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.