Chlorine/pH/CYA Relationships

[chem geek]

Simmons99 makes an excellent point. I don't know what the recommended pH range is for vinyl liners, but as the numbers show below it doesn't make much sense to try and vary pH to improve chlorine effectiveness rather than simply adding more total chlorine.

The following shows the amounts of disinfecting chlorine (HOCl) and non-disinfecting chlorine (OCl-) when the total chlorine is 3 ppm at various levels of pH with different CYA amounts for each table. The "missing" chlorine (to add up to 3) when CYA is present is bound to CYA, better protected from sunlight (UV) destruction, but not immediately available to do work (disinfect or oxidize). It's like being in reserve and can be called up rather quickly (in seconds).


for Total Free Chlorine = 3 ppm

. . . . . . pH
0 ppm CYA 7.0 7.5 8.0
ppm HOCl 2.243 1.451 0.685
ppm OCl- 0.742 1.520 2.271

. . . . . . pH
15 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.142 0.107 0.087
ppm OCl- 0.047 0.112 0.289

. . . . . . pH
30 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.060 0.045 0.038
ppm OCl- 0.020 0.047 0.126

. . . . . . pH
70 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.023 0.017 0.015
ppm OCl- 0.008 0.018 0.050


You can see that though the ratio of HOCl to OCl- is only a function of pH and is independent of CYA, the presence of CYA cuts the total amount of both HOCl and OCl- way, way down and also makes the disinfecting form of chlorine, HOCl, less variable with pH changes while the non-disinfecting form, OCl-, becomes more variable.

Specifically, without CYA, changing the pH from 7.5 to 7.0 increases chlorine effectiveness by 55% while with CYA present (in any amount much larger than the total amount of chlorine), this same pH change only increases chlorine effectiveness by 33%.

If you were to increase the total chlorine from 3 ppm to 4 ppm, you would get the following.


Total Free Chlorine = 4 ppm

. . . . . . pH
0 ppm CYA 7.0 7.5 8.0[/B]
ppm HOCl[/B] 2.991 1.934 0.913
ppm OCl-[/B] 0.990 2.027 3.028

. . . . . . pH
15 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.219 0.167 0.134
ppm OCl- 0.072 0.175 0.442

. . . . . . pH
30 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.086 0.064 0.055
ppm OCl- 0.028 0.067 0.181

. . . . . . pH
70 ppm CYA 7.0 7.5 8.0
ppm HOCl 0.032 0.024 0.021
ppm OCl- 0.011 0.025 0.068


Notice how without CYA you get the expected 33% increase in chlorine effectiveness (4/3 = 33%), but with a CYA of 30 ppm you get a larger 42% increase in chlorine effectiveness when you increase total chlorine from 3 to 4. This is because having the total free chlorine level closer to the CYA level makes the CYA's buffering of chlorine less effective.

Yes, that's a lot of information, but the bottom line is that when you have CYA in your pool it is much easier to increase chlorine's effectiveness by adding more chlorine than it is by lowering the pH (which, as pointed out earlier, has other side effects).

Richard