LaMotte DPC-52 Water Testing Laboratory - Manual (Page 5)

FREE AVAILABLE, TOTAL & COMBINED CHLORINE
DPD METHOD
Chlorine is added to pool water as a sanitizing agent to kill bacteria and other disease
producing organisms. Chlorine is also used to control algae and to remove undesirable
odors and colors.
Depending on what commercial chlorine product is used, chlorine may be added to pool
water in a variety of forms - chlorine gas, liquid sodium hypochlorite, granular calcium
hypochlorite, or organic chlorine compounds. When chlorine is added to water, it
quickly forms hypochlorous acid, HOCl, the active ingredient which is responsible for
chlorine’s sanitizing capabilities. The hypochlorous acid is also known as the Free
Available Chlorine. The presence of other compounds in the pool may consume the
hypochlorous acid, or free available chlorine, and form other chloro compounds, such as
chloramines or other organic nitrogen compounds. As chlorine combines with other
compounds in the pool water, much of its sanitizing capability is lost, and higher levels of
chlorine are necessary to achieve the same germ free water. Therefore, it is essential to
chlorinate to the point of establishing a free available chlorine residual and then
maintain it at the recommended level. For most pools, the free available chlorine
residual should be maintained between 1.0 - 3.0 ppm.
The LaMotte DPD liquid test method distinguishes levels of Free Available Chlorine,
Combined Chlorine and Total Residual Chlorine, using a single test sample.
RANGE:
0.1 - 5.0 ppm
METHOD:
In the absence of iodide, free available chlorine reacts instantly
with the buffered diethyl-p-phenylenediamine indicator
(DPD) to produce a red color in proportion to the amount of
chlorine present. Subsequent addition of potassium iodide
produces a rapid color response from the combined forms of
chlorine (chloramines).
INTERFERENCE:
The only interfering substance likely to be encountered in pool
water is oxidized manganese. The extent of this interference
can be determined by treating a sample with sodium arsenite
to destroy the chlorine present, so that the amount of
interference can be measured.
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