CO2 in Pool Water #3 of 4
While some of the following information has already been mentioned, let’s specifically address the question raised at the beginning of this update series. Why does the pH eventually rise after acid initially makes it drop? Again, the answer lies in the fact that whenever the pH is below 8.2-8.3, there is generally more CO2 in water than its natural equilibrium level with the atmosphere. Because of this, the extra CO2 will off-gas into the atmosphere over time. The pH rises because CO2 is off-gassing from the pool water. Unless checked, the pH will continue to rise until the CO2 reaches its equilibrium or about 8.3. The more alkalinity (over 80 ppm), the stronger pull to a pH of 8.3. This is one of the difficulties that pool service techs have to deal with. Generally, there is no getting around this.
But there are exceptions to the above rule. For instance, a common but occasionally unrecognized factor which interferes with CO2 off-gassing or absorption is a pool cover. When pools are covered with non-gas permeable covers, such as the common blue bubble solar blankets or solid vinyl or plastic automatic safety covers, the exchange of gas from water to air and air to water is blocked.
With newer plaster pools, a vast supply of hydroxide (a component of the plaster surface) is exposed to the water and its chemistry. If a pool cover blocks the otherwise natural process of CO2 off gassing, the CO2 reacts with the plaster surface and, together with hydroxide, form carbonate, thereby reducing CO2 in the water. Eventually, all aqueous CO2 could be depleted, causing the pH to climb to 8.4, and the pool cover would not allow more CO2 from the atmosphere to dissolve into the water to keep the pH from rising even higher than 8.4. At this point, dissolved calcium in the water would probably begin to precipitate and produce scale on the floor and walls.
In vinyl, painted, and fiberglass pools, on the other hand, no ready source of hydroxide is available, so pool covers on these pools can keep the pH artificially low when inhibiting the ability of CO2 to off-gas. When acid or an acidic sanitizer is added to this type of pool that has a non-permeable cover on it, the CO2 generated (by the acid) will stay in the water, and will not be able to off-gas. Therefore, the pH will probably remain low and unchanged until either other chemicals are added, or the cover is removed.
When non-permeable covers are used, pH needs to be watched carefully. Ideally, the cover should be removed for enough time to allow gases to equilibrate (perhaps 6 to 8 hours, twice a week for residential pools). When this is not an option, careful control of pH using acids and bases must be maintained. How will you know if the pool contains the right amount of CO2? The pH will be balanced.