Ponderings on SWCG pH chemistry and TA

[dschlic1]

The Hayward/Goldline automatic pH control, Sense and Dispense, recommends using CO2 to lower the pH. I have several questions and points to ponder on using CO2 to correct pH in pools.

1. Won't the addition of CO2 raise the TA? After all to lower the TA we are advised to aerate the water to remove CO2.

2. My pool maybe an exception, but I am adding about 16 oz of muriatic acid per week to maintain pH stable. I am using an acid pump that runs for a set time each day. I have adjusted the feed rate of the pump to the point where my pH changes less than 0.1 units per week.

3. I have been running this setup for about 6 months, and during that six months, my TA has changed from 100 to 90 ppm. This seems to indicate that my pH change is due to processes other than the loss of CO2 in the water.

It would seem that the use of CO2 to correct pH changes, over a long period of time, would result in a continuous rise in the TA.

 

[Aquaman95]

I can't explain the chemistry behind it...perhaps chemgeek will be along, but I can tell you from practical experience that it seems highly dependent on how the C02 is introduced. If it is injected with little or ineffective diffusion the alkalinity increase is greater. The theory is that the larger the bubble the greater the alkalinity increase. In the early days many systems just injected into the return through a standard gas or chemical injection fitting. Then venturi injectors were used frequently. Now, most systems use a good quality sintered stainless diffuser of less than 10 microns with an offline contact chamber.

I have no idea what Hayward is using in the Sense & Dispense, but knowing that it is targeted at the residential market I bet it's cheap. Probably along the lines of an aquarium stone.

I can tell you that I've worked with these systems for many years on commercial pools and I even installed one on my home pool for a couple years. They work, but frankly I don't see a practical advantage other than eliminating the "fear factor" of handling/storing acid. C02 is marketed as the safe alternative, but in some scenarios it may actually be more dangerous (storage & transport of a compressed gas cylinder, gas heavier than air that will displace oxygen in a poorly ventilated pump room).

There is certainly no cost advantage.

 

[chem geek]

I'm going to tell you the theory and what may be happening in your situation. I don't have an explanation for any TA change from CO₂ injection or the way it is injected.

Removing CO₂ by aeration or adding it by injection does not change the TA. The technical reason for this is that removing or adding CO₂ removes or adds carbonic acid, H₂CO₃ and this is composed of two pieces that cancel each other out in terms of TA. The CO₃^2- is doubly charged and can accept two hydrogen ions (protons) so counts double adding to TA, but the two hydrogen ions, 2H⁺, each subtract from TA. Another way to look at this is that there are two things going on with CO₂. One is a change in total carbonates while the other is a change in pH. The changing carbonates changes TA in one direction while the changing pH changes TA in the other direction and these cancel each other out.

The removal of CO₂ by aeration does have the pH rise and likewise injecting CO₂ into the water lowers the pH -- that is, if nothing else is going on.

Your TA should be dropping in your pool not because of the aeration of CO₂, but due to the addition of acid. In a 7500 gallon pool, adding 16 ounces of Muriatic Acid (31.45% Hydrochloric Acid) would lower the TA by 8.3 ppm if there were no base being added elsewhere. You should need to be adding baking soda over time in order to maintain the TA level. If you haven't been doing that and have only seen the TA go from 100 to 90 ppm over 6 months, then you are right that there must be something very basic getting added to the pool. If the SWCG were generating chlorine gas that was escaping, then that could account for what you are seeing. Specifically, if I assume the production of 10 ppm FC from your SWCG where 4 ppm becomes chlorine in your pool but 6 ppm escapes as chlorine gas (i.e. doesn't get dissolved in the pool water), then that results in almost perfectly stable pH and TA with your acid addition.

By any chance is the amount of piping from your SWCG to your pool fairly short? Are your returns pointed slightly upward?

In your situation, if you were to use CO₂ injection instead of acid, then you would see the TA rise, but technically this is due to the chlorine gas outgassing since the net result from the SWCG is basic and the acidity of chlorine gas dissolving and chlorine consumption is what normally compensates for that. If the chlorine gas doesn't dissolve in the water and instead escapes, then the SWCG would be raising the pH and TA. The CO₂ injection would lower the pH so the net pH change would be zero, but the net TA would rise. Now it's true that the injection of CO₂ can be seen as increasing the total carbonate in the water, but TA and total carbonate are not the same thing. I know this is confusing. I guess the easiest way to look at it is that the amount of total carbonate in the water IS related to TA when comparing two waters at the same pH. If the pH is different, then that has an effect on TA by itself that needs to be taken into account.

Richard

 

[dschlic1]

It was my assumption (you know what happens when your assume something) that the pH is held constant in all of the senarios. After all that is the purpose of the pH control systems.

As far as my pool, it has a very low chlorine demand. It is on the order of 1 to 2 ppm per day. Even in the heat of summer with the solar covers off 24/7, my chlorine demand never exceeded 3 ppm per day. I am currently running the filter 5 hours which gives 3 chlorination cycles. Chlorinator is set at 8%.

 

[chem geek]

My example of 10 ppm FC generated was a per-week number so would be roughly consistent with your low demand.

What is the rough length of plumbing from your SWCG into your pool? Is it very nearby or is it 50+ feet away? I'm just figuring that your main source of pH rise is chlorine gas escaping that doesn't get dissolved into the pool water. When the SWCG is on, do you smell any chlorine above the return, more so than when the SWCG is off? It sounds like you've got a pool cover on now which I would think makes it less likely for gas to escape. A mystery...

Richard

 

[chem geek]

Another possibility (that I often forget about) is that your PebbleTek may be new -- less than a year old. New plaster or similar material will cure over time and this process raises the pH, especially in the first few months. Is that the case with your pool? You talk about this here and also mention how lowering your TA did lower your acid demand, but clearly you've still got some and it doesn't appear that it's due very much to outgassing of carbon dioxide since your TA level is staying fairly stable (assuming you aren't increasing TA via chemicals such as baking soda).

The chart below shows the net effect of various combinations of products, assuming that the pH is made to be stable over time. It shows the effect on TA.

............................................... pH ..... TA
Trichlor or Dichlor .................. - ....... -
CO₂ outgassing ...................... + ....... 0
----------------------------------
Net Effect ................................ 0 ....... -
shows that a higher TA level is useful when using acidic sources of chlorine to keep pH stable

Hypochlorite chlorine .............. 0 ....... 0
CO₂ outgassing ...................... + ....... 0
Acid addition ........................... - ....... -
----------------------------------
Net Effect ................................ 0 ....... -
shows that for hypochlorite sources of chlorine if one lowers the TA level to reduce CO₂ outgassing, then one can lower acid addition

SWG chlorine ........................... 0 ....... 0
CO₂ outgassing ...................... + ....... 0
Acid addition ........................... - ....... -
----------------------------------
Net Effect ................................ 0 ....... -
shows that for an SWG if one lowers the TA level to reduce CO₂ outgassing, then one can lower acid addition

SWG chlorine ........................... 0 ....... 0
Plaster Curing ......................... + ....... +
Acid addition ........................... - ....... -
----------------------------------
Net Effect ................................ 0 ....... 0
shows that plaster curing requires acid addition (or very acidic sources of chlorine or a combination); TA is stable

SWG chlorine ........................... 0 ....... 0
Chlorine outgassing ................ + ....... +
Acid addition ........................... - ....... -
----------------------------------
Net Effect ................................ 0 ....... 0
shows that chlorine outgassing (pretty unique to SWG pools and to a lesser extent in spas) is like plaster curing requiring acid addition; TA is stable

The general rule is that if you are in a situation regularly adding acid where you have net stable pH but the TA is dropping, then lowering the TA level should help lower the amount of acid required since the lowering of TA over time indicates CO₂ outgassing. On the other hand, if the TA is stable over time in spite of acid being added regularly, this means that the source of pH rise requiring acid addition is a pure base so lowering the TA won't help. [EDIT] As noted in my post below, this rule isn't true if there is some source of increasing TA such as from evaporation and refill. [END-EDIT]

Richard

 

[dschlic1]

Plaster is about 14 months old. It was installed in October of 2007.

At this point I am not too worried about the amount of acid I am adding. My numbers are very stable, and the water is great! If we don't get heavy rains this summer, I will probably do a partial drain and refill to get my CH down. As for now, with a pH of 7.6 my CSI is 0.

 

[mas985]

Don't forget about fill water.

High TA fill water keeps my TA above 90 ppm even though I am adding quite a bit of acid each week. Given that you also have a high CH level makes me think that your city may be using well water like mine which comes with both high CH and high TA.

 

[chem geek]

More good points. Thanks, Mark. It would be a good idea to measure the TA and CH of the fill water since whatever is in that will get added to the pool whenever water evaporates from the pool. I even have the TA slowly rise in my own pool due to the TA being 80 ppm in the fill water though it's a very slow rise due to my use of a pool cover.

This means that my rule-of-thumb about TA being stable implying that lowering the TA wouldn't help isn't exactly true if the reason the TA is stable is that additional TA is being added, say, by fill water. In that case, lowering the TA level could still help reduce the rate of pH rise. I know that you're happy with your current situation and that's fine -- I just want to try and get some basic principles nailed down for others.

The weekly amount of acid should have been lowering the TA by 8.3 ppm per week so at 100 ppm TA in 7500 gallons it would take an evaporation and refill with 200 ppm TA fill water of 311 gallons which with 22' x 15' would be 1.5" which is certainly plausible as a weekly water loss in Florida (see this map for 60" per year annual evaporation in Florida -- about 1.2" per week). By the way, 7500 gallons with 22' x 15' implies an average pool depth of 3' which is shallower than most pools -- is that really your situation?

Richard

 

[dschlic1]

It has been many months since I filled the pool. With solar covers on, I have very little loss from evaporation. My pool is "L" shaped, outside dimensions are 15 x 22. The legs of the "L" are about 7.5 feet wide. Total pool area is 242 sq ft. Average depth is about 4.5 ft. My fill water has very high CH, around 200 ppm, so I am trying to avoid filling the pool :-D

 

[cliff_s]

I have used a CO2 system for a couple of years now and here are my thoughts.

Does it control pH, yes. I have noticed that when CO2 is used the lowering pH effect
is not as long lasting as using acid. At best the effect is short lived. You must add it several
times a day to achieve any sort of stability in the pH levels. I add the CO2 into my floor
cleaning system so it gets spread around the pool and introduced to the bottom of the
pool. This gives the maximum amount of time for the CO2 to be in contact with the
water. Since the CO2 is in the form of small bubbles they soon rise to the surface
of the water and are gone.

The cost of using CO2 is higher than using acid. A 20lb bottle will last about
a month. The amount used is directly proportional to pool water temperature.
This is also the case with Chlorine consumption.

I did notice that the TA does rise with time requiring additional acid addition.
CO2 does not eliminate the use of acid for TA control.

I have come to the conclusion that using a peristaltic adjustable pump to
add diluted acid is a better system. The reason is TA control. By carefully adjusting
your acid addition you can achieve reasonable pH and TA control with one system.
Using a 15 gallon plastic drum and diluting the acid 3 to 1 the drum will last over 10+
weeks without any refill.

I am fortunate in that I can control the addition of either CO2 or acid with my home automation.
This way I can have very fine control of the amounts added and I can have a
schedule that changes throughout the year. It does take a while to determine the length
of the SWG run and the amount to CO2 or acid added. Since the pool temperature changes
rather slowly over the months this is not hard to determine. Another advantage is the you
don't need any special measuring system to control pH and Chlorine addition. With proper timing
you can achieve stable parameters throughout the year. Running the filter pump and SWG while the pool
is in use compensates for added chlorine consumption cause by bathers during the summer.

On my former pool I used the filter run time to also control the acid addition, since this pool didn't have
in floor cleaning I could adjust the filter timing for the time of the year and also adjust the acid addition at the
same time. With a in floor cleaning system this won't work because of the additional pump run time needed for pool
cleaning. I have found that you need to run the in floor cleaning system about the same length of time for
the entire year to keep the pool clean, so the acid pump needs to be adjusted separately.

Using the above systems you can reduce your pool maintenance to a minimum.

I also might note, I live in Arizona and don't cover my pool in the winter(I think covers are ugly).

Cliffs

 

[Guest]

I did notice that the TA does rise with time requiring additional acid addition.
CO2 does not eliminate the use of acid for TA control.

This effect is seen in almost all systems that use CO2 for pH control and I have commented on it before in this and other forums.

 

[chem geek]

I just don't know where this effect comes from -- the rising TA when using CO₂ -- unless there is evaporation and refill with water that has TA. That seems to be the primary source of rising TA in my own pool over a season.

 

[Guest]

I just don't know where this effect comes from -- the rising TA when using CO₂ -- unless there is evaporation and refill with water that has TA. That seems to be the primary source of rising TA in my own pool over a season.

Equilibrium shift. As you increase the 'carbonic acid' of the water by injecting CO2 would not the equilibrium start to shift to increase the bicarbonate ions present to maintain equilibrium? I would think that, even though the effect is slight, it would be noticeable over time. Adding CO2 is adding carbonation to the system which, as we know, will exist as the bicarbonate/carbonic acid buffer system at normal pool pH.

A similar effect is seen in planted aquariums that use CO2 injection, btw.

 

[chem geek]

Equalibrium shift. As you increase the 'carbonic acid' of the water by injecting CO2 would not the equalibrium start to shift to increase the bicarbonate ions present to maintain equalibrium? I would think that, even though the effect is slight it would be noticable over time. Adding CO2 is adding carbonation to the system which, as we know, will exist as the bicarbonate/carbonic acid buffer system at normal pool pH.

The problem is that this is the exact opposite of what happens when CO₂ outgasses which causes the pH to rise with no change in TA. So adding CO₂ should likewise just change the pH without changing the TA.

You are correct that adding CO₂ shifts the equilibrium increasing total carbonates in the water and creating more bicarbonate, but it also creates more hydrogen ion which is why the pH drops and these exactly cancel each other out in terms of the TA measurement. TA doesn't just measure bicarbonate (and the small amount of carbonate), but is also influenced by the starting pH. When the starting pH is lower, the TA is lower because it's closer to the end-point of the titration. Basically, when the pH is lower, you've already done the equivalent of adding acid to have a lower TA level while the increase in bicarbonate is equivalent to increasing the TA level and these two exactly cancel each other out.

CO₂ + H₂O ---> H₂CO₃
Carbon Dioxide + Water ---> Carbonic Acid
No change in TA because none of these species contribute to TA

H₂CO₃ ---> H⁺ + HCO₃^-
Carbonic Acid ---> Hydrogen Ion + Bicarbonate Ion
No change in TA because Hydrogen Ion decreases TA while Bicarbonate Ion increases TA

HCO₃^- ---> H⁺ + CO₃^2-
Bicarbonate Ion ---> Hydrogen Ion + Carbonate Ion
No change in TA because Hydrogen Ion decreases TA and the loss of Bicarbonate Ion also decreases TA while Carbonate Ion increases TA twice (because it can accept two hydrogen ions)

Richard

 

[Guest]

How about the CHANGE of pH and it's effect on the EXISTING buffer system because of the CO2 injection. You are not keeping the system static and if you have a starting ph of, say, 8.0 and lower it to 7.4 by injecting CO2 this will change things a bit, will it not? You cannot introduce something into a system and expect it to remain at the same equalibrium. If an observed effect cannot be explained by the chemical equations you are using then your model is too simplistic to accurately explain a real life situation.
(Richard, we haven't had a discussion like this in a very long time! I don't think we've ever had one on this forum before! )

 

[chem geek]

That's the whole point. When you inject CO₂, you are changing two things at the same time. You are increasing the amount of carbonates in the system including bicarbonate which increases TA AND you are decreasing the pH which lowers the TA (similar to adding acid) because of the equilibrium shift that lowers the amount of bicarbonate. These two TA effects exactly cancel each other out (accounting for hydrogen ion as well), just as they do when CO₂ outgasses though in that case the pH rises with no change in TA. If you were adding a pure acid, then the lowering of pH would shift the equilibrium to have less bicarbonate and therefore a lower TA, but when you add CO₂ you are doing the same thing as acid in terms of the equilibrium shift due to lower pH AND you are increasing carbonates.

Basically, the amount of bicarbonate in the system isn't changing very much at all. The lower pH has the bicarbonate level decrease (which is what happens when you add a pure acid which lowers both pH and TA) but the increase in total carbonates has the bicarbonate level increase and these two mostly cancel out. The small difference is canceled out by the difference in hydrogen ion concentration. The TA does not change. Again, it is exactly the opposite of what occurs when CO₂ outgasses and we already see that the TA doesn't change in that case.

Richard

 

[JasonLion]

If CO2 outgassing is causing your PH to rise, then it makes sense that CO2 injection would exactly cancel that out. But what if some other effect is causing a small portion of the PH increase? For example, plaster curing would raise both PH and TA, and the CO2 addition would only lower the PH and not the TA. Plaster doesn't fully cure for decades, though the rate of curing gets very very slow after the first year or two.

I wouldn't be surprised if there were some additional processes with similar effects that are going on slowly in the background. CYA breaks down very slowly, a variety of things get left over after the various organics that fall into the pool are broken down by chlorine, there is sometimes excess lye in bleach, and on and on. Several of those can be ruled out, but some of them have rather complex pathways that are difficult to follow to a conclusion.

 

[chem geek]

Your absolutely right Jason. If there are other reasons for the rise in pH other than CO₂ outgassing, then the net result of balancing pH with that method would be a rise in pH. Plaster curing is one example, though any addition of a pure base would cause that effect.

I think I need to explain the CO₂ effect, be it outgassing or injection, a little more since it is confusing and keeps coming up. The following is the main equation of pH buffering in the pool since the the majority species are dissolved carbon dioxide and bicarbonate ion.

CO₂(aq) + H₂O <---> H⁺ + HCO₃^-
Carbon Dioxide + Water ---> Hydrogen Ion + Bicarbonate Ion

[H⁺] * [HCO₃^-] / [CO₂(aq)] = 10^-6.3

taking log10 of both sides one has

log₁₀([H⁺]) + log₁₀([HCO₃^-]) - log₁₀([CO₂(aq)]) = -6.3

and since pH = -log₁₀([H⁺]) and combining some of the above we have

log₁₀([HCO₃^-]/[CO₂(aq)]) = pH - 6.3

The hydrogen ion concentration is much smaller than the bicarbonate and aqueous carbon dioxide concentrations. The following table shows the differences at three pH's all with the same total carbonate in the water (I ignore ion pairs to simplify the discussion) so reflects what happens when one adds a strong acid (moving lower in pH) or a strong base (moving higher in pH).

pH ......... [H⁺] ......... [HCO₃^-] ....... [CO₂(aq)] ... Total Carbonate
6.5 ..... 3.5x10^-7 ..... 1.3x10^-3 ..... 8.3x10^-4 ..... 2.1x10^-3
7.5 ..... 3.5x10^-8 ..... 2.0x10^-3 ..... 1.3x10^-4 ..... 2.1x10^-3
8.5 ..... 3.5x10^-9 ..... 2.1x10^-3 ..... 1.3x10^-5 ..... 2.1x10^-3

When one adds a pure (strong) acid, one is adding hydrogen ions which lowers the pH which lowers the ratio on the left-hand side ratio so bicarbonate goes down and therefore the TA drops. When the acid is added, most of the hydrogen ions are absorbed by the bicarbonate ion to form more aqueous carbon dioxide. So the bicarbonate drops a lot which is why the TA drops. Though the hydrogen ion leftover does increase, it is small in absolute amount compared to the change in bicarbonate.

On the other hand, when one adds carbon dioxide, one is increasing total carbonates. The increase in aqueous carbon dioxide does shift a very small amount to produce more bicarbonate, but this also produces more hydrogen ion and these cancel each other out with respect to TA. Even if one ignores the hydrogen ion change, the change in bicarbonate is very small in absolute terms. This is because the ratio is mostly affected by the amount of aqueous carbon dioxide (i.e. changing a small denominator has a large effect on the ratio).

pH ......... [H⁺] ......... [HCO₃^-] ....... [CO₂(aq)] ... Total Carbonate
6.5 ..... 3.5x10^-7 ..... 2.0x10^-3 ..... 1.3x10^-3 ..... 3.3x10^-3
7.5 ..... 3.5x10^-8 ..... 2.0x10^-3 ..... 1.3x10^-4 ..... 2.1x10^-3
8.5 ..... 3.5x10^-9 ..... 1.9x10^-3 ..... 1.3x10^-5 ..... 2.0x10^-3

The small error in the last line above is from [CO₃^2-] that is 3.4x10^-5.

You can see above how the bicarbonate amount remains essentially unchanged while it is the amount of aqueous carbon dioxide that changes the most (and carbonate ion, when the pH is higher). It also shows that it takes a lot of carbon dioxide addition to lower the pH and far less outgassing of it to raise the pH (when starting at a pH of 7.5).

The bottom line is that the change in TA is not due to addition or outgassing of carbon dioxide, but to other factors (evaporation and refill with TA in fill water, addition of a strong base such as calcium hydroxide from plaster curing, etc.).

Richard

 

[Guest]

If an observed effect cannot be explained by the chemical equations you are using then your model is too simplistic to accurately explain a real life situation.

Which is why I made this statement earlier! The bottom line is this, the use of CO2 for pH control does not eliminate the need for acid for TA management in the majority of installations whatever the reason for the TA increase. Therefore the use of peristaltic acid pumps seems a more cost effect solution in the long run since there is only a need for one chemical instead of two. As far as safety, it's really a toss up as to which is more dangerous, CO2 or acid (and many commercial pools use sulfuric (H2SO4) instead of muriatic (HCl) acid since it does not fume. I know that both Hasa in CA. and Dumont in FL. both offer sulfuric acid for pool acid. The sulfuric is sold in a somewhat diluted form to the approximate acid strenth of muriatic so it can be substituted on basically a volume for volume measure and be 'in the ballpark'. I believe it is just slightly more effective and requires slightly less than muriatic to achieve the same pH decrease.)
IMHO, CO2 for pH control is not the first choice. It is more expensive in the long run and does not eliminate the need for acid. It is also not inherently any safer, although the dangers are different.