clean. Do not touch the dropper. Never use the dropper to stir the sample. As soon as the solution has been introduced
into the tube, replace the dropper in the bottle of indicator solution.
Color and turbidity. To eliminate errors due to natural color and turbidity of the sample, make sure that
water is added to the right-hand tube before making color comparison.
Sunlight. Do not allow direct sunlight to fall on the samples being tested. Sunlight causes the color
developed by the indicator solution to fade.
See general test precautions above.
Place the chlorine color comparator disc in the comparator.
Thoroughly rinse two rectangular comparator tubes and fill both tubes with the water sample exactly to the
etched mark on the tubes.
Hold the comparator in one hand with the eyepiece facing you, and place the two water filled tubes into the
openings at the top of the comparator. Hold the sodium arsenite and orthotolidine bottles between the thumb and
forefinger of the hand holding the comparator.
Fill the droppers in the caps of the sodium arsenite and orthotolidine bottles and let the dropper caps sit
loosely in bottles. Do not screw the caps back on the bottles.
Preparing the blank: Squirt with force, one dropper full (not less than 0.75 ml) of sodium arsenite into the
tube in the right opening of the comparator. Forcefull addition is very important in order to get good mixing of the
chemicals in the water. Then squirt with force, one dropper full (not less than 0.75 ml) of orthotolidine into the same
Refill the sodium arsenite and orthotolidine droppers, and let the droppers sit loosely in the bottles.
Preparing the indicator: Squirt with force the dropper full of orthotolidine into the tube in the left opening of
the comparator. Immediately squirt with force, the dropper of sodium arsenite into the same tube. The immediate
addition of the sodium arsenite is important to stop any combined chloride that may be present from erroneously
increasing the yellow color. Push both tubes down.
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). Be sure your fingers do not cover the light
window in the back of the comparator. Rotate the chlorine color comparator disc until a color