Vacation Strategy

[chem geek]

I'm still a bit confused as to what the HOCl (ppm Cl2) label at the top of the chart means.

The "HOCl (ppm Cl2)" is the concentration of hypochlorous acid, what I've been calling "disinfecting chlorine", and the "ppm Cl2" just says that all chlorine measurements are measured in part-per-million as weight of chlorine gas (that's just the convention).

With no CYA and at a pH of 7.5, the Free Chlorine (FC) concentration is double the disinfecting chlorine (hypochlorous acid) concentration because the FC (with no CYA) measures both the hypochlorous acid (HOCl) and the hypochlorite ion (OCl-) and these are roughly equal in concentration at a pH of 7.5. So that's why I doubled the disinfecting chlorine (hypochlorous acid) concentration to get the FC level when there was no CYA.

Richard