Why does Alkalinity rise in a pool using a CO2 feeder for pH control?

CO2 reduces pH but not alkalinity. Alkalinity tends to rise in CO2-fed pools. Here's why.

Alkalinity rises due to the excess hydroxides (lye) from non-stabilized chlorine that would otherwise be negated when using acid.

Acid reduces both pH and alkalinity (by converting bicarbonate into carbonic acid), so the excess hydroxide (OH^-) from liquid chlorine or cal hypo is negated. But when using CO₂, CO₂ does nothing to these excess hydroxides, so they accumulate. Eventually, those hydroxides build up enough to create a noticeable increase in total alkalinity.

pH is largely determined by the amount of carbon dioxide (CO2) in your water

The more CO₂ dissolved in your water, the lower the pH. This is why injecting CO2 lowers pH. Some of the dissolved CO₂ converts into carbonic acid (H₂CO₃), which lowers the pH.  The opposite is also true. The loss of CO₂ raises the pH. This is why aeration raises the pH.

↑ CO₂ = ↓ pH

↓ CO₂ = ↑ pH

Acid, on the other hand, creates carbonic acid by converting bicarbonate (HCO₃-), thus reducing the amount of bicarbonate (and therefore lowering total alkalinity).

Learn more about these equilibria here.