LaMotte Pool Manager Water Quality Handbook - Manual (Page 26)

The DPD test method for free
chlorine uses either a single foil
stripped tablet, identified as
DPD #1, or a dual liquid
reagent system. In the tablet
system one Chlorine DPD #1
tablet is added to a measured
water sample in a tube and
mixed to disintegrate the tablet.
Color develops from a faint
pink to a vivid red, depending
on the concentration of free chlorine, and is matched to
standards with known values in a comparator.
The dual liquid reagent system produces virtually the
same color reaction after drops of each solution are
added to the test sample.
To obtain a reading for Combined Chlorine the DPD #3
tablet or DPD #3 liquid is added to the original free
chlorine test sample. The result is read as Total Chlorine
and the increase in color represents combined chlorine.
Example:
Total Chlorine Free Chlorine Combined Chlorine
2.0 — 1.5 = 0.5
If the combined chlorine is above 0.2 ppm,
superchlorination or a shock treatment should be
considered to achieve “breakpoint chlorination”
†
. A
chlorine-based treatment should be used if free chlorine
is too low. If additional free chlorine is not needed, an
oxygen-based shock treatment can be utilized to
eliminate combined chlorine.
† Breakpoint is achieved by multiplying the combined chlorine
level times 10. Dose the resulting amount in ppm.
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