Determination of the Iodine Number of Raw Rubber

In the course of an investigation dealing with the nature of the state of unsaturation of raw rubber, it became necessary to have available a precise and convenient method for determining the iodine number of raw rubber. Though iodine reacts only slowly with unsaturated compounds, chlorine and bromine form both addition and substitution products. In the substitution reactions, the hydrogen halide is evolved, and by addition of potassium iodate, e.g., in the method of MacIlhiney, it is possible to determine the hydroden halide formed and thus the quantity of halogen substituted. In all methods, however, where extensive substitution as well as addition take place, the results are variable and uncertain. Accordingly in the chemistry of fats where the iodine number is of the greatest importance, iodine halides have been used for determining the iodine number, for under certain conditions there is very little substitution. Thus Hübl, Waller and Wijs solutions contain iodine chloride and Hanus solution contains iodine bromine. The Hanus method has in fact to a great extent replaced all other methods For determining the iodine number of rubber, the Wijs method in the form in which it was modified by Kemp is probably used more than any other. In the Kemp method about 0.1 gram of rubber is swollen in 75 cc. of carbon disulfide, and is then allowed to react for 2 hours at 0° C. with 25 cc. of Wijs solution, i.e., 0.5 N iodine chloride in acetic acid.