188 QUANTITATIVE AGRICULTURAL ANALYSIS
Effects of Soluble or Volatile Acids.—It should be noticed that
whether the distillation or the filtration process is employed,
the standard base required finally to titrate the acid will include
that equivalent to acids other than acetic. That is, the distilla-
tion process will yield a distillate of acetic acid and volatile
organic acids while the filtration process will yield a filtrate con-
taining acetic acid and soluble organic acids. The close relation
between soluble acids and volatile acids has already been dis-
cussed (page 183). To correct for the presence of these acids in
the solution containing the acetic acid one may either subtract
the volume of base used in the determination of soluble (or vola-
tile) acids, or a different method may be used. As a rule this
correction will be small except with oils showing a high soluble-
acid number or Reichert-Meissl number and in these cases the
acetyl value is nearly zero, so that it is of little use as a means for
identifying the oils.
The "acetyl value7' is defined to be the number of milligrams of I
potassium hydroxide required to combine with the acetic acid lib- I
erated from 1 gm of acetylated fat or oil. Certain oils are charac- |
terized by unusually high acetyl values and it is only in these
cases that the determination will be of value for oil testing. \
Castor oil is the most noteworthy of these, having a value of
about 150. Another class of oils having high acetyl values is
composed of "blown" or "oxidized" oils. By blowing air
through oils at somewhat elevated temperatures (70° to 115°) the \
viscosity and specific gravity are considerably increased and they •
become suitable for use as lubricating oils. The chemical
changes that take place are not thoroughly understood but j
oxidation is known to occur. This is partly due to combination
with unsaturated acids (evidenced by a diminished iodine absorp- \
tion number) and partly to the formation of hydroxyl radicals 1
from hydrogen. The latter change results in an increased j
acetyl value and this may even reach a number as great as that j
for castor oil. \
The large variation in acetyl values recorded in Table X will I
indicate the value of this determination for the identification of f
certain oils and fats. In other cases the determination will have ''
little value.