Chlorine/pH/CYA Relationships

[chem geek]

I’m full of questions tonight. What is the consequence of running the pH a bit low to try to make up for the “Chlorine/CYA lock?

I know the “ideal” is 7.5 pH & 40 ppm CYA but--------

I’ve seen charts that put chlorine effectiveness at around 50% at 7.5 pH, but up around 75% at 7.0 pH. Then the next Chart says Chlorine effectiveness is reduced by high CYA. (>50ppm).

What I am getting at is can the two conditions (low pH & high CYA) off-set the effects of each other or doesn’t it work that way??

27000 gallon In ground Vinyl DE filter

Jakebear,

These are good questions and it turns out that because CYA acts like a chlorine buffer (similar to the way that alkalinity acts like a pH buffer), the presence of CYA reduces the pH effect on chlorine's effectiveness. Of course, the presence of CYA itself significantly reduces the absolute amount of disinfecting chlorine, but once that is done it acts to somewhat keep that amount more constant when pH swings compared to not having any CYA at all. Of these two effects that CYA has -- 1) reducing chlorine's effectiveness and 2) buffering chlorine against pH changes, it turns out that the reduction of chlorine's effectiveness is huge. The bottom line is that you have absolutely no hope of restoring significant amounts of chlorine effectiveness when CYA is present by simply changing the pH.

I have to leave right now, but will add another post with some numerical examples to give you a better feel for this. It turns out that this question comes up quite a bit as people think they should run at lower pH to get greater chlorine effectiveness and don't realize that when CYA is present this inhibits their ability to do this (i.e. to lower pH to increase chlorine's effectiveness).

Richard