After adding acid to the pool water, the pH goes down (at first) because of the effect on pH of the increasing amount of CO2 (aq) produced by the acid. Shortly 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 CO2 (aq) in the water than is dictated by the equilibrium, most of the CO2 (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.
It may be instructive at this point to mention that 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 8.3. This pH range is dependent on total alkalinity.
Since it is dissolved CO2 in water that keeps calcium soluble in water, we want to maintain pool water 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 creates high pH conditions that are scale forming to pool surfaces. Of course, etching and scaling are something service techs are always trying to prevent from happening.
CO2 can also be added to pool water by simply injecting pure CO2 into the water, or by adding “dry ice” (frozen CO2) to water. The pH will drop, and if a large amount of CO2 is absorbed into the water, the pH could go below 6.0. Despite the fact that the pH could be lowered dramatically, no change or reduction will occur in the alkalinity under this scenario. This fact bears repeating because it is incorrectly being taught in some seminars. The amount of CO2 in water does not affect the content of alkalinity, only the pH is affected.
The process of injecting CO2 into water (especially spas) is becoming more and more popular. Systems have been designed to add CO2 into the water when sanitizers such as bleach are used. The concept behind this is to use CO2 to offset the high pH effects of bleach. This system works quite well to keep the pH in check and in balance (7.2 – 7.8) except for the fact that the total alkalinity will gradually increase over time, but this is only due to the small amount of alkaline properties of the bleach, not the CO2.
Therefore, small amounts of acid will eventually need to be added to control total alkalinity levels, and it won’t be necessary to experience the vicious cycle of adding larger doses of acid followed by offsetting doses of sodium bicarbonate or soda ash to maintain pH and alkalinity. But, because the content of the CO2 being injected in the water is higher than the natural level (equilibrium, or 8.2-8.3), most of it will off-gas over time. Therefore, a continual addition of CO2 will be needed to keep the pH from rising. Although this may be done manually, it is much easier by dosing the CO2 with a pH controller.