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Carbon Dioxide in Pool Water

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The Role of CO2 in Pool Water #1
While some service techs go about their business taking care of various swimming pools, and specifically after they lower pH by adding acid, they may ask themselves why the pH of the water begins to rebound (rise back up again) afterwards. Also, they may wonder why this pH rebound happens faster in some pools than in others.

The answer lies in the behavior of carbon dioxide in the water. Carbon dioxide (also known as CO2 & carbonic acid) is formed when acid is added to swimming pool water, and it is this compound that affects the changes in pH of pool water.

Carbon dioxide (CO2) is a common, essential compound in nature. It is found almost everywhere, from what flowers and trees breathe in, to what humans and animals breathe out; and to the bubbles in the soda you drink. In its warmer phase it is a gas, and in its colder phase it becomes a solid – dry ice. Our atmosphere (the air we breathe) contains a relatively small amount of gaseous carbon dioxide – only about 0.03% to 0.06% – which is fortunate, since levels around 10% or higher would cause us all to lose consciousness! Because it exists in the air, a slight amount of carbon dioxide can be absorbed by water.

In water, CO2 primarily exists as aqueous CO2 (gas in, but not fully reacted with the water), but a small amount also combines with water to form carbonic acid: CO2 + H2O = H2CO3, and the slightly acidic nature of this compound lowers the pH somewhat.

Carbon dioxide plays an important role in the make up and balancing of pool water. When dissolved in water, carbon dioxide has a direct effect on the water’s pH. The more CO2 in the water, the lower the pH, and the less CO2, the higher the pH. Pool water with no dissolved CO2 (and with a minimum alkalinity of 100 ppm) will have a pH of about 8.4 (as long as no other chemicals have been added). On the other hand, pool water that is saturated with CO2 will have a pH down around 5. Rain water can pick up CO2 (acid rain) from the air which affects pool water by lowering the pH.

Although CO2 can be introduced to water from the air, it is also produced in pool water by simply adding acid. As we all know, when acid is added, both the alkalinity and the pH are lowered. The alkalinity is lowered because, with normal pool water parameters, the added acid reacts with bicarbonate alkalinity in the water, converting it to carbonic acid and aqueous CO2 – which is then no longer alkalinity.

For you who enjoy formulas, bicarbonate and acid form carbonic acid and chloride, or HCO3 + HCl = H2CO3 + Cl, and then all but a fraction of a percent of the carbonic acid shifts to aqueous CO2: H2CO3 <==> CO2 (aq) + H2O. Depending on the amount of acid added, a specific and calculatable amount of alkalinity is eliminated.

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Comment by Richard A. Falk on June 29, 2009 at 1:06pm
I disagree as to the risk. One can increase Calcium Hardness (CH) to keep the saturation index near zero (if needed, as with plaster pools) when the TA is lower. Also, one would not use a lower TA unless one were using hypochlorite sources of chlorine -- with acidic sources of chlorine such as Trichlor, a higher TA is needed to compensate for the constant net acid introduction from the Trichor and chlorine consumption/usage.

In practice, a TA of 70 ppm in hypochlorite-chlorine-pools would be the typical low side, though pools with lots of aeration from waterfalls, spillovers, fountains, etc. could go lower (depending on CYA level which also contributes to TA) though not by much.

Borates don't contribute very much to TA. 50 ppm Borates at a pH of 7.5 only adds 5 ppm to TA. Most of the borates are in the undissociated form of boric acid rather than borate ion.
Comment by Kim Skinner on June 25, 2009 at 2:58pm
Yes, lowering the total alkalinity may help resolve some high pH issues, but it still considered a risky alternative.

Borates are part of total alkalinity, so that has to be factored in.
Comment by Richard A. Falk on June 25, 2009 at 1:22pm
The use of 50 ppm Borates in a pool definitely adds to the pH buffering, especially against a rise in pH, but it does not combat the primary SOURCE of rising pH which is carbon dioxide itself. The way to address that, in pools using hypochlorite sources of chlorine that tend to rise in pH, is to lower the TA level. This is counter-intuitive, but it works because TA not only provides a pH buffer, but higher TA itself CAUSES a faster pH rise because it increases the amount of carbon dioxide in the water. It turns out that this latter effect overwhelms the former effect so as strange as it seems, a lower TA level results in greater pH stability IF the cause of rising pH is carbon dioxide outgassing. Pools with a pool cover wouldn't outgas much so a higher TA wouldn't show a problem. Pools with more aeration, such as waterfalls, spillovers, fountains, etc., have more carbon dioxide outgassing so a lower TA is more helpful.

If the TA is lowered, then the use of Borates is helpful to supplement the pH buffering. However, though the borates buffer pH, they do not affect the amount of acid that is added over time. The borates simply reduce the frequency of acid addition, but it takes more acid to move the pH so you end up adding more acid less frrequently -- the net over time is the same. That is, unless you lower the TA level. With a lower TA level you not only slow down the pH rise, but you reduce the total amount of acid needed over time because you are attacking the SOURCE of the pH rise problem.
Comment by Kim Skinner on May 15, 2009 at 11:33am
Adding borates does buffer the pH somewhat, but has little to do with removing or adding CO2. I dispute that it softens the water. However, there is some evidence that borates reduce eye irritation and also can act as a herbicide or algaestat. In that way, it could reduce chlorine demand, but regular chlorine additions will still be required.

There will be a few more updates upcoming on this topic about carbon dioxide's role in pools.
Comment by shaun mclane on May 14, 2009 at 7:59pm
Fantastic article!

Just went to an IPSSA meeting last night, and we discussed Borates (sp?). The conversation was started by a chemical rep, but a lot of people joined in. I guess these borates remove the CO2 from the water, making the pH levels remain stable, but also softening the water. He claimed the water felt more like saline solution (what you use to clean contacts). He pitched it as a cheap alternative to a salt system - soft water, and less chlorine consumption.

Anyone ever heard of this? The Borate's product name is "Foundation," if that helps.

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