c. Procedure for Determining Free Available Chlorine.
(1) Modified modified orthotolidine-sodium arsenite method. The procedure
for determining FAC residual in a water sample by means of the color comparator is
referred to as the OTA method. The two comparator tubes are filled with the water to
be tested. A control tube, called a "blank," is first prepared by adding sodium arsenite,
then orthotolidine. Any color formed is due to the natural color of the water or interfering
substances, such as iron or manganese. Orthotolidine is added to a second tube
(called the "indicator"). Then, within 5 seconds, sodium arsenite is added to the
indicator. Any difference in color between the blank and the indicator is due to the
reaction of orthotolidine with FAC. The difference may be read by means of the chlorine
disk in the comparator. Figures 2-4 A through H illustrate the step-by-step procedure. It
is important in conducting the test that:
(a) The comparator tubes be chemically clean before filling them with
the sample water.
(b) The reagent droppers and comparator tubes be filled exactly to the
(c) The reading not be made in direct sunlight since this will cause
fading of the color.
A. Place the two water-filled tubs into the openings
at the top
of the comparator.
Fill droppers of sodium arsenite and orthotolidine bottles and let droppers set
loosely in the bottles.