Diffusing CO2 to control PH

[chem geek]

Remember what I said in my original post replying to yours: "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."

I didn't say that there couldn't be TA rise, just that the carbon dioxide injection wasn't the direct source of it. If the primary cause of pH rise was carbon dioxide outgassing, then adding carbon dioxide would be the exact opposite so where would the increasing TA over time come from? It's not from the carbon dioxide. Regardless of all the chemistry that I referred to, if you add something to the water and then remove it (or vice versa), it is back to where it started. Carbon dioxide injection for lowering the pH makes sense to do IF the reason for rising pH is carbon dioxide outgassing in the first place. If the source of pH rise is from other sources that are essentially strong bases, then the net result WILL be a rise in TA. I never disputed that. The bottom line is that it is best to balance carbon dioxide outgassing with carbon dioxide injection and best to balance pH rise from a strong base by adding a strong acid. Actually, if one has carbon dioxide outgassing, it is even better to just lower the TA level since that will reduce the rate of such outgassing in the first place.

If part of the pH rise is due to curing of plaster, for example, or from "excess lye" in hypochlorite sources of chlorine or from side reactions or outgassing of chlorine from SWG systems, then using carbon dioxide for pH control results in a TA rise from these "strong base" sources. It's the opposite of the "strong acid" example I gave that lowered both pH and TA. With a "strong base", both pH and TA are raised so if you lower the pH with carbon dioxide, then the net result is a rise in TA.

The point is that the part about carbon dioxide directly adding to bicarbonate and thereby increasing the TA is false. It's not that the TA can't rise from strong bases, but rather that the source of the rise isn't the carbon dioxide injection directly. The first link you gave says "Can sometimes cause an increased Total Alkalinity level." which is consistent with what I've said above. It doesn't say it always causes an increase though it would be better if they said something like "can sometimes have a net result of an increased Total Alkalinity level" since the carbon dioxide itself isn't a direct cause. The second link says "While lowering pH, CO2 also raises total alkalinity since it forms a bicarbonate salt." is incorrect. The third link says "While it is lowering pH slightly, it may also be increasing the total alkalinity into an unacceptably high range." Again the key word here is "may" since it won't always happen since it depends on why the pH was rising in the first place. I didn't see in the fourth link any reference to a rising TA and the part about a bicarbonate buffer is already there -- adding carbon dioxide does not lower that buffer, unlike adding a strong acid. The pH, in fact, could go below 6.8 if outgassing of carbon dioxide were limited (such as with a pool cover) -- after all, soda water has a pH as low as 2.5 or so (see this link), but that's done under pressure. There is no magic low pH number of 6.8, but it does get harder to lower the pH since the outgassing of carbon dioxide is faster at lower pH (and higher TA).

Next time you make soda water by injecting carbon dioxide into water, measure the TA before and after. You will find that the TA did not increase, but that the pH did get lowered.

[CO2]

Well, I'm still not sure, but you seem to be pretty knowledgeable about these things so I will have to do a little more investigating. There seems to be quite a lot of misinformation in the pool chemistry industry, even from the so-called experts. Thank you for providing such a comprehensive and logical explanation, that helps a lot.

[smcrea]

I'm very interested in using CO2 to control my Ph. I have a Pentair System with Intelichlor chlorine generator, and successfully use acid and baking soda to keep my Ph and TA spot on. (I normally have a scaling index of hovering around +0.2.

I have read in this post about the Goldline Pro Logic and AQL-CHEM sensing kit and how it wires into a Pro Logic controller.

I'm guessing that this kit requires that you have a complete Hayward system with a Hayward controller. Is this true?

Is there a way that I can employ this system on my Pentair controller?

What is the overall cost of the system?

Is there a way to 'simplify' the system by having the injection of C02 on a timer? rather than a closed loop Ph monitoring system? I've found that my Ph is as reliable as the phases of the moon so long as I keep my chlorine generator on the same % and run times. I'm sure that I could also determine the right amount of C02 to inject! (6fl oz of 30% each and every day)

I've also looked at the poolsmith technologies unit.. does anyone have any recommendations on that one?.. it looks very expensive at $1300. Not sure that I want to spend that kind of money.

Any other units on the market worth discussing?

Thanks for your help!

Steve.

[smcrea]

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. 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.

Richard

Richard,

Phewwww I'm an electrical Engineer not a chemist, but you certainly convinced me that you know what you are talking about. I know enough about my pool water chemistry to keep my SI in good shape and that's it!

For a novice like me, can you tell me if I should get a CO2 system or not?. It seems that you are saying that the CO2 will not raise TA.

I read in one of the reports that were linked further down in the discussion that you end up adding as much acid to keep TA in check as you would to keep Ph down. I have no problem with slugging acid once a month to get my TA down, but I want to stop this daily pilgrimage of dumping 7fl oz of acid (diluted) into my 10K pool every morning.

BTW. I have a 10K gallon pool. I keep my Ph at 7.5 with the 7fl oz of acid each day. I get a lowering of TA of about 10ppm every 7 to 10 days. Hence add about 1.5 pounds of baking soda every 7 to 10 days. I try to keep my TA around 90ppm as CH is around 320ppm. At my 88 degrees this keeps my SI around +0.2

My main concern is when I go away for a week or so.. my Ph goes off the chart, so the automation of CO2 is an attractive alternative. But the $1,300 for a PoolSmith unit is a big investment http://www.poolsmith.com

Bottom line is: You seem to really know what you are talking about. I respect your expert opinion: Should I get a Co2 unit or not?

Thanks,

Steve.

[mas985]

I have a SWG system and was constantly fighting PH rise but I decided that muriatic acid was much more economical than CO2 gas. Also, if you live in a region like I do that has a lot of evaporation and have to rely upon fill water with high TA, you will need acid anyway to keep the TA in check. I ended up building my own cost effective acid dosing system that I have been using for over two years now.

[chem geek]

Steve,

Don't try and raise the TA to as high as you are doing. If it's lower, the rate of pH rise should go down. You can compensate for the saturation index by increasing the calcium hardness (CH) and/or having a higher pH target (say, 7.7). I also suggest you considering using 50 ppm Borates in the pool as an additional pH buffer, especially if you let the TA get rather low, down to 60 ppm or so. If there are sources of aeration of water in your pool, such as waterfalls, spillovers, fountains, etc. I'd consider having them turned off longer (if possible). Also, use of a pool cover should significantly reduce the rate of pH rise and also save on water (from evaporation) and on rising TA and CH (from evaporation and refill).

My point about CO2 was that it was not a direct cause of TA increasing, but there are other sources of TA increasing such as evaporation and refill so overall you may find that using only CO2 for pH control does result in a slow increase in TA over time.

I can't really recommend one thing or another since it's a personal choice and there are pros/cons with each. Personally, I'd try the techniques that reduce the rate of pH rise first, then see how much acid you need to add.

Read more about how to maintain your pool at the Pool School.

Richard

[Plutonium]

In the typical case of someone using carbon dioxide to lower their pH from 8.0 to 7.6, the pH is lowered primarily by the removal of the excess hydroxide ions.

CO2 + OH --> HCO3

Therefore, as you can see, the pH is lowered but the bicarbonate alkalinity is raised.

[Plutonium]

The carbon dioxide also raises pH by removing carbonates. The carbon dioxide combines with the carbonates in the water like this:

CO2 + CO3 + H2O --> 2 (HCO3)

Therefore, as it should be apparent, adding carbon dioxide to reduce pH does increase bicarbonate alkalinity in several ways.

[chem geek]

You are forgetting that the measure of Total Alkalinity (TA) includes hydroxyl ion so your equation

CO2 + OH- --> HCO3-

takes away hydroxyl ion thus reducing alkalinity while increasing bicarbonate ion thus increasing alkalinity the same amount with a net result of no change in alkalinity.

In your equation

CO2 + CO32- + H2O --> 2HCO3-

this takes away a carbonate ion which counts twice towards alkalinity since it can accept two hydrogen ions while increasing bicarbonate ion twice which exactly cancels out the change so there is no net change in alkalinity.

[Plutonium]

OK, I can agree that you are correct about this equation:

CO2 + CO32- --> 2HCO3-

As far as the hydroxyl ion, I thought that Total Alkalinity was a measure of the buffering ability of the water. I thought that it was the ability of the water to accept acid without changing pH. If you reduce the hydroxyl ion, you increase the pOH and you decrease the pH by an equal amount. pOH + pH = 14. Is this incorrect?

[Plutonium]

OK, based on titrating with a strong acid down to a pH of 4.2, then I will agree that the hydroxyl ion will count toward the total alkalinity and you are correct on both counts.

I had considered Total Alkalinity to be based on any movement of the pH.

[chem geek]

Yes, that's exactly how the TA test works -- titrating with a strong acid down to a pH of around 4.5 when the indicator dye changes color. The indicator dye is typically a combination of methyl red and bromocresol green that goes from green to red and stops changing at around a pH of 4.5.

However, it's not just the specifics of the test but rather the technical definition of TA itself. Though I don't always like Wikipedia since it is sometimes inaccurate, it currently does describe the definition reasonably well here. You can see that the definition includes the terms [OH-]-[H+] since hydroxyl ion can (weakly) buffer against acid addition while hydrogen ion itself is the opposite and represents negative alkalinity. The difference between these is the pH buffering capacity of pure water which is zero at a pH of 7.0.

Technically, in pure water, the TA test is off since one drop of titrant in the test would change the sample from green to red and be recorded as 10 ppm though further analysis would show that an extremely small amount of titrant would do the same so in practice the TA is near zero in the test. If one adds TA indicator dye to a water sample and it immediately turns red, then this means that the pH is at or below 4.5 and the TA is zero (technically, it's negative).

[Plutonium]

OK, some references use a double endpoint titration method where the sample is titrated to a pH of 4.5 and then to 4.2 for extra accuracy. That's where I got the pH of 4.2 as shown in these references.

Total alkalinity is measured by measuring the amount of acid (e.g., sulfuric acid) needed to bring the sample to a pH of 4.2.

At pH 4.5, it is certain that all carbonate and bicarbonate are converted to carbonic acid. Below this pH, the water is unable to neutralize the sulfuric acid and there is a linear relationship between the amount of sulfuric acid added to the sample and the change in the pH of the sample. So, additional sulfuric acid is added to the sample to reduce the pH of 4.5 by exactly 0.3 pH units (which corresponds to an exact doubling of the pH) to a pH of 4.2.

http://www.epa.gov/volunteer/stream/vms510.html

A known volume of the sample aliquot is titrated with a standardized solution of H2SO4 (or HCl), to pH=4.5 then to pH=4.2, using an automatic titrator and a pH meter calibrated for 25oC. The total alkalinity is found from both titration volumes. A two endpoint technique is employed to determine the actual inflection point.
http://www.ungiwg.org/openwater/?q=book/export/html/113

Total alkalinity is measured by collecting a water sample, and measuring the amount of acid needed to bring the sample to a pH of 4.2.
http://www.umass.edu/tei/mwwp/phalk.html

I'm assuming that the standard pool test kit probably uses a pH of about 4.5 or just slightly below to ensure accuracy.

[Plutonium]

The Wikipedia reference that you give contains this:

Dissolution of carbonate rock
Addition of CO2 to a solution in contact with a solid can affect the alkalinity, especially for carbonate minerals in contact with groundwater or seawater . The dissolution (or precipitation) of carbonate rock has a strong influence on the alkalinity. This is because carbonate rock is composed of CaCO3 and its dissociation will add Ca+2 and CO3−2 into solution. Ca+2 will not influence alkalinity, but CO3−2 will increase alkalinity by 2 units.

This could explain some of the increase in alkalinity reported by some pool operators. If some of the calcium carbonate or calcium hydroxide in the plaster is dissolved, then the total alkalinity could increase.

[chem geek]

Thanks for the links and explaining where the 4.2 came from. The standard TA tests in pool kits use a single transition point at 4.5 and are only accurate to within one drop (i.e. 10 ppm). That's more than good enough for the purposes of calculating the saturation index.

If the rise in TA is from the dissolving of plaster (which would also raise the pH) then that's not a good thing, of course. The saturation index should be kept close to zero and adding carbon dioxide to the pool should not be done with the carbon dioxide getting too close to plaster surfaces. This would be similar to adding acid near such surfaces.

Fill water almost always has a reasonably high amount of TA in it -- usually at least 50 ppm. My tap water has close to 80 ppm. If there is evaporation and refill, then whatever is in the fill water gets added to the pool (evaporation does not remove anything but water from the pool) so generally that causes an increase in both TA and CH, though this can be a slow rise. My own pool tends to slowly have the TA rise over a season and this is probably the reason -- it's that slow because I have a pool cover so the pool is only open for 1-2 hours every weekday plus longer on weekends. My tap water has a CH of 50 ppm so the rise in CH is not as noticeable and splash-out/refill (I have a cartridge filter, so no backwashing) would tend to lower the CH so the net is pretty constant.

You can see annual pan evaporation rates here where 50" (more than 4 feet) is not uncommon. The average depth of a typical 6 foot deep pool is around 4.5 feet so that's around 90% of the pool volume being added with fill water so roughly increasing the pool's TA by that of the fill water. Even indoor pools evaporate quite a lot of water since this has a lot to do with warm water and drier air.

The use of carbon dioxide for lowering the pH would make any sources of rising TA more noticeable. When using a strong acid to lower the pH, the TA would get lowered as well, usually faster than such sources of rising pH so that baking soda needs to be added on occasion to raise the TA.

Richard