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QUANTITATIVE AGRICULTURAL ANALYSIS

f, '11

I

Iodine monobromide is absorbed at a double bond thus:
—C = C 4- IBr -»—C—C—
I 1
I Br
Acid Value.—Fresh oils sometimes contain small amounts of
free fatty acids produced during the process of extraction.
Rancid fats and oils, contain free acids as products of hydroly-
sis of the glycerides composing them. The acid value is defined
as the number of milligrams of potassium hydroxide required to
neutralize the free fatty acids in 1 gm of oil or fat. Acidity is also
sometimes expressed in terms of oleic acid as per cent, or as "acid
degree," which is cubic centimeters of normal base equivalent to the
free acids in 100 gm of oil or fat. The determination of acid
value is made for the purpose of determining the condition of the
oil and its fitness for a given use, rather than for the purpose of
identifying it, since the acid value is a variable within rather wide
limits for any oil.
Determination of Acid Value.—Weigh 20 gm of oil or fat into a 200-cc
flask and add 50 cc of 95-per cent alcohol which has been made neutral to
phenolphthalein by a dilute solution of sodium hydroxide. Heat to the
boiling-point in a steam bath and agitate thoroughly. Titrate with a tenth-
normal solution of sodium or potassium hydroxide, using phenolphthalein.
Shake vigorously during the titration and add the standard solution until
the pink color persists for a short time. An absolutely permanent color
cannot be obtained because any excess of base will finally saponify the oil
and thereby become neutralized.
Saponification (Kottstorfer) Number.—The saponification
number is the number of milligrams of potassium hydroxide
required to saponify 1 gm of oil or fat. Different oils show differ-
ent saponification numbers because of variation in the molecular
weight of the esters contained in them, those of relatively low
average molecular weights requiring more base for the saponifica-
tion of a given weight of oil than those of higher molecular
weights. The variation is, however, not as great as is the case
with iodine absorption numbers and the saponification number
is consequently not as valuable for use in identifying oils as is
the iodine number.
Notable exceptions to this rule are butter and cocoanut fat,
on the one hand, and the true waxes on the other. Of these the