.This determination, as commonly performed on samples of oils, fats, and waxes, is used to indicate the degree of unsaturation of the compound that is, the number of pairs of carbon atoms in which two valency bonds are concentrated between the two carbon atoms. Any such double linkage is in a state of strain, and when the substance containing it comes in contact with certain other substances, one of the bonds breaks and two atoms of the new substance are attached, one to each of the carbon atoms originally joined by the double linkage. Thus every two atoms of the reagent absorbed represent one double linkage in the original substance. The reagent used in the Hubl or Wij method is iodine monochloride, which adds on one atom of iodine to the one carbon atom, and one atom of chlorine to the other. Thus in this case, one atom of iodine absorbed indicates the presence of one double linkage. The method of Hubl is to make up an iodine monochloride solution from iodine and mercuric chloride, and add excess of this to a known weight of the substance. After standing for some lime, the excess of iodine chloride is estimated by the potassium iodide and thiosulphate method: a blank test is carried out alongside, using an equal quantity of the reagent without the substance, and the difference indicates the amount of iodine absorbed by the substance. The iodine value may be defined as the amount of iodine chloride absorbed by 100 grms. of the substance, expressed in terms of iodine. The drying properties of oils are in almost direct ratio of their iodine values. Saturated substances such as the paraffins have iodine value nil.
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