the pH during or shortly after aerating. Aerating disturbs the equilibrium between carbonic acid, bicarbonate and bicarbonate, and the pH will go up, but when aeration ceases, the equilibrium will gradually be re-established. You should run the pumps for a few minutes after adding acid, but leave the air injection off.

When you add acid (assuming you're adding enough to make a difference) you permanently destroy some of the alkalinity, and the pH **WILL** go down. The equilibrium shifts away from carbonate (high pH) towards bicarbonate (moderate pH) and carbonic acid/carbon dioxide (low pH). Aerating will always raise the pH temporarily, but the new balance will take effect after aeration ceases for a while.

When I add acid to my spa, I don't aerate, but just run the pumps for a few minutes. The pH always goes down, and stays down unless I check it during or shortly after aerating. It is, after all, the equilibrium pH that you are interested in, since most of the time (I'm assuming here) you are not aerating your spa. It is that equilibrium pH that determines the effect that your water has on bathers and on the equipment. Try checking it first thing in the morning, and don't aerate after adding chemicals to adjust the pH. The carbon dioxide fluctuations that can cause rapid temporary pH changes can mislead you.

[chk57]

I'm trying to follow NITRO'S approach for water chemistry. Here is my problem.

It doesn't seem to want to balance?

I open up everything to aerate...get it around 8.0...throw Dry Acid in there...bring it back down. Aerate it goes back up...Dry Acid again...

it just doesn't seem like I'm getting any closer though? I've got up and down probably four or five times now....how many of these ups/downs does it normally take?

It IS my first time trying to do this stuff CORRECTLY...maybe its just my inexperience?

You shouldn't be checking the pH during or shortly after aerating. Aerating disturbs the equilibrium between carbonic acid, bicarbonate and bicarbonate, and the pH will go up, but when aeration ceases, the equilibrium will gradually be re-established. You should run the pumps for a few minutes after adding acid, but leave the air injection off.

When you add acid (assuming you're adding enough to make a difference) you permanently destroy some of the alkalinity, and the pH **WILL** go down. The equilibrium shifts away from carbonate (high pH) towards bicarbonate (moderate pH) and carbonic acid/carbon dioxide (low pH). Aerating will always raise the pH temporarily, but the new balance will take effect after aeration ceases for a while.

When I add acid to my spa, I don't aerate, but just run the pumps for a few minutes. The pH always goes down, and stays down unless I check it during or shortly after aerating. It is, after all, the equilibrium pH that you are interested in, since most of the time (I'm assuming here) you are not aerating your spa. It is that equilibrium pH that determines the effect that your water has on bathers and on the equipment. Try checking it first thing in the morning, and don't aerate after adding chemicals to adjust the pH. The carbon dioxide fluctuations that can cause rapid temporary pH changes can mislead you.

Okay that makes sense.

Will try again this evening once I get home...thanks for everyone's help.

[chk57]

thanks again for everyone's help.

Got home and had SUCCESS!

Not sure if this is right...or bad...or what...but my TA is pretty low...40. The pH is holding pretty well right around 7.6 after aeration...is that okay?

Now onto Chlorine....

[PaulR]

If your pH is pretty steady then the TA is in the right place.

--paulr

[Nitro]

It is, after all, the equilibrium pH that you are interested in, since most of the time (I'm assuming here) you are not aerating your spa. It is that equilibrium pH that determines the effect that your water has on bathers and on the equipment. Try checking it first thing in the morning, and don't aerate after adding chemicals to adjust the pH. The carbon dioxide fluctuations that can cause rapid temporary pH changes can mislead you.

It's not necessarily the "Equilibrium pH" I'm interested in so much. It's the pH while I'm in the tub. After all, that's when we want the pH as close to 7.5 (Our Tears) as possible.

When I check my pH before I run the jets/air, it's around 7.4. After I get out of the tub (for long soaking periods) the pH is around 7.6-7.8. That means my pH hovers between 7.4-7.8. That's as ideal as you're going to get in a spa, and feels the best.

[Nitro]

thanks again for everyone's help.

Got home and had SUCCESS!

Not sure if this is right...or bad...or what...but my TA is pretty low...40. The pH is holding pretty well right around 7.6 after aeration...is that okay?

Now onto Chlorine....

Adjust your TA so that your pH is within 7.4-7.8, and you'll be fine. That means your TA should be around 50-60 ppm. If your tub doesn't have many jets/air, then you should raise your TA to 60-80 ppm.

Good luck!

[chem geek]

The chemical reactions between carbonate ion, bicarbonate ion and carbonic acid are VERY, VERY fast. It is only the reaction between carbonic acid and dissolved (aqueous) carbon dioxide that is slower, but even then it is still relatively fast reaching equilibrium in seconds. The slowest reaction, of course, is a physical process -- that between dissolved carbon dioxide in the water and carbon dioxide gas in the air -- that is, outgassing of carbon dioxide.

The reactions and rate constants for the equilibrium between carbonic acid and aqueous carbon dioxide are as follows (most of what is present in the water between these two species is aqueous carbon dioxide where the carbonic acid is far less at around 1/600th the concentration):

H₂CO₃ ---> CO₂(aq) + H₂O

Carbonic Acid ---> Aqueous Carbon Dioxide + Water

Rate = 23 s^-1 * [H₂CO₃]

CO₂(aq) + H₂O ---> H₂CO₃

Aqueous Carbon Dioxide + Water ---> Carbonic Acid

Rate = 0.039 s^-1 * [CO₂(aq)]

At the equilibrium amounts with pH 7.5, TA 100 (CYA 30 ppm), the forward and reverse rates are equal and are at least (because the above rate constants are at 77F, not at 104F spa temperatures) about 4% of the total concentration amounts per second. That is, 23x of the amount of carbonic acid is getting converted to aqueous carbon dioxide (so its total concentration is converted in around .04 seconds) and 4% of the amount of aqueous carbon dioxide is getting converted to carbonic acid every second. In human time, this is a fast reaction that largely gets to equilibrium rather quickly. Carbon dioxide doesn't outgas that fast.

When there is vigorous aeration, or even when there is still water and no circulation, then the pH of the water closer to the surface will rise more than the water elsewhere, but with circulation this gets mixed in a matter of minutes. So yes, if you are measuring the pH by just dipping below the surface while the water is being aerated, then you will get an artificially high reading relative to the average of the bulk spa water as a whole, but this is not due to chemical equilibrium, but rather to physical mixing (or lack thereof).

The process of lowering TA goes faster if you keep the pH lower so not wait until the pH rises to 8.0 before adding acid. The rate of outgassing increases dramatically at lower pH and, of course, with more aeration. Outgassing is at least 3 times faster at 7.0 than at 7.5 and at least 3-1/2 times faster at 7.5 than at 8.0. When you add acid, this lowers both the pH and the TA. It should not be necessary to check TA at every pass if you are keeping track of the amount of acid you are adding. One tablespoon of dry acid will lower the TA by about 6 ppm in 350 gallons.

Richard

[DonR6702]

Good analysis, Richard. I would add one thing to it which you mentioned in one of its contexts but failed to mention in the other context - the equilibrium between dissolved carbon dioxide and carbon dioxide in the air. You mentioned that outgassing of carbon dioxide to the air is slow, which it is. But you didn't mention that dissolution of carbon dioxide from the air is also quite slow, and it is this dissolution that re-establishes the equilibrium after carbon dioxide has been stripped from the water with vigorous aeration.

When you add acid to the spa, it reacts with bicarbonates (there are practically no carbonate species in aqueous solution below pH 8.5) to produce the salt of the acid used plus carbonic acid, as you noted. The alkalinity drops and the pH drops, and a new equilibrium between carbon dioxide, carbonic acid and bicarbonate is established quickly. By the time the added acid is mixed thoroughly with the water that reaction is finished. But the new, lower pH means that carbon dioxide is less soluble, so some of it escapes to the atmosphere via that slower mechanism you mentioned. Though slower than the initial reactions, that mechanism too will be complete within minutes, and will be accelerated by mixing and/or aeration.

But what happens now if you turn on the pumps at high speed with air injection? Carbon dioxide is stripped from the water fairly quickly. The carbonic acid --> carbon dioxide equilibrium is pushed to the right with carbonic acid decomposing to produce more carbon dioxide, which is stripped from the water, etc., etc., and the pH rises. It is possible with vigorous aeration at pH in the mid 7's to drive most of the carbon dioxide and carbonic acid out of the water. But in this process the alkalinity remains unchanged, so the equilibrium is disrupted. After the vigorous aeration ceases, the equilibrium is re-established by the reaction of atmospheric carbon dioxide with water at the water surface. This reaction is relatively slow, depending on the water temperature and pH, and the ratio of water volume to surface area. It could easily take more than an hour for the pH to return to its new (lower) equilibrium value. In short, vigorous aeration will always raise the pH, but it doesn't change the alkalinity, so the pH will gradually return to its equilibrium value.

Don

[chem geek]

But what happens now if you turn on the pumps at high speed with air injection? Carbon dioxide is stripped from the water fairly quickly. The carbonic acid --> carbon dioxide equilibrium is pushed to the right with carbonic acid decomposing to produce more carbon dioxide, which is stripped from the water, etc., etc., and the pH rises. It is possible with vigorous aeration at pH in the mid 7's to drive most of the carbon dioxide and carbonic acid out of the water. But in this process the alkalinity remains unchanged, so the equilibrium is disrupted. After the vigorous aeration ceases, the equilibrium is re-established by the reaction of atmospheric carbon dioxide with water at the water surface. This reaction is relatively slow, depending on the water temperature and pH, and the ratio of water volume to surface area. It could easily take more than an hour for the pH to return to its new (lower) equilibrium value. In short, vigorous aeration will always raise the pH, but it doesn't change the alkalinity, so the pH will gradually return to its equilibrium value.

Don,

The part you wrote above isn't true. Aeration will not drive out more carbon dioxide than the equilibrium amount that would normally be in the water itself. Aeration is not a "push" that makes anything go further out of equilibrium. It is simply an accelerator, like a catalyst, that makes the out-of-equilibrium situation go closer to equilibrium. Remember that pool and spa water is intentionally over-carbonated. The equilibrium TA in water with normal amounts of carbon dioxide in air is only about 10 ppm (with no CYA and at a pH near 7.5).

All that happens when the aeration is stopped is that the acceleration is stopped. Carbon dioxide in the air doesn't go back into the water again (except in the normal back and forth that always occurs -- I'm talking about a NET going back into the water more than it is leaving). The lower pH has there be more dissolved (aqueous) carbon dioxide so it makes things MORE out-of-equilibrium than it already was and then when aeration occurs more carbon dioxide is outgassed and the pH rises. This higher pH lowers the amount of aqueous carbon dioxide near the surface and this slows down the rate of carbon dioxide outgassing somewhat. If the water is churned and mixed, the pH doesn't get too high. When more acid is added, then this keeps the rate of outgassing higher (due to lower pH). When aeration and acid addition are stopped, all that happens is that, with circulation, the water gets mixed so the somewhat higher pH water closer to the surface is mixed with the lower pH water below, but it's not as if even the water below didn't rise in pH -- with any decent circulation, it does rise as well.

So after aeration stops, carbon dioxide continues to outgas, but at a much slower rate. There is no net "reversal" of carbon dioxide going back into the water. The water was way over-saturated with carbon dioxide when the TA was higher and now after aeration and acid addition it's not as over-saturated since the TA is lower, but it is STILL over-saturated with dissolved (aqueous) carbon dioxide.

In practice, I have never heard of anyone doing the TA lowering procedure where they found that after stopping the procedure at a pH they want (usually near 7.5), that the pH would then drop in an hour or so. I suppose that with poor circulation or measuring too close to the surface that there could be the effect I described earlier, but it's not carbon dioxide going back into the water lowering the pH.

Richard

[DonR6702]

Richard, you can in fact strip dissolved carbon dioxide from water with aeration, as you can strip almost any dissolved gas from water. It is done all the time in industry. Carbon dioxide is different from some gases in that it reacts with water, forming an equilibrium between carbon dioxide and carbonic acid, but the stripping of carbon dioxide from water is not dependent on that equilibrium. It does, in fact, disrupt that equilibrium, which is quite weak at normal spa pH values. The great majority of carbon dioxide in water exists as the dissolved gas at pH 7.5.

Stripping of gases via aeration is a purely mechanical process, so it can't be described by equilibrium equations. With vigorous enough aeration for a long enough time you can strip out practically all of the carbon dioxide, including that which is in the carbonic acid form - in this case, the equilibrium actually aids the stripping process because as carbon dioxide gas is purged from the water, carbonic acid dissociates to produce more carbon dioxide, which is subsequently stripped from the water. This is why the pH goes up when you aerate water. With moderately alkaline water having moderately high calcium hardness, it is possible to raise the pH high enough, with aeration only, to precipitate calcium carbonate.

Your statement:

Remember that pool and spa water is intentionally over-carbonated. The equilibrium TA in water with normal amounts of carbon dioxide in air is only about 10 ppm (with no CYA and at a pH near 7.5).

is quite confusing. I guess it indicates that your experience is limited to pool and spa water starting from water with relatively low alkalinity. Water chemistry is much more complex than that. I just refilled my spa, and the tap water had pH 8.0 with 35 ppm alkalinity. This pH is higher than normal for my location, but the alkalinity is right in line. My tap water normally runs from pH 6.8 to 7.5, with alkalinity between 30 and 40. I have been testing and logging my tap water chemistry for years (for aquaria use as well as for my spa), and the alkalinity has never been below 30, while the average pH is about 7.2.

There are some parts of the US where tap water alkalinity runs to 150 ppm or higher. There is no "normal" value of alkalinity in water, and your term "over-carbonated" is misleading. The carbonate species in spa water are in equilibrium by definition except when steps have recently been taken to make adjustments. In some parts of the US it is necessary to lower the alkalinity of tap water after adding it to a pool or spa. This water, even if the natural alkalinity approaches 200 ppm, is not "over carbonated" or "over-saturated" with carbon dioxide. It is in fact in equilibrium, and the amount of dissolved carbon dioxide is very low compared to the bicarbonate and carbonate species. If the spa water was "over-saturated" with carbon dioxide, you wouldn't need to aerate in order to purge the carbon dioxide, it would happen naturally.

After you strip carbon dioxide, or any other gas present in the atmosphere, from water by vigorous aeration, those gases diffuses back into the water and dissolve according to Henry's Law and Graham's Law. In most cases this is a slow process, though it's faster for carbon dioxide than for many other gases (19 times faster than oxygen, for example). Carbon dioxide continues to diffuse into the water from the atmosphere and to dissolve until equilibrium is reached, and the pH drops.

If you don't believe it, don't take my word for it, try it. Measure your spa's pH, then aerate vigorously for an hour or so. Measure the pH - it will have risen, of course. Then let the spa sit, with no chemical additions and normal water circulation without air injection for 24 hours or so. The pH will have fallen back towards the first value. How could it be any other way, when the alkalinity and temperature remain unchanged?

[chem geek]

Don,

Aeration does not remove gas from water unless the amount of gas in the water relative to the amount in air is higher than its equilibrium amount. Think of what aeration does -- it simply mixes air, which has some carbon dioxide already in it -- with the water so that whichever one has more than it is supposed to for equilibrium moves to the other. If the air has more carbon dioxide than the water, then it moves into the water; if the water has more carbon dioxide than the air, then it moves into the air. Aeration is not a "one-way" forcing from water into the air unless you are using "air" that does not contain carbon dioxide. Now in practice, as you say, most water has a higher TA than the equilibrium amount with air so any aeration will move carbon dioxide out of the water into the air, but the water still has more carbon dioxide in it than the equilibrium amount in air.

So long as aeration isn't at higher pressures or concentrations than normal, then it is simply a physical process increasing surface area between two phase states. There is no "forcing" of equilibrium in one direction or the other; there is only an acceleration of movement in the direction towards creating equilibrium. Stopping aeration doesn't all of a sudden "reverse direction" of the equilibrium.

You cannot completely "strip" a gas from water by aerating IF the air you are using has that same gas in it. If you aerated with pure carbon dioxide gas, for example, then obviously the water would gain carbon dioxide instead of losing it. It wouldn't get stripped of carbon dioxide. Are you claiming that it would? The whole idea that aeration always strips gasses from water doesn't make any sense at all. It completely depends on the content of the air and air has carbon dioxide in it, but an amount that is less than the amount that would exist in equilibrium with water that typically is higher in TA than 10 ppm at a pH of 7.5.

In fact, in aquariums, you aerate the water with air to ADD oxygen to the water, not to strip such oxygen gas from the water. I suggest you think about your stripping argument a little more. Again, aeration does nothing more than increase the surface area between the air/water boundary in order to accelerate the movement of gas between the air and the water -- the net direction of movement is towards getting to equilibrium (Henry's Law). The aeration simply makes this go faster but does NOT change that direction which is solely a function of the concentrations of the gas in the air and in the water (and on temperature, since that changes the equilibrium constant).

Aeration does indeed increase the pH of the water because carbon dioxide is outgassed faster. I said that before. What I am disputing is that if you stop the aeration that the carbon dioxide in the air will go back into the water lowering its pH. That won't happen because there is STILL more carbon dioxide in the water than in the air even after aerating unless you get the TA very, very low (which no one does). Even then, you can't go past equilibrium; at best you end up AT equlibrium. As I wrote before, if you find that the pH drops, it is because the water isn't thoroughly mixed (or as described below, an acidic process occurs such as a drop in FC). Measure the pH near the bottom or middle of the spa away from the surface (i.e. in the bulk water) right after you stop aerating -- don't just measure it near the surface where the pH will be a bit higher.

There are lots of people who have done the TA lowering procedure and they don't all of a sudden experience the pH dropping after they stop aerating. In fact, some people just stop adding acid and aerating when they reach their TA target and don't bother continuing to aerate to raise the pH because it will rise on its own -- it doesn't drop. If the spa has significant FC and it drops (from oxidizing bather waste), then that is an acidic process so the pH will go down slightly, but not by a lot unless the FC drop is larger. For example, a drop in FC of 4 ppm when the TA is around 80 ppm results in a pH drop from 7.5 to 7.36 with no chemical additions.

The equilibrium amount of TA with no CYA and low CH and TDS and at 77F at various pH is 85 at pH 8.5, 25 at pH 8.0, 8 at pH 7.5, 2.5 at pH 7.0. Tap water typically has higher TA than these equilibrium amounts because water sources typically go through some limestone or similar rock that has calcium carbonate that gets dissolved. This is why the water has calcium in it as well. So such water is somewhat in excess of equilibrium of carbon dioxide, but it doesn't get mixed with air enough to get to equilibrium. The movement of carbon dioxide to/from water is relatively slow unless it's quite a ways out of equilibrium or there is quite extensive aeration. This is why in practice a TA of 50-60 ppm (with CYA 30 ppm so carbonate alkalinity it 40-50 ppm) results in a fairly stable pH in a spa with respect to carbon dioxide outgassing; it's not at equilibrium, but it's not so far out-of-equilibrium to have a much faster outgassing and therefore pH rise over time.

I suppose the term "over-carbonated" is misleading in terms of what natural waters contain, but as noted above such water has higher TA because it has had calcium carbonate dissolve in it (which is also why high TA water is typically high in CH as well) and does not get a chance to equilibrate with air. Perhaps I should use a phrase such as "out-of-equlibrium" with respect to air or "has extra carbon dioxide beyond equilibrium" with respect to air. Do you have a suggestion for a better term other than "over-carbonated"?

Richard

[chem geek]

The great majority of carbon dioxide in water exists as the dissolved gas at pH 7.5.

:

With moderately alkaline water having moderately high calcium hardness, it is possible to raise the pH high enough, with aeration only, to precipitate calcium carbonate.

:

If the spa water was "over-saturated" with carbon dioxide, you wouldn't need to aerate in order to purge the carbon dioxide, it would happen naturally.

The first statement is not true if you are talking about ALL carbonate species in the water. It is only true if you are talking about carbonic acid relative to carbon dioxide. At a pH of 7.5 with a TA of 80 (CYA zero so the TA is the same as carbonate alkalinity) at 77F and ignoring ion pairs (such as calcium bicarbonate), 0.17% of the total carbonates (inc. carbon dioxide) is in the form of carbonate ion, 93.61% is in the form of bicarbonate ion, 0.01% is in the form of carbonic acid and 6.21% is in the form of carbon dioxide. So the vast majority of carbon dioxide in the water exists in the form of bicarbonate ion, not as aqueous carbon dioxide. Also, the vast majority of TA is also bicarbonate ion.

It is true that you can raise the pH via aeration to precipitate calcium carbonate, but you mention this in the context of having carbon dioxide removed from the water which is the source of pH rise. I never disputed that and in fact is what I've been saying all along -- that aeration accelerates the process of carbon dioxide outgassing and this causes the pH to rise with no change in TA. What I am disputing that you wrote is that after aeration carbon dioxide starts going (net) back into the water lowering the pH. That would only happen if you aerated using air that didn't have carbon dioxide gas in it (or had a lower concentration of it than normal air).

As for spa water "over-saturated" with carbon dioxide, it DOES naturally outgas carbon dioxide without aeration which is precisely why the pH tends to rise in spas even if their jets aren't turned on. If the TA is higher, then this happens faster. It's only stopped if you seal off the spa to air circulation such that the air above the spa gets more saturated with carbon dioxide and the pH rises in the water (lowering aqueous carbon dioxide concentration) at which point equilibrium is reached. It is precisely this principle which allows for pools having a higher TA when using acidic sources of chlorine such as Trichlor such that with a high enough TA one can get fairly stable pH and have the TA drop over time.

The TA lowering procedure that works by lowering the overall pH and by aeration (along with regular acid addition to keep the pH low), simply ACCELERATES A NATURAL PROCESS (for the carbon dioxide outgassing; obviously acid addition isn't a natural process). It does not force a shift in the direction of an otherwise normal equilibrium. It just makes what is already occurring happen more quickly. In practical terms, as I noted in the post above, one doesn't have to get all the way to equilibrium to get to reasonable pH stability. The process slows down to the point where it is very tolerable and the pH rise from carbon dioxide outgassing becomes less than the "noise" of pH fluctuations and drifts from other sources (i.e. chemical additions, chlorine consumption, chlorine outgassing, etc.).

In a typical spa where Dichlor is used, the pH may drop, not rise, especially if the TA isn't high and this is because the use of Dichlor is a net acidic source of chlorine. It is close to pH neutral upon addition, but the usage/consumption of chlorine is acidic as described in this post. Bleach and other hypochlorite sources of chlorine are net pH neutral. The pH rises upon addition, but then drops as the chlorine gets used up (except for some "excess lye" in some bleaches and chlorinating liquid, but that's fairly small). So the "natural" process of pH rise shows up more when using hypochlorite sources of chlorine. This occurs in pools as well, even when there is no "extra" aeration that would accelerate the process. Lowering the TA reduces the rate of carbon dioxide outgassing and makes the pH more stable in such pools. This has been proven over and over again in hundreds if not thousands of pools (reported on forums) and also works in saltwater chlorine generator (SWG) pools where pH rise is a common problem (though that also has other sources of that rise including undissolved chlorine outgassing).

You can look at the Pool Equations spreadsheet that I wrote, though that has a lot more in it than just the carbonate buffer system.

Richard