I am at the understanding that pH has relatively no affect on Chlorines ability to
kill algae and organisms, is this correct?
Weather a pool has a pH of 6 or a pH of 8, it should not make any difference on how well a maintained Chlorine level of 10 should be at destroying organisms, is this correct?
What you wrote is not quite true or certainly needs more explanation. pH does affect the active chlorine (hypochlorous acid) level that is responsible for most of the disinfection of the water. However, when Cyanuric Acid (CYA) is in the water, it buffers this active chlorine so that pH has far less of an effect on it. To be specific, with no CYA in the water, going from a pH of 7.5 to 8.0 has the active chlorine level drop by 50%. With CYA in the water, it drops by only 15%. Going the other direction, going from a pH of 7.5 to 7.0 the active chlorine level with no CYA increases by a little more than 50% but with CYA in the water it increases by 35%. So we generally ignore the pH effect on chlorine except for a SLAM because the larger amounts of chlorine have the pH rise significantly so we recommend lowering the pH first before a SLAM, especially when it's a lot of FC due to high CYA levels.
As for chlorine outgassing, this will be mostly from the active chlorine and it's a very low amount and not changing much with pH so that is not an issue.
On the other hand, hypochlorite ion (OCl
-) breaks down from sunlight faster than hypochlorous acid (HOCl). With CYA in the water, while it buffers the active chlorine (HOCl), this makes the hypochlorite ion concentration vary more than if there were no CYA in the water. What this means is that
the rate of chlorine loss from sunlight is higher at higher pH. HOCl breaks down in sunlight at a rate of around 32% per hour (half-life of 2 hours 10 minutes) while OCl
- breaks down in sunlight at a rate of around 204% per hour (half-life of 20 minutes). So the following table shows what happens with 3 ppm FC with 40 ppm CYA just looking at these two forms of chlorine at different pH:
[EDIT]
NOTE: The following tables do not take into account any CYA shielding effect protecting chlorine from breakdown from UV in sunlight at higher CYA levels. It only accounts for the direct unshielded loss from the amount of chlorine unbound to CYA and assumes no breakdown of chlorine bound to CYA. Also, the per hour and total losses assume the FC level is maintained, not a one-time dose. I have added another column for one dose 8 hour loss and 8 hours is 8 noontime-equivalent hours so effectively a full day of sunlight. [END-EDIT]
3 ppm FC with 40 ppm CYA
pH ...
HOCl ...
per hour loss...
OCl- ...
per hour loss ...
Total 8 hour loss ...
One-time Dose Loss
7.0 ... 0.042 ...... 0.013 ......... 0.014 ...... 0.029 ..................... 0.34 ..................... 0.32
7.5 ... 0.033 ...... 0.011 ......... 0.031 ...... 0.063 ..................... 0.59 ..................... 0.54
7.8 ... 0.028 ...... 0.009 ......... 0.059 ...... 0.120 ..................... 1.03 ..................... 0.87
8.0 ... 0.026 ...... 0.008 ......... 0.089 ...... 0.182 ..................... 1.52 ..................... 1.19
8.5 ... 0.023 ...... 0.007 ......... 0.248 ...... 0.506 ..................... 4.10 ..................... 2.24
Now there are additional chlorine losses not related to sunlight but to temperature and there may also be chlorine breakdown from chlorine that is bound to CYA but I don't have any definitive numbers for that. What the above shows, though, is that at a pH of 8.0 one would expect 0.9 ppm FC per day higher chlorine loss than at pH 7.5. Note that a pH of 7.8 would have only 0.44 ppm FC per day higher chlorine loss than pH 7.5 so the loss accelerates more from 7.8 to 8.0 and even faster above that. This is, in part, why one wants to lower the pH before a SLAM -- not only to have more active chlorine and reduce the risk of metal staining but also to lessen the amount of chlorine loss from sunlight during a SLAM (of course, the loss is much higher than I show above which only has 3 ppm FC). I show below what happens during a SLAM with 16 ppm FC with 40 ppm CYA.
16 ppm FC with 40 ppm CYA
pH ...
HOCl ...
per hour loss...
OCl- ...
per hour loss ...
Total 8 hour loss ...
One-time Dose Loss
7.0 ... 0.382 ...... 0.122 ......... 0.129 ...... 0.263 ....................... 3.1 ....................... 2.8
7.5 ... 0.301 ...... 0.096 ......... 0.320 ...... 0.653 ....................... 6.0 ....................... 5.0
7.8 ... 0.273 ...... 0.087 ......... 0.578 ...... 1.179 ..................... 10.1 ....................... 7.5
8.0 ... 0.256 ...... 0.082 ......... 0.860 ...... 1.754 ..................... 14.7 ....................... 9.6
8.5 ... 0.206 ...... 0.066 ......... 2.188 ...... 4.464 ..................... 36.2 ..................... 14.3
So this shows clearly that letting the pH get high during a SLAM will result in much faster chlorine losses from sunlight. In the above example, if I use chlorinating liquid or bleach (or any other hypochlorite source of chlorine such as Cal-Hypo or lithium hypochlorite), then if I start at a pH of 7.5 and TA of 80 ppm, I end up at 8.31 where the loss/usage of chlorine to sunlight is around 26 ppm FC (if one were to maintain the 16 ppm FC level during the SLAM) while if I start at a pH of 7.2 I end up at 7.67 then the loss of chlorine to sunlight in one day during the SLAM is only 8 ppm FC so considerably less.