Total Chlorine in water exists in two forms: COMBINED and FREE. OTO test kits are no longer recommended as they measure total chlorine only, i.e. Free and Combined Chlorine together. DPD test kits must be used to test for PPM of Free Chlorine.

There are two forms of FREE Chlorine:
a) Hypochlorite ion (ClO-)
b) Hypochlorous acid (ClOH)

a) The ionic form ClO- which is a slow acting sanitizer, and b) the molecular form, hypochlorous acid ClOH, which is the fast acting sanitizer. ClOH acts about 80 to 100 times faster than ClO-. It is therefore essential to the maintenance of sanitized conditions in the water.

As shown on the curve below, the concentration of ClOH, the fast acting sanitizer decreases very rapidly with the increase in pH.

At low pH levels, almost all the Free Chlorine is ClOH. At a pH level of 7.5, all the Free Chlorine is divided 50/50 between ClOH and ClO-. At a pH level of 8.0, the fasting acting ClOH is only 20 and 80% is ClO-.

Unfortunately, DPD and other test kits cannot differentiate between the two forms of Free Chlorine. They do not show the decrease in ClOH (the fast acting sanitizer) when the pH is increased.