The Role of Carbon Dioxide (CO2) in Swimming Pool Water, part 1

May 21, 2016

Reprinted with permission from Kim Skinner of OnBalance:

While 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 rise back up again afterwards.  Also, they may wonder why this pH rise happens faster and higher in some pools than in others.

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

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.

When dissolved in water, CO₂ primarily exists as aqueous CO_2, but a very small amount also combines with water to form carbonic acid: CO₂ + H₂O = H₂CO₃, and the slightly acidic nature of this compound lowers the pH.

The more CO₂ in the water, the lower the pH. The less CO₂, the higher the pH. Pool water with no dissolved CO₂ (and alkalinity of 80 ppm) will have a pH of about 8.4.

As we all know, when acid is added, both the alkalinity and the pH are lowered.  The alkalinity is lowered because the added acid reacts with bicarbonate and carbonate alkalinity in the water, converting it to CO₂ and carbonic acid – which is then no longer alkalinity.

After adding acid to pool water, the pH goes down (at first) because of the effect on pH of the increasing amount of CO_{2 (aq)} produced by adding the acid.  Afterwards, the pH begins to rebound and eventually can return to its original level.  This is due to the equilibrium relationship between the amount of CO₂ in the water and the amount of CO₂ in the air above the water. This is known as Henry’s Law.

Since the acid addition forms more CO_{2 (aq)} in the water than is dictated by the equilibrium, most of the CO_{2 (aq)} created by the addition of acid will then begin to release and off-gas into the atmosphere – which will gradually raise the pH level but not the alkalinity. The alkalinity remains the same.

The “natural” level of CO₂ in balanced pool water after it has had sufficient time to reach equilibrium with the atmosphere is from about 0.5 ppm to 2.0 ppm and the pH will be around 8.0 to 8.5 depending on the amount of carbonate alkalinity.

Since CO₂ in water helps keep calcium soluble in water, pool water should be maintained with just enough CO₂ to keep the pH down in the mid to high 7 range. Too much CO₂ in water creates low pH conditions that are aggressive to pool plaster, and no CO₂ may create high pH conditions that can be scale forming to all pool surfaces. Of course, etching and scaling are something service techs are always trying to prevent from happening.

Next: The Role of CO2 in Swimming Pool Water, part 2

onBalance May 2016