Thermo Fisher Scientific C101 Colorimeter - Manual (Page 18)

Measurement: Chlorine, Free & Total
C401/301/201Colorimeter
14
Bromine and Ozone, for details.
2.4.5 Chemistry
Chlorine exists in water as Un-Combined (Free) Chlorine and Combined
Chlorine. The concentration of Free Chlorine can be determined directly
and the concentration of Combined Chlorine may be calculated by
subtracting the concentration of Free Chlorine from that of Total Chlorine.
In the Free Chlorine test method Un-Combined Chlorine reacts directly
with DPD (N,N-diethyl-p-phenylenediamine) in a conditioned sample to
give a pink color. In the Total Chlorine test method Combined Chlorine
reacts with potassium iodide to form tri-iodide ions. The tri-iodide ions and
the Free Chlorine present react with DPD in a conditioned sample to give
a pink color.
The depth of the pink color produced is proportional to the concentration of
Chlorine. The amount of 525nm light absorbed by the pink color is
accurately measured by the meter and then converted into the Chlorine
concentration.