2-10. DETERMINING CHLORINE RESIDUAL WITH THE COLOR COMPARATOR
a. General. The method for determining the chlorine residual of drinking water
by means of the chlorination kit was discussed in paragraph 2-9. This procedure is only
an approximate method, because (1) it does not distinguish between free available
chlorine (FAC) and combined available chlorine (CAC); (2) there are only three color
standards (1.0, 5.0, and 10.0ppm) between which the observer must interpolate; and (3)
there is no way to compensate for the natural color of the water. The color comparator,
using the modified orthotolidine-sodium arsenite (OTA) method or N, N-diet'hyl-P-
Phenyi-enediamine (DPD), will provide an accurate measure of the free available
chlorine from 0.1 to 10 ppm and compensate for the natural color of the water. All
readings should be taken within 1 minute.
b. Chemical Principles of the Modified Orthotolidine-Sodium Arsenite. The
chemical principles involved in determining the chlorine residual by means of the color
comparator are based upon the reactions of orthotolidine and sodium arsenite with
(1) When orthotolidine is added to water containing chlorine, the two react
to produce a yellow color. The more chlorine, the deeper the yellow.
(a) Orthotolidine reacts rapidly with free available chlorine, producing
color within five seconds.
(b) Orthotolidine reacts more slowly with combined available chlorine,
developing additional color over 5 minutes.
(2) Sodium arsenite also reacts with chlorine, but its reaction is much more
rapid than that of orthotolidine. Sodium arsenite, when added to chlorinated water,
immediately combines with both FAC and CAC. However, the combination of sodium
arsenite and chlorine produces no color.
(a) Sodium arsenite added to chlorinated water before the addition of
orthotolidine will "tie up" (react with) the chlorine and prevent color formation.
(b) Sodium arsenite added to chlorinated water after the addition of
orthotolidine will not change any color that has formed, but it will prevent the formation
of additional color.
(3) Therefore, if orthotolidine is added to a sample of chlorinated water, and
after about 5 seconds, the reaction is "stopped" with sodium arsenite, and the color that
develops is due to FAC.