Chlorine bound to CYA has a small oxidizing capability. In the
Certified Pool Operator (CPO) training -- what is not taught thread in the "Chlorine/CYA Relationship" section there is
this paper referenced when describing CYA's effect on chlorine's rate of oxidizing organics. The reaction rate of oxidation of monochlorodimedone (MCD) by H
2ClCy was estimated at 0.5×10
5 M
−1 s
−1 compared to that from free chlorine of >7.6×10
6 M
−1 s
−1. That's a factor of over 152 difference. So we normally ignore this. For example, with an FC that is 3 ppm and a CYA of 30 ppm, the HOCl and OCl
- together at pH 7.5 are 0.087 ppm while the chlorine bound to CYA is 2.91 ppm. If we divide the chlorine bound to CYA by a factor or 150, then it is equivalent in oxidizing power to 0.019 ppm so a fraction of the free chlorine amount.
From a disinfection point of view, we know it's even a bigger difference since the dominant species at pool pH is HClCy
- which like OCl
- is negatively charged so has a harder time getting into cells (it's also a bigger molecule and unlike HOCl doesn't "look like" water). It is unknown whether the killing of algae is more similar to the HOCl effects of getting inside cells and disrupting metabolic and reproductive processes or whether it is more related to the oxidizing capability, but I suspect it's the former. For the pH test, however, it would be about oxidation rates for oxidizing the pH dye.
However, using your 35 ppm FC with 300 ppm CYA numbers we have HOCl and OCl
- together at 0.11 ppm while chlorine bound to CYA is 34.9 ppm so dividing by 150 we have 0.23 ppm which is actually larger than the free chlorine amount. So it's possible that the pH dye gets oxidized almost 3 times faster than at the same FC/CYA ratio but lower absolute FC and CYA levels. Of course, even this 3 times faster is still slower than not having any CYA at all where the numbers would be the FC level itself. Keep in mind that this is very speculative since the rate of chemical reactions is species-specific so the rate of reaction with dye may not be the same as the relative rates of reaction with MCD.