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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