182

QUANTITATIVE AGRICULTURAL ANALYSIS

weights themselves, it is not to be expected that there would be a
large variation in either the Hehner value or the per cent of
soluble acids. The former has an average value of about 95 and
the latter of considerably less than 1. Therefore these numbers are
without any great significance in most cases and their determina-
tion will give little assistance in the task of identifying most
oils. A few exceptions to this statement should be noticed.
Butter has already been mentioned as containing unusually
large quantities of butyric, caproic, caprylic and capric acids.
Consequently its Hehner value falls to 88-90 and its per cent of
soluble acids rises to about 5. Other notable exceptions are
cocoanut, palm nut and croton oils. Practically, it is in these
cases only that the determination of soluble and insoluble acids
will be of any great use.
Determination of Saponification Number.—Prepare the following
solutions:
(a) Alcoholic Base.—Purify 2 liters of alcohol by heating on a steam
bath for 3 hours with about 10 gm of sodium hydroxide, using a reflux
condenser. Distill and make 1000 cc of a solution of 40 gm of potassium
hydroxide in the alcohol. The potassium hydroxide should be as nearly
free from carbonate as is possible. Allow the solution to stand until
the small amount of potassium carbonate that is always present has settled
out, then decant into another bottle. The concentration does not remain
constant for long and the solution should not be standardized, except by a
blank determination, made at the time saponification number is determined.
(6) Prepare also a half-normal solution of hydrochloric acid in water.
Select two ordinary flasks of 250-cc capacity having, if possible, necks of
slightly larger diameter at the top than at the bottom, though this feature
is not essential. Clean with alcohol. Weigh into each flask about 5 gm
of oil or fat, using a small bottle and glass rod as in the determination of
iodine number. Add to each flask 50 cc of the alcoholic solution of potas-
sium hydroxide'from a calibrated pipette or burette, place in the neck of the
flask a funnel having a short stem and warm on the water bath until the
alcohol boils, though it should not be evaporated more than is necessary.
The oil is usually saponified in about 30 minutes. A homogeneous solution
must be produced, so that no separation will occur when boiling is inter-
rupted. Measure 50 cc of the alcohol solution of potassium hydroxide into
each of two other flasks, for standardization. While saponification of the oil
is proceeding titrate these solutions with the half-normal acid, using phenol-
phthalein. Cool the flasks in which the oil was saponified, add a drop of
phenolphthalein and titrate the excess of base with half-normal acid, deduct
from the volume used for 50 cc of the base in the standardization and calcu-
late the saponification number.