SAPONIFIABLE OILS, FATS AND WAXES 173
kept at exactly 20°. After 30 minutes take off the drop of water from
the tip of the stopper, remove the bottle and wipe perfectly dry with a
clean towel but without warming the bottle to above 20°. Place in the
balance case and weigh after 15 minutes. Calculate the weight of con-
tained water.
Empty the bottle and dry inside and out, then fill with oil and ma-
nipulate as before, calculating the weight of contained oil. This weight
divided by the weight of contained water gives the specific gravity of the
-1 4.2°°
oil at 2Qo-
If the specific gravity has been determined at any other temperature or if
it is desired to calculate the specific gravity at any temperature from the
determination at 20°, use the following formula:
0 = Gf 4- 0.0007 (*' - *), where
G — specific gravity at temperature t,
Gr = specific gravity at temperature t'.
20°
Determination of Specific Gravity at -70--—Multiply the specific gravity
20°
at SQO by 0.99897, which is the density of water at 20°. The product is the
20°
specific gravity of the oil at-jr- (See page 94.)
Determination at the Temperature of Boiling Water.—Fill a 25-cc
picnometer, dried and weighed as above described, with freshly boiled
hot water. Nearly immerse in a bath of briskly boiling water and leave
for 30 minutes, replacing evaporated water with boiling distilled water.
Insert the stopper, previously heated to 100°, remove the picnometer
from the bath, wipe dry, cool to room temperature and weigh. Cal-
culate the weight of contained water.
Fill the flask, dried at 100°, with the dry, hot, freshly filtered fat or oil,
which must be entirely free from air bubbles. Keep in the boiling
water bath for 30 minutes then insert the stopper, which has been heated
to 100°, wipe dry, cool to room temperature and weigh. Calculate the
weight of contained oil and from this and the weight of water contained
at boiling temperature calculate the specific gravity of the oil at the temper-
ature of boiling water.
This determination is necessarily less accurate than the one
at 20°, on account of the difficulty involved in keeping the bath
at any constant temperature. Superheating may easily occur
with distilled water and less pure water may have a boiling
point above 100°. Variation in barometric pressure will also
change the temperature of the bath so that it becomes necessary
to carry out both parts of the experiment at the same atmos-
pheric pressure. However the determination is sanctioned and
has been made official by the Association of Official Agricultural
Chemists.