100 QUANTITATIVE AGRICULTURAL ANALYSIS
Glycerol and acetoglycerol are easily soluble in water and they
would therefore be removed in the process of washing the
acetylated oil, so that no error would result from this source.
On the other hand both distearin and monostearin, as well as
their acetylated products, are insoluble in water. On this
account the acetyl value of the partly hydrolyzed oil would be
materially increased.
The free acids produced by hydrolysis, themselves containing
a hydroxyl group, will combine with acetic anhydride to a
varying degree and this will still further increase the acetyl value
of such rancid materials.
It is to be noted also that many of the waxes contain certain
quantities of free higher alcohols and free acids. In consequence,
these waxes will show moderately high acetyl values, as will be
noted in table XII, page 198.
It will be obvious from these considerations that acetyl values
cannot be used with safety for identifying oils unless these are
reasonably fresh. This will be indicated by the acid value, which
should be low.
The most important application of this determination is in
the identification of castor oil. This oil is nearly pure ricinolein,
a glyceride of ricinoleic acid. The latter is hydroxylated oleic
acid,
CHa(CHa) 8CH-OH-CHa-CH = CH(CH2) 7COOH,
and the glyceride, ricinolein, has a theoretical acetyl value of
159.1. Its abundance in castor oil gives the latter an actual
acetyl value of about 150, a value which is far above that of
any other natural oil, only blown oils approaching it in this
respect.
Lastly may be mentioned the occurrence of certain quantities
of free alcohols, especially in the waxes which have, on this
account, appreciable acetyl values. Cholesterol, C27H46OH, in
fats, oils and waxes of animal origin, and its isomers, the phytos-
tcrols, in vegetable oils, are the most important of such alcohols.
Determination of Acetyl Value.—Place about 20 gin, approximately
weighed, of oil or fat in a 100-cc flask, add an equal volume of acetic anhy-
dride, insert a short-stemmed funnel and boil gently for two hours. Cool
and pour into 500 cc of water contained in a beaker. Pass a current of car-