The most effective range for FC is 7.2-7.5. When you get to the extremes like 6.8 & down and 7.9 up, chlorine loses a significant amount of "punch". This is what I was taught in my training anyway. Over the years, I've dealt with many Chemical Engineers and they generally agree with it. Once you put CYA in the mix things improve a great deal, mostly how fast the degradation of FC takes place. I am sure Richard (Chemgeek) can line out real particulars for you, but that's a simple definition.
I've read through the technical explanation (pool-water-chemistry-t628.html#p4366) that chem geek put up with all the charts. I think I'm starting to get it. With no CYA present, the sanitizing ability of Chlorine is greatly impacted by pH. With CYA present, it appears that the sanitizing ability of Chlorine is fairly constant. It does vary, increasing with lower pH and gradually decreasing as pH rises, but it looks like it only changes by about 6% or so. So it seems to me, to put it in very simplistic terms, once you bring CYA into the situation, the effect of pH on the sanitizing ability is pretty minimal, and in most situations can be ignored.
I'm still wondering if there is a specific point at which the CYA level becomes significant enough to have the influence that it does. Is it as soon as it is introduced? Is it at a specific concentration? Is it more dependent on the ratio of FC to CYA? In the example used for the chart, the CYA was 30ppm, which isn't all that high, so it would seem that in just about any properly maintained pool, where CYA levels should be at or above 30ppm, pH shouldn't have much impact on the sanitizing ability of Chlorine.
With no CYA in the water, going from a pH of 7.5 to 8.0 lowers the active chlorine (hypochlorous acid) level by roughly 50%. With CYA in the water, not just at 30 ppm as in what I showed in the graphs bug higher levels as well, the same pH change only has the active chlorine level drop by 15%. So you can generally ignore the pH effect on chlorine. Cyanuric Acid is a buffer for hypochlorous acid so resists changes in the active chlorine concentration.
Now the fact that without CYA the active chlorine level drops so much at higher pH is really a moot point anyway since the active chlorine level is way too high to begin with when there is no CYA in the water. So if it drops from an effective 1.0 to 0.5, who cares? This is still at least 7 times higher than the active chlorine level at the minimum FC/CYA ratios in the chlorine/CYA chart that are needed to prevent algae growth and yet still kill pathogens very quickly.
There's a flip side to this CYA hypochorous acid buffering effect and that is that adding a hypochlorite source of chlorine to a pool with CYA in the water has the pH rise more than if there were no CYA in the water. This is one reason I suggest lowering the pH to 7.2 or so before shocking, especially before shocking to a high FC level as when the CYA level is higher. It's not to have the chlorine be more effective so much as to prevent the pH from rising so much where metal staining or scaling/cloudiness could be a problem.
Thank you all for your input. Little by little, I'm chipping away at some of the mythology I keep hearing about pool chemistry. The more I learn, the more things start to make sense, and the more I see it happening with my pools. Its like a camera coming into focus...all of a sudden everything becomes clear.
Extreme PH (well out of the normal range either high or low) can impact the effectiveness of chlorine significantly, even with CYA. If your PH is anywhere in the normal range and you have an outdoor pool I would just ignore this effect completely.
Relative to a pH to 7.5 with 100% chlorine effectiveness, a pH of 8.4 has roughly 75% the effectiveness (i.e. is 25% less effective) while a pH of 6.6 has roughly 200% the effectiveness (i.e. roughly twice as effective). Of course, any difference in effectiveness can be compensated by simply changing the FC so perhaps with your > 8.0 pH pools you could target somewhat higher FC (roughly 30% higher) to prevent algae growth if such pools are close to the minimum FC/CYA levels.
Remember that every pool is different so in general if you have a pool that isn't responding as well as you'd like in preventing algae growth, then bump up the FC somewhat to see if that makes a difference (getting rid of any existing algae by shocking first). Factors such as poor circulation will often outweigh the effects of pH.