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Determination of Iodine Value


Learn how to determine the Iodine Value using different methods - Iodine Monochloride Method or Wijs Method, Iodine Monobromide Method or Hanus Method and Pyridine Bromide Method.

The iodine value is the number which expresses in grams the quantity of halogen, calculated as iodine, which is absorbed by 100 g of the substance under the described conditions. It may be determined by any of the following methods.

Method A

(Iodine Monochloride Method or Wijs Method)
Place an accurately weighed quantity of the substance under examination in a dry 500-ml iodine flask, add 10 ml of carbon tetrachloride and dissolve. Add 20 ml of iodine monochloride solution, insert the stopper and allow to stand in the dark at a temperature between] 5° and 25° for 30 minutes. Place 15 ml of potassium iodide solution in the cup top, carefully remove the stopper, rinse the stopper and the sides of the flask with
100 ml of water, shake and titrate with 0.1 M sodium thiosulphate using starch solution, added towards the end of the titration, as indicator. Note the number of m] required
(a). Repeat the operation without the substance under examination and note the number of ml required (b).
Calculate the iodine value from the expression
Iodine value = 1.269 (b - a)/w
where, w = weight, in g, of the substance.
The approximate weight, in g, of the substance to be taken may be calculated by dividing 20 by the highest expected iodine value. If more than half the available halogen is absorbed, the test must be repeated with a smaller quantity of the substance.

Method B

(Iodine Monobromide Method or Hanus Method)
Unless otherwise specified, weigh accurately the quantity of the substance under examination, stated in the table (see below), place it in a dry 300-ml iodine flask or which has been rinsed with glacial acetic acid unless otherwise specified in the monograph. Add 15 ml of chloroform and dissolve. Add slowly from a burette 25.0 ml of iodine monobromide solution, insert the stopper, allow to stand in the dark for 30 minutes, unless otherwise specified in the monograph, shaking frequently. Add 10 ml of potassium iodide solution and 100 ml
Iodine Value and Substance Weightof water and titrate with 0.1 M sodium thiosulphate using starch solution, added towards the end of the titration, as indicator. Note the number of ml required (a). Repeat the operation without the substance under examination and note the number of ml required (b). Calculate the iodine value from the expression given under Method A.
The approximate weight, in g, of the substance to be taken, unless otherwise specified in the monograph, may be calculated from the table.

Method C

(Pyridine Bromide Method)
Place an accurately weighed quantity of the substance under examination in a dry iodine flask, add 10 ml of carbon tetrachloride and dissolve. Add 25 ml of pyridine bromide solution, allow to stand for 10 minutes in the dark and complete the determination described under Method A beginning at the words "Place 15 ml of...". The approximate weight, in g, of the substance to be taken may be calculated by dividing 12.5 by the highest expected iodine value. If more than half the available halogen is absorbed, the test must be repeated with a smaller quantity of the substance.



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