Monochloramine, Chlorine, CYA and pH

[chem geek]

If you are going to be shocking with chlorine as opposed to adding ammonia to form monochloramine or adding bromide to from bromine (similar arguments for its effectiveness since it does not combine with CYA), then I would lower the pH to around 7.2 IF you are shocking at very high levels. This is generally the case when fighting mustard/yellow algae since such shocking is usually done with an FC that is 60% of the CYA level. Though the pH dependence of chlorine when CYA is present is less, it's not zero, so heavy shock levels would be better if the pH is lowered first.

As a concrete example using a TA of 80 for reference (and assuming no Borates), adding chlorine to raise the FC by 10 ppm would go from a pH of 7.5 to 8.15 with a hypochlorous acid concentration of 0.17 ppm. Starting with a pH of 7.2 you would get to 7.53 and 0.21 ppm so 24% higher in chlorine. However, if you raised the FC by 20 ppm, then starting with a pH of 7.5 goes to 8.56 and 0.43 ppm vs. starting with a pH of 7.2 going to 8.1 and 0.64 ppm which is 49% higher. A 30 ppm FC increase would be 7.5 to 8.72 and 0.69 ppm vs. 7.2 to 8.4 and 1.08 ppm which is 57% higher (and a starting pH of 7.0 would go to 7.95 and 1.83 ppm which is 165% higher than starting with a pH of 7.5). So I generally tell people to lower the pH first if they are going to be shocking with 20 ppm or more incremental FC. Don't forget that the pH will drop back down to where you started as the chlorine gets used up (assuming not much pH rise from outgassing).

The above heavy shock levels are for pools that can more readily handle it such as plaster pools. I would be more conservative using lower chlorine levels with vinyl pools to be on the safe side. And as waterbear mentioned, different types of algae require some different methods. Black algae is most effectively killed by frequent brushing to remove the slime layer "head" so that lower layers get exposed to the shock level of chlorine.

Richard