H. Read free chlorine residual directly (5.0) ppm).
Figure 2-4. Modified OTA method, color comparator determination of free
available chlorine (concluded).
(2) N, N-diethyl-p-Phenyl-enediamine Test. The modified OTA method has
been replaced by the DPD test. The OTA test method is currently being phased out.
When new equipment is required, the DPD test equipment should be obtained to
replace the OTA test equipment. The DPD test procedure is similar to the OTA
procedure with minor exceptions. The two comparator tubes are filled with water to be
tested. Place the two water-filled tubes into the openings at the top of the comparator.
Add one DPD-free available chlorine tablet (DPD tablet No. 1) to the tube in the right
opening of the comparator. Cap this test tube and shake in order to disintegrate the
tablet. For those kits with hard to dissolve tablets, a stirring rod is usually provided to
"crush" the tablets. In "crushing" the tablets, be sure to take the tube from the
comparator, and place the tube on a tabletop, and hold firmly with one hand while using
the rods in the other to "crush" the tablet. Hold the comparator close to your eye and
face a good light source (daylight, but not the direct rays of the sun; daylight illuminator;
or artificial light reflected from a white surface). Rotate the chlorine color comparator
disc until a color on the disc matches the color of the indicator (left) tube. The reading
can be made directly from the window in front of the comparator. The value is
expressed in miIIigrarns per liter (mg/ I).
d. Procedure for Determining Total Available Chlorine.
(1) The procedure for determining FAC residual in a water sample by
means of the color comparator method (OTA). The procedures for determining TAC