The efficacy of chlorine, that is, the power of
it to have an effect, is greatly influenced by the care with which you
manage your pH levels. As the pH of your pool increases, the killing
power of your chlorine decreases. At a pH of 6.0, we'll get 96% or so
of the potential out of each lb of chlorine, but at what cost? Such
a low pH would wreck havoc on all of the surfaces the water comes in
contact with, including swimmers and pool equipment. It's just too corrosive.
Move the pH up to 7.0 and the efficacy of the chlorine drops to 73%,
but raise it up to 8.0, where many a pool seems to drift to, and it
drops dramatically... down to 21%! At a perfect pH level of 7.5, we
can expect to have about 50% of our chlorine in the molecular structure
of hypochlorous acid, the active, killing form. The remaining half is
in the form of a hypochlorite ion, which is also an active form of chlorine,
but very weak and slow to kill.