172

QUANTITATIVE AGRICULTURAL ANALYSIS

Specific Gravity.—In a general way the specific gravity of oils
increases with the per cent of (a) glycerides of unsaturated acids,
(6) glycerides of soluble acids and (c) free fatty acids. Old
oils also usually have higher specific gravities than the normal,
on account of oxidation. The specific gravity of the waxes
and of solid fats is usually higher than of liquid oils. These
rules do not hold in all cases and the determination of specific
gravity, like that of the other constants of oils, is made for com-
paring with recorded data for the purpose of identification more
often than for throwing light upon the chemical constitution
of oils of known purity.

The principles underlying the modes of expression and deter-
mination of specific gravity have been discussed on pages 94 to
102, Part II. Unfortunately there has been a great lack of
uniformity in selecting conditions and modes of expression for spe-
cific gravities of oils as they are recorded in the literature. Tem-
peratures of 15.5°, 17.5°, 20°, 25°, 40°, 60°, 100° and others are
commonly used. In favor of the higher temperatures it may be
said that the fats and waxes are all liquid at these temperatures
so that determinations may readily be made. It has been found1
that a fair degree of approximation may be made in correcting the
specific gravity to another temperature by using the coefficient
0.0007 as the change for each Centigrade degree. This is the
average value for a considerable number of oils between tem-
peratures of 15.5° and 98°. Of course this does not remedy the
lack of uniformity of expression, noted above.

For the determination use a picnometer, a Westphal balance or an
accurately calibrated hydrometer. "If a Westphal balance is used the
displacement of the plummet in pure boiled water should be accurately
determined at the temperature at which the balance is to be used. The
thermometer in the plummet should be compared with a standard ther-
mometer. The picnometer method is recommended.

20°

Determination of Specific Gravity of Oils at r^p—Use a 25-cc specific

gravity bottle (picnometer). Clean with chromic acid, followed by distilled
water/then rinse with alcohol and dry in an oven at 100°. Cool in the bal-
ance case (in which the air should be at a temperature not above 20°) and
weigh. Fill with distilled water which has been recently boiled to expel
dissolved gases and cooled to a few degrees below 20°. Insert the stopper
and nearly immerse the stoppered bottle in a bath of distilled water which is
1 WRIGHT, J. 8nc. Chew. Intl., 26, 513 (1907).