High alkalinity and pH drift
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The pH drifts upward due to the outgassing of carbon dioxide because essentially pools are "over-carbonated", just like a lovely tasty beverage! When carbon dioxide leaves the pool, the pH rises while the Total Alkalinity remains the same. This process will continue until the equilibrium concentration of carbon dioxide that would normally be dissolved in the water matches that in the air. Fortunately, this process slows down as the amount of carbon dioxide in the pool gets closer to its proper equilibrium amount.
The pH numbers you specified are not magic or special. The actual pH number where there will be equilibrium is a function of the TA level (or more technically, the carbonate alkalinity level). If you start with standard conditions of a pH of 7.5, TA of 100, CYA of 30 (because TA includes both carbonate alkalinity and the alkalinity from CYA), then outgassing of carbon dioxide will occur until the pH goes up to 8.45 and about 12.75% of the total carbonate in your pool will have outgassed, but if instead you started with a TA of 80, the pH would only go up to 8.34 and about 11.66% of the total carbonate in your pool will have outgassed.
However, you could just as easily lower your TA level and have an equilibrium at a pH of 7.5. A TA level of 19 will be in equilibrium and will no longer outgas carbon dioxide at a pH of 7.5. Of course, much of this TA is composed of the 30 ppm CYA so if we instead look at having no CYA in the pool, then the amount of TA for equilibrium at a pH of 7.5 would be only 10 ppm. Such a low TA would not be a very good buffer so the pH in your pool would be very, very sensitive to pH swings.
So you might wonder whether you could operate your pool at a higher pH to avoid the carbon dioxide outgassing. The answer is yes and Ben describes a lot about this at this link. In practice, operating a pool using chlorinating liquid or bleach and having the TA level no higher than 80 (but no lower than 50) is usually sufficient to have the pH rise very slowly and be quite manageable with infrequent additions of acid. It's only when you have significant aeration features (or an SWG that aerates with hydrogen gas) that it becomes more difficult. One option is to use an additional pH buffer such as having 50 ppm Borates (from Borax) in your pool. The Borates are a good buffer for preventing the pH from rising, but they are not very good for keeping the pH from dropping (a lot -- the Borates become "exhausted" rather quickly) so this approach wouldn't be very good if acidic chlorine sources were used (e.g. Trichlor).
Richard
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