12. DETERMINATION OF IODINE NUMBER
The iodine number of a fat or oil represents
the number of grams of iodine capable of being absorbed under certain fixed conditions
by 100 g. of the substance. It is an indication of the degree of unsaturation in
the fatty acid radical of the glycerides.
The use of iodine numbers for the evaluation of
essential oils has never attained practical significance. This is due primarily
to the unpredictable behavior of such oils in the presence of iodine solutions.
It has been shown frequently that the iodine numbers of many essential oils vary
with the size of the sample as well as with the period of contact with the reagent.
Furthermore, the results do not correspond with the theoretical values
expected.
In the case of fixed oils the iodine number is
an important criterion of purity. Since the essential oil chemist occasionally is
faced with the evaluation of such fixed oils as persic oil, sweet almond oil, olive
oil, castor oil, and sesame oil, a procedure for the determination of iodine numbers
is included here.
Procedure:127 Introduce about 0.25
g. of the oil, accurately weighed,128 into a glass stoppered Erlenmeyer
flask of 250 cc. capacity, dissolve it in 10 cc. of chloroform, add 25 cc. of
iodobromide solution, accurately measured from a burette, stopper the vessel
securely, and allow it to stand for 30 min.129 protected from light. Then add in
the order named 30 cc. of a 1 N potassium iodide solution and 100 cc. of distilled
water, and titrate the liberated iodine with tenth-normal sodium thiosulfate, shaking
thoroughly after each addition of thiosulfate. When the iodine color becomes quite
pale, add 1 cc. of a 1% starch indicator solution and continue the titration with
thiosulfate until the blue color is discharged. Carry out a blank test at the same
time with the same quantities of chloroform and iodobromide solution, allowing it
to stand for the same length of time and titrating as directed. The difference between
the number of cc. of thiosulfate consumed by the blank test and the actual test,
multiplied by 1.269 and divided by the weight of sample taken, gives the iodine
number. If more than half of the iodobromide solution is absorbed by the sample
of the substance taken, the determination must be repeated, using a smaller sample
of the substance under examination.
The iodobromide solution may be prepared by the
following method:
Dissolve 13.2 g. of reagent iodine in 1,000 cc.
of glacial acetic acid with the aid of gentle heat if necessary. Cool the solution
to 25 and determine the iodine content in 20 cc. by titration with tenth-normal
sodium thiosulfate. Add to the remainder of the solution a quantity of bromine equivalent
to that of the iodine present. Preserve in glass-stoppered bottles, protected from
light.
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