eck in case of dispute or doubt.

_Acetin Method._--This consists in converting the glycerol into its
ester with acetic acid, the acetic triglyceride, or triacetin being
formed. This is then saponified with a known volume of standard alkali,
the excess of which is titrated with acid, and the percentage of
glycerol calculated from the amount of alkali absorbed.

From 1 to 1.5 grammes of the glycerine is weighed into a conical flask
of about 150 c.c. capacity, 7 or 8 c.c. of acetic anhydride added,
together with about 3 grammes of anhydrous sodium acetate, and the whole
boiled on a sand-bath under a reflux condenser for one to one and a half
hours, after which it is allowed to cool, 50 c.c. water added, and the
ester dissolved by shaking, and gently warming, the reflux condenser
still being attached as the acetin is very volatile. The solution is
then filtered from a white flocculent precipitate, which contains most
of the impurities, into a larger conical flask, of some 500-600 c.c.
capacity, and after cooling, rendered just neutral to phenol-phthalein
by means of N/2 caustic soda solution, the exact point being reached
when the solution acquires a reddish-yellow tint; 25 c.c. of a strong
caustic soda solution is then added, and the liquid boiled for about
fifteen minutes, the excess of alkali being titrated after cooling, with
N/1 or N/2 hydrochloric acid. A blank experiment is carried out
simultaneously, with another 25 c.c. of the soda solution, and the
difference in the amounts of acid required by the two, furnishes a
measure of the alkali required to saponify the acetin formed, and hence
the amount of glycerol in the crude glycerine may be calculated.

_Example._--1.4367 grammes crude glycerine, after treatment with acetic
anhydride, and neutralising, was saponified with 25 c.c. of a 10 per
cent. caustic soda solution.

The blank experiment required 111.05 c.c. N/1 hydrochloric acid.
Flask containing acetin " 75.3 c.c. " "
-----
35.75 c.c. " "

Hence, the acetin formed from the glycerol present in 1.4367 grammes of
the crude glycerine required 35.75 c.c. N/1 caustic alkali for its
saponification, so that the percentage of glycerol may be calculated
from the following formula:--

35.75 x 0.03067 x 100
Per cent. glycerol = --------------------- = 76.3.