Diffusing CO2 to control PH

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.

Diffusing CO2 to control PH

Postby jwdavis » Wed 19 Mar, 2008 19:26

Interested if anyone has any experience with using CO2 to control pH and what potential issues that may arise with TA, etc...





Postby jdlloyd1183 » Fri 16 May, 2008 01:37

Carbon dioxide is the most effective way to control your pH in your swimming pool...new to the makret for residential use , a tank of CO2 is used to pump it into the pool to lower ph, since it forms carbonic acid when injected into the water... Goldline controls makes an automation system (Aqua Plus - sense and dispense kit) that has total pool automation( timers, freeze protect, salt water chlorination) and ORP and pH control. We've installed several where I work and have even retrofitted a few old goldline systems with the new dispense kit. It works well at curbing the sodium hydroxide produced by the salt system that naturally raises the ph of the pool water.. better than your other option... peristaltic acid pump, messy and dangererous, especially if you use tablets and shock. There is a slight decrease in TA with just about every acid you use, especially if you dont evenly distribute your acids when your intention is to lower pH. If the Goldine kit isnt for you, I recommend using sodium bisulfate, since this can be easily broadcast over the pool surface, lowering pH and not TA... united chemicals' Easy Acid is my favorite for lowering pH with little effect on TA. If you need pH up, use united chemicals' Easy pH powder, 2 times as powerful as soda ash, with little effect on TA
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Goldline Sense and Dispense

Postby rcvjmv » Wed 30 Jul, 2008 17:41

I want to upgrade my Aqua Logic PS-8 to use the Sense and Dispense. How difficult is the upgrade kit to install and how well does it perform? I have to use well water to top off my pool so TA is always creeping up. I go through a lot of Muric Acid duing the season and I was looking to automate and use CO2. Anyones thoughts would be appreciated.
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Diffusing CO2 to control PH

Postby Tammy Bond » Sat 29 Aug, 2009 14:00

I have the Goldline Sense and Dispense system. The chlorine is generated using salt so the PH is always high. The Sense and Dispense system is connected to a CO2 tank. When it was first installed last year, a CO2 tank would last about a month to month and a half. I am struggling this summer to get a tank to last 2 weeks. I don't know what is going on. Also, the ORP level is staying too low and going into extend mode and not allowing the automatic timer to shut the filter off. I have had my water tested a local pool store and everything is in balance.
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Diffusing CO2 to control PH

Postby Me... » Sat 29 Aug, 2009 16:40

Tammy, sounds typical. And a reason I choose other methods of automation.

If your water somehow creates a chlorine demand it will of course call for lots of chlorine to be generated. One of the most common scenarios could be the combined chlorine reading climbing because of high bather loads and organics. The salt system can not deal with this by itself unless you have a vastly over sized system. Almost never happens commercially and I doubt ever happens residentially. Combined chlorine is like a magnet to free chlorine and if the free chlorine isn't high enough to oxidize the combined, it simply forms more combined. And your controller of course keeps chugging away trying to create enough chlorine to satisfy its set-point.

You calibrate controllers with the water balanced to a certain level. It was interesting to read one post hear actually stating the salt system raises pH, this is true but most will adamantly stick by the myth they are pH neutral. So if your system is generating chlorine constantly the pH is always rising creating the substantial need to reduce it. If the pH is actually higher than the calibrated set point then the ORP reading will be FALSELY low. This of course makes the controller try to generate even more chlorine, which in turn continues trying to raise the pH.

Vicious circle, similar to to trying to reduce pH while keeping the Alkalinity from dropping through the floor and ion fact this same scenario can get worse exactly that way.

Solution. If you have a good test kit, test your water, A LOT until you yourself can determine if this is in fact happening to you. Make sure the sensor probes are cleaned and calibrated in water that is BALANCED. Might be easiest to just manually add acid to get the pH down and manually shock to get rid of excess organics and cloramines. And try not to let it happen again.

I find situations like that have gone on for months and months before people finally call for help. These are not the forgiving "turn on and forget" systems some people think they are. No automatic system is. It's just a much simpler and easier way to maintain the pools but the systems need to be understood.

Diffusing CO2 to control PH

Postby CO2 » Sat 29 Aug, 2009 18:06

Adding carbon dioxide lowers the pH, but it raises the alkalinity. When carbon dioxide dissolves into the water, it becomes carbonic acid. The carbonic acid is in equilibrium with the bicarbonate. As the carbonic acid increases, the bicarbonate also increases.

This is the reverse of the aeration method of reducing alkalinity where you aerate the water to cause carbon dioxide to outgas.
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Diffusing CO2 to control PH

Postby chem geek » Sun 30 Aug, 2009 14:41

Though you are correct that injecting carbon dioxide gas into water is the reverse of the aeration method, you are incorrect that it changes the Total Alkalinity (TA). Neither aeration nor injection of carbon dioxide changes the TA; aeration raises the pH while injecting carbon dioxide lowers the pH but neither change the TA. The reason is that, as you point out, carbon dioxide becomes carbonic acid in water, but this is a weak acid and not a strong acid. Adding a weak acid increases the TA because TA is a measure of chemical species (ions) that accept a hydrogen ion down to the pH limit of the TA test (around a pH of 4.5) and technically hydroxyl ion counts positively towards this while hydrogen ion counts negatively. When carbonic acid dissociates to produce a hydrogen ion, which lowers pH and counts negatively towards (i.e. lowers) TA, it also produces a bicarbonte ion, which counts positively towards (i.e. raises) TA.

Adding a strong acid, such as Muriatic Acid (31.45% Hydrochloric Acid) lowers both pH and TA. Adding a salt of a weak acid, such as baking soda (sodium bicarbonate) raises the TA and in the case of bicarbonate getting added near pool pH, it has minimal effect on the pH (unless it is added in a way to promote outgassing).

Now over time, one often notices the TA rising slowly in pools whose pH is managed by carbon dioxide injection, but this is due to other sources of rising pH other than carbon dioxide outgassing. For example, chlorinating liquid has some excess lye in it and will cause a slow pH rise over time even if there were no carbon dioxide outgassing and such addition of a strong base increases pH and TA while the carbon dioxide injection just lowers the pH with the net result being a slow rise in TA. Similarly with SWG systems, there may be some net base addition possibly from greater loss of undissolved chlorine gas or aeration of hypochlorous acid in addition to carbon dioxide. The addition of hypochlorite sources of chlorine, including generation of chlorine from SWG systems, raises the pH, but normally the consumption/usage of chlorine lowers the pH back down for no net pH change -- but if the chlorine escapes in some way without getting converted to chloride salt, then there is a net pH increase (i.e. a net base addition).


Diffusing CO2 to control PH

Postby CO2 » Mon 31 Aug, 2009 04:03

CO2 + H2O --> H2CO3
Carbon Dioxide + Water -->Carbonic acid
H2CO3 --> HCO3− + H+
Carbonic acid dissociates into Bicarbonate and Hydrogen ions.

The Bicarbonate raises the Alkalinity and the hydrogen ions lower the pH.
The hydrogen ion does not count against alkalinity. Carbonate Alkalinity is a measure of the amount of carbonate and bicarbonate anions in solution.
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Diffusing CO2 to control PH

Postby chem geek » Mon 31 Aug, 2009 11:02

You are only half-right. Total alkalinity (which is what the TA measures and not carbonate alkalinity alone) does measure bicarbonate ions and twice counts carbonate ions (since they can accept two hydrogen ions), but it also includes the difference between hydroxyl ions and hydrogen ions. It counts any ions that accept a hydrogen ion down to a pH of 4.5, including Cyanuric Acid (CYA) ions.

The Total Alkalinity (TA) test adds acid to the solution until the pH gets to 4.5 at which point the indicator changes from green to red in color. ANY ions that can accept a hydrogen ion (from the acid) will get measured.

Look at the formula for alkalinity here and notice the "[OH-] - [H+]" part of the equation. Any weak acid, such as carbonic acid from carbon dioxide, added to water will lower the pH with no change in TA because the (negatively charged) anion increases TA since it can accept a hydrogen ion but the (positively charge) cation which is hydrogen ion lowers TA since it is a hydrogen ion itself (so is already "on the way" towards the transition point in the TA test). A strong acid lowers both the pH and TA because there is no anion remaining that can accept a hydrogen ion (i.e. chloride ion remains chloride ion and does not combine with hydrogen).

Also, see this link (from Rutgers University) which describes (total) alkalinity as a net increase in weak acid anions and says "Because dissolution of CO2 by itself adds equal concentrations of HCO3- and H+ and does not affect alkalinity".

You said, correctly, that injecting carbon dioxide into the water is the opposite of it outgassing from the water. So what happens when carbon dioxide outgasses from the water? In reality, the pH rises with no change in TA and this is how this procedure is able to work and how the pH rises in pools using hypochlorite sources of chlorine with little change in TA (there is some rise in TA due to "excess lye" in some chlorine products, but it's fairly small).

Let's look at some actual examples of adding carbon dioxide which produces a weak acid (carbonic acid) to lower pH vs. adding a strong acid such as Muriatic Acid. To simplify this example I assume zero Cyanuric Acid (CYA) and Free Chlorine (FC) in the water and ignore ion pairs (e.g. CaCO3o, CaHCO3+). What I show below is what happens starting with a TA of 100 ppm and going from a pH of 7.5 to a pH of 7.0 in terms of each of the carbonate (and some other) chemical species. In the following I am using common units of ppm CaCO3 that is used in measuring Total Alkalinity (TA) so converting from mole/liter to this ppm is a factor of 50,043.5 (1000 times half the molecular weight of calcium carbonate since it counts twice towards alkalinity -- that is, the TA ppm measurement is the equivalent carbonate, not bicarbonate).

pH 7.5
CO2(aq) 5.4699 ppm
H2CO3 0.0084 ppm
HCO3- 99.5215 ppm
CO32- 0.2260 ppm
OH- 0.0279 ppm
H+ 0.0017 ppm
Total Alkalinity (TA) = 99.5215 + 2*0.2260 + 0.0279 - 0.0017 = 99.9997 ppm
Total Carbonate = 5.4699 + 0.0084 + 99.5215 + 0.2260 = 105.2258 ppm

Adding CO2 to Lower pH
pH 7.0
CO2(aq) 17.3369 ppm
H2CO3 0.0267 ppm
HCO3- 99.8518 ppm
CO32- 0.07190 ppm
OH- 0.0088 ppm (not exactly same as H+ due to temperature)
H+ 0.0055 ppm
Total Alkalinity (TA) = 99.8518 + 2* 0.0719 + 0.0088 - 0.0055 = 99.9989 (same as before, ignoring rounding error since molar concentrations had 5 significant digits)
Total Carbonate = 17.3369 + 0.0267 + 99.8518 + 0.07190 = 117.2873 ppm

Adding Muriatic Acid to Lower pH
pH 7.0
CO2(aq) 15.5676 ppm
H2CO3 0.0240 ppm
HCO3- 89.5679 ppm
CO32- 0.0643 ppm
OH- 0.0088 ppm
H+ 0.0055 ppm
Total Alkalinity (TA) = 89.5679 + 2*0.0643 + 0.0088 - 0.0055 = 89.6998 ppm
Total Carbonate = 15.5676 +.0240 + 89.5679 + 0.0643 = 105.2238 (same as before, ignoring rounding error)

So you can see from the above that what happens when carbon dioxide is added to the water is that it mostly stays in the water as aqueous carbon dioxide (dissolved gas) and a very small amount of it directly lowers the pH and this causes some of the carbonate to shift towards bicarbonate. Overall, the TA does not change. Essentially, carbon dioxide addition has two effects that cancel each other out -- 1) it increases carbonates which if the pH didn't change would increase TA (just as adding baking soda increases TA with little change in pH) and 2) it lowers pH which also lowers the TA (just as a strong acid does). These two effects cancel each other out.

When a strong acid is added, both the pH and the TA get lowered because you get a shift from bicarbonate ion to dissolved (aqueous) carbon dioxide. That is, a strong acid just shifts equilibrium and this not only lowers the pH but also lowers the TA. When you add a weak acid, such as carbonic acid from carbon dioxide, the lowering of the TA from the lower pH cancels out the increase in TA from the increased total carbonate.

Conceptually, because the amount of hydrogen ion is so much smaller than the amount of bicarbonate and aqueous carbon dioxide, very little of the added carbonic acid needs to dissociate and the resulting lower pH prevents any further dissociation hence the pH is lowered while the TA is not. With a strong acid, the dissociation is complete so affects the equilibrium reactions so that the TA is lowered along with the pH.

I'm not sure I can think of any other way of phrasing what is going on.


Diffusing CO2 to control PH

Postby CO2 » Mon 31 Aug, 2009 17:15

"Carbon Dioxide Gas - Principle residual - Bicarbonates
Can sometimes cause an increased Total Alkalinity level."

"While lowering pH, CO2 also raises total alkalinity since it forms a bicarbonate salt. Because CO2 increases total alkalinity of the water, it should not be used in pools where high alkalinity is a problem,"
http://www.pested.msu.edu/Resources/bul ... 1chap6.pdf

"Carbon dioxide at one time was thought to be a simple solution to control pH safely. However, you should be aware of what else is happening. Carbon dioxide is a weak acid and is short lived. While it is lowering pH slightly, it may also be increasing the total alkalinity into an unacceptably high range. If this occurs a great deal of time is spent lowering the total alkalinity with the same amount of acid that would have been required to simply lower the pH in the first place."

"CO2 generates a natural bicarbonate buffer that makes the water more stable and prevents the pH from dropping below 6.8."

It seems that your theoretical calculations are asynchronous with the real world results.

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