SAPONIFIABLE OILS, FATS AND WAXES

175

Determination of Index of Refraction by Means of the Abbe Refrac-
tometer.—Set up the instrument in front of a window or any artificial light
•source, noting, that, monochromatic light is not essential. Connect a
constant temperature apparatus furnished with the instrument and adjust
the flow of water and the height of the flame until the desired temperature
(20° for oils, 40° or higher for fats or waxes) is attained. Open the prism
so that the lower half is in a horizontal position and place two or three
drops of oil or melted fat or wax upon it; using a glass rod or pipette but
avoiding scratching the prisms. Quickly close and lock the system, allow
time for the temperature to become constant and then adjust the com-
pensator and focus until the line of division of the field is sharply defined and
bring this to the cross hairs. Read the index of refraction upon the scale.
Clean the prisms by applying a mixture of equal volumes of alcohol and
ether, using a tuft of absorbent cotton.
Melting Point of Fats.—From the fact that fats are mixtures
and not pure compounds, it will be seen that they cannot have
definite and sharp melting points. The observation will there-
fore be a somewhat arbitrary one. The following is Wiley's
method.
Determination.—Prepare discs of fat as follows: Allow the melted and
filtered fat to fall a distance of about 20 cm, from, a dropping tube to a
piece of ice or to the surface of cold mercury. The discs thus formed should
be 1 to 1.5 cm in diameter and they should weigh about 200 mg. Since a
recently melted and solidified fat does not have its normal melting point the
discs should stand two to three hours before testing.
Prepare an alcohol-water mixture of graduated density, as follows: Boil,
separately, water and 95-per cent alcohol for ten minutes to remove dis-
solved gases. While still hot pour the water into a 20 cm test tube until
it is almost half full. Nearly fill the tube with the hot alcohol, pouring
down the side of the inclined tube, to avoid too much mixing.
Place the test tube containing the alcohol-water mixture in a tall beaker
containing ice water, until cold. Drop the disc of fat into the tube and it
will at once sink to a point where the density of the mixture is exactly equal
to its own. Lower an accurate thermometer, graduated to tenths, into the
test tube until the bulb is just above the disc, stirring very gently. Slowly
heat the water in the beaker, stirring constantly with an air blast or
mechanical stirrer.
When the temperature of the alcohol-water mixture has risen to a point
about 6° below the melting point of the fat the disc will begin to shrivel and
roll into an irregular mass. Now lower the thermometer until the fat particle
is even with the center of the bulb. Rotate the thermometer gently and
regulate the temperature so that about 10 minutes is required for the last
increment of 2°. As soon as the fat becomes a spherical globule read the
thermometer. This serves as a preliminary determination of melting point.