It should be understood that neither low total alkalinity (below 60 ppm) nor higher levels than 180 ppm will detrimentally affect the efficacy of chlorine or other types of sanitizers. As long as the pH is in the range of 7.2 to 7.8, the sanitizer will still be able to effectively destroy bacteria and algae and keep the pool safe to swim in. Unless the pool is cloudy, there is no need for Health Inspectors to close the pool just because the alkalinity is out of range. As mentioned above, simple steps can be taken to correct the out of range alkalinity content.The one unfortunate aspect regarding alkalinity is that when total alkalinity is in the proper operating range of 60 ppm and 180 ppm, it causes the pH to seek a natural level of about 8.0 to 8.3. Consequently, acid is needed to constantly lower the pH back to the operating parameters of 7.2 to 7.8. Periodic additions of sodium bicarbonate are needed to replenish the alkalinity lost due to acid addition. Another approach to pH control is to add carbon dioxide, rather than acid, for pH control. In such a case regular alkalinity replenishment may not be required.Adding carbon dioxide lowers the pH, but does not affect the total alkalinity content. The loss of carbon dioxide increases the pH toward 8.4, and also does not affect total alkalinity.If pool water contains cyanuric acid (from the addition of Trichlor and Dichlor, or from direct additions), it also contributes to the total alkalinity. This is why the term “total alkalinity” is used when discussing pH buffering capabilities and for establishing the minimum and maximum levels. However, the alkalinity contributed by cyanuric acid is not a “carbonate” type of alkalinity, which is the type of alkalinity needed to determine proper water balance using the Langelier Saturation Index (LSI). “Carbonate” alkalinity consists of bicarbonate and carbonate ions. Sodium bicarbonate, sodium carbonate, and sodium hydroxide are chemicals that can be added that contribute to “carbonate” alkalinity.
I think it should be noted that a higher Total Alkalinity (TA) results in faster carbon dioxide outgassing leading to a faster pH rise. The natural higher pH level you describe is only for when the TA is higher, near 100 ppm. At a pH of 7.5, for example, the amount of carbon dioxide in the air and the equilibrium amount in water would result in a TA of 10 (with no CYA). So it's not true that the pH will always migrate upwards, though in practice it does because pools and spas are intentionally over-carbonated in order to provide a pH buffer and, for plaster/gunite/grout, to saturate the water with calcium carbonate to prevent pitting/dissolving of such surfaces.
Hypochlorite sources of chlorine (chlorinating liquid, bleach, Cal-Hypo, lithium hypochlorite) are all close to NET pH neutral when one accounts for the consumption/usage of chlorine which is an acidic process (technical details are in this link: http://www.troublefreepool.com/post4367.html#p4367 ). In pools using hypochlorite sources of chlorine, one can significantly cut down the rate of pH rise if one lowers the TA level. This is counter-intuitive, but it is true because TA, specifically the carbonates portion of TA (i.e. "adjusted TA" reduced to remove the CYA component of TA) acts not only as a pH buffer, but as a SOURCE of rising pH itself since it is proportional to the amount of over-saturation of water with dissolved carbon dioxide.
Of course, if one lowers the TA level to reduce the rate of pH rise, then one may need to increase the Calcium Hardness (CH) level to keep the saturation index near zero in plaster/gunite/grout pools.
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Hypochlorite sources of chlorine (chlorinating liquid, bleach, Cal-Hypo, lithium hypochlorite) are all close to NET pH neutral when one accounts for the consumption/usage of chlorine which is an acidic process (technical details are in this link: http://www.troublefreepool.com/post4367.html#p4367 ). In pools using hypochlorite sources of chlorine, one can significantly cut down the rate of pH rise if one lowers the TA level. This is counter-intuitive, but it is true because TA, specifically the carbonates portion of TA (i.e. "adjusted TA" reduced to remove the CYA component of TA) acts not only as a pH buffer, but as a SOURCE of rising pH itself since it is proportional to the amount of over-saturation of water with dissolved carbon dioxide.
Of course, if one lowers the TA level to reduce the rate of pH rise, then one may need to increase the Calcium Hardness (CH) level to keep the saturation index near zero in plaster/gunite/grout pools.