TM 5-4610-218-12
parator to a reading position, with the prism 8 to 12
the disc until the best match is obtained between
inches (20 to 30 cm) in front of the eyes.
the sample and one of the glass standards. Hold the
comparator so that prism is approximately 12
(9) Match as closely as possible, the color de-
inches (30 cm) from the eye. Read the residual value
veloped in the second tube with a glass color stan-
in the opening in the front upper left-hand corner of
dard by revolving disc containing the permanent
the comparator. If the sample color falls between
glass color pH standards. The figure appearing in
two of the glass standards, the residual value must
the upper left-hand opening in the comparator will
be estimated.
be the pH glass standard value. If the sample color
falls between two of the glass standards, the pH is
(8) When the test has been completed, remove
obtained by interpolation.
both sample tubes from the comparator, empty out
the samples and wash the tubes with clean water.
(10) When the test has been completed, re-
move both sample tubes from the comparator,
2-20. Determination of pH Value.
empty out the samples and wash the tubes with
clean water.
parator measures them as simple numbers con-
2-21. Test Procedure, Free Chlorine Residual.
forming to the hydrogen ion scale. The pH color disc
furnished with this comparator contains nine per-
a. Place a chlorine color comparator disc of an
manent values. The reagent bottle with color coded
appropriate range in the comparator.
label and cap is kept with the pH indicator solution.
b. Thoroughly rinse two rectangular comparator
The pH color disc being used in the comparator is
tubes and fill both tubes with water sample exactly
marked to show the solution to be used.
to the etched mark on the tubes.
b. Test Procedure.
(1) See general test precautions in paragraph
piece facing you, and place the two water-filled
2-18.
tubes into the openings at the top of the compara-
tor. Hold the sodium arsenite and orthotolidine bot-
(2) Remove two knurled screws on front of the
tles between the index and middle fingers of the
comparator and lift off front cover. Install pH range
hand holding the comparator.
disc on comparator body so that the recess at the
center of the disc fits over a hub on the body, replace
d. Fill the droppers in the caps of the sodium ar-
front cover and install knurled screws.
senite and orthotolidine bottles and let the dropper
caps sit loosely in the bottles. Do not screw the caps
back on the bottles.
gaging two pins in the prism in matching holes in
the front cover.
e. Squirt with force, one dropper full (not less
than 0.75 ml) of sodium arsenite into the tube in the
(4) Wash two sample tubes and fill one tube to
right opening of the comparator. The forceful addi-
the graduation mark with the water to be tested.
tion of the orthotolidine and sodium arsenite to the
Place this tube in the right-hand compartment of
tubes is very important in order to get good mixing
the comparator. This tube when filled to the mark
of the chemicals in the water. Then squirt with
with water under test, compensates for color and
force, one dropper full (not less than 0.75 ml) of or-
turbidity. The omission of this step may cause seri-
thotolidine into the same tube. This is the blank.
ous errors.
f. Refill the sodium arsenite and orthotolidine
(5) Fill the graduated dropper with 0.50 ml of
droppers, and let the droppers sit loosely in the bot-
the proper pH indicator solution, 0.60 ml is indi-
tles.
cated by the lower line of the dropper. Do not touch
the sample of the tube with the dropper or permit it
g. Squirt with force, the dropper full of ortho-
to touch other objects. Add this solution to the sec-
tolidine into the tube in the left opening of the com-
ond tube.
parator. Immediately squirt with force, the dropper
of sodium arsenite into the same tube. The immedi-
(6) Fill the second tube to the graduation
ate addition of the sodium arsenite is important to
mark with water to be tested.
make a correct residual reading. This is the indi-
(7) Place the second tube in the center com-
cator.
partment of the comparator. Make sure that both
h. Hold the comparator to your eye, and face a
sample tubes are pushed down as far as they will go.
good light source (daylight but not the direct rays of
(8) To obtain a pH reading, bring the com-
2-55
