this 1st statement comes from "professional pool operators of America" 12 most common myths of pool water chemistry,
5. The ideal pH for any pool water is 7.4.
Impossible to know. Out of context, there is no “ideal” anything. The water’s hardness and other CSI variables, even the values read from the make-up (fill) water, help you determine the most appropriate value. In general, the best pH is “the lowest pH you can get away with” as chlorine works much better at lower pH values. That actual number is, of course, clearly limited by the bottom of the state health-code ranges, and otherwise chosen by pool operators considering the influences and trends of their sanitizer and their make-up water – wisely keeping that CSI thing in balance all along. Curiously, a pH of 7.4 is not even legal in South Dakota where pH 7.6 is dictated, and almost never used in Germany (the birthplace of water-chemistry science) where their national code’s working range is 6.5 to 7.3! Typically, they hold 7.0 or 6.9.
and this chart is from "Blue science"
Chlorine Effectiveness Scale as Depending on pH (Potential Hydrogen):
Active Chlorine------
97% --------------------
91%---------------------
76%--------------------
66%--------------------
50%--------------------
33%--------------------
17%--------------------
10%--------------------
5. The ideal pH for any pool water is 7.4.
Impossible to know. Out of context, there is no “ideal” anything. The water’s hardness and other CSI variables, even the values read from the make-up (fill) water, help you determine the most appropriate value. In general, the best pH is “the lowest pH you can get away with” as chlorine works much better at lower pH values. That actual number is, of course, clearly limited by the bottom of the state health-code ranges, and otherwise chosen by pool operators considering the influences and trends of their sanitizer and their make-up water – wisely keeping that CSI thing in balance all along. Curiously, a pH of 7.4 is not even legal in South Dakota where pH 7.6 is dictated, and almost never used in Germany (the birthplace of water-chemistry science) where their national code’s working range is 6.5 to 7.3! Typically, they hold 7.0 or 6.9.
and this chart is from "Blue science"
Chlorine Effectiveness Scale as Depending on pH (Potential Hydrogen):
Active Chlorine------
pH97% --------------------
6.091%---------------------
6.576%--------------------
7.066%--------------------
7.2 50%--------------------
7.5 33%--------------------
7.817%--------------------
7.910%--------------------
8.0