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5.6 IODINE VALUE

 ​            The iodine value represents the number of g of iodine absorbed, under the prescribed conditions, by 100 g of the substance. Unless otherwise specified in the individual monograph, determine the Iodine Value by Method I and use the following quantities of the substance for both methods:

Method I

 ​            Transfer an accurately weighed quantity of the substance, as determined from the accompanying table, into a 250-ml iodine flask, dissolve it in 10 ml of chloroform, add 25.0 ml of iodine bromide TS, insert the stopper in the vessel securely, and allow it to stand for 30 minutes protected from light, with occasional shaking. Then add, in the order named, 30 ml of potassium iodide TS and 100 ml of water, and titrate the liberated iodine with 0.1 M sodium thiosulfate VS, shaking thoroughly after each addition of thiosulfate. When the iodine colour becomes quite pale, add 3 ml of starch TS, and continue the titration with 0.1 M sodium thiosulfate VS until the blue colour is discharged. At the same time, perform a blank test in exactly the same manner (Residual Titrations, Appendix 6.17).

 ​            Calculate the iodine value from the equation: 

where 126.9 = the atomic weight of iodine,

 ​           b = the volumes, in ml, of 0.1 M sodium thiosulfate VS consumed by the blank,

 ​           a = the volumes, in ml, of 0.1 M sodium thiosulfate VS consumed by the actual test,

 ​           M = the exact molarity of the sodium thiosulfate VS, and

 ​           W = the weight, in g, of the substance.

(Note If more than half of the iodine bromide TS is absorbed by the portion of the substance taken, repeat the determination, using a smaller portion of the substance under examination.)

Method II

 ​           Preparation of the sample Melt the substance, if it is not already liquid. (Note The temperature during melting should not exceed the melting point of the substance by more than 10º.) Pass through two pieces of filter paper to remove any solid impurities and the last traces of moisture. The filtration may be performed in a hot air oven at 100º but should be completed within 5 minutes ±30 seconds. The substance must be absolutely dry. All glassware must be absolutely clean and completely dry. After filtration, allow the filtrate to achieve a temperature of 68º to 72º befores weighing the substance.

 ​           Procedure Immediately weigh the sample into a 500-ml iodine flask, using the weights and weighing accuracy noted in the accompanying table. (Note The weight of the substance must be such that there will be an excess of iodochloride TS of 50 to 60 per cent of the amount added, that is, 100 to 150 per cent of the amount absorbed.) Add 15 ml of a fresh mixture of cyclohexane and glacial acetic acid (1:1), and swirl to dissolve the sample. Add 25.0 ml of iodinechloride TS, insert the stopper securely in the flask, and swirl to mix. Allow it to stand at 25º±5º, protected from light, with occasional shaking, for 1.0 or 2.0 hours, depending on the Iodine Value (IV) of the sample: IV less than 150, 1.0 hour; IV equal to or more than 150, 2.0 hours. Then, within 3 minutes after the indicated reaction time, add, in the order named, 20 ml of potassium iodide TS and 150 ml of recently boiled and cooled water, and mix. Within 30 minutes, titrate the liberated iodine with 0.1 M sodium thiosulfate VS, while stirring by mechanical means after each addition of thiosulfate. When the yellow iodine colour has almost disappeared, add 1 to 2 ml of starch TS, and continue the titration with 0.1 M sodium thiosulfate VS until the blue colour is discharged. At the same time, perform a blank test in exactly the same manner (Residual Titrations, Appendix 6.17). Calculate the iodine value from the equation as described under Method I.