STANDARDIZATION OF SOLUTIONS 77
Materials: 12 N hydrochloric acid.
sodium hydroxide, pellet form.
sodium carbonate, anhydrous.
solution of methyl orange.
Procedure: (a) Prepare an approximately 0.5N HC1 solution
by diluting 21 cc. of 12 N HC1 with 479 cc. of distilled water.
After thoroughly mixing, store it in the glass-stoppered bottle.
. (6) Prepare an approximately 0.5N NaOH solution by weighing
out 10.5 grams of the solid fused NaOH (0.25 F.W., allowing
5 per cent for impurities). After thoroughly mixing with 500 cc.
of distilled water, store it in the rubber-stoppered bottle.
(c) Pour about 10 cc. of the hydrochloric acid solution into one
of the burettes. Rinse it back and forth, and let the acid drain
out of the tip. Repeat the rinsing with two more 10-cc. portions
of acid. Fill the burette with the acid solution and clamp it to a
ring stand. In the same manner rinse and fill the other burette
with the sodium hydroxide solution. Clamp this
burette to the same ring stand. Make sure that
there are no bubbles of air in the tip of the burettes.
If any are found, they must be removed. This
can be done by opening the stop cock and allowing
the solution to run out rapidly. Drain enough of
the solution from the burette to bring the menis-
cus on the graduated scale. Allow the solution
to stand a minute until the film of liquid on the
wall of the tube has time to drain. Read the
position of the lower Gurve of the meniscus. This
should be done by putting a strip of white paper
around the burette with the upper straight edge
two small divisions below the bottom of the menis-
cus and bringing the eye to a position exactly hori-
zontal with upper edge of the paper (Fig. 16).
Record this reading as the initial volume. Do not try to set the
meniscus on any particular line, but read it exactly wherever it
comes to rest. Run approximately 40 cc. of the HC1 into the
300-cc. Erlenmeyer flask; add 100 cc. of water and 2 drops of
methyl orange. Then run in NaOH until the color changes from
pink to yellow. Rotate the flask to insure thorough mixing, and
rinse down the wall of the flask with distilled water from the wash
bottle. Since the "end point" is probably overstepped a little,
FIG. 16