Title: Determination of Iodine value
1. Objective: To determine the Iodine value of the sample.
- Principle: 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.
3. Procedure:
- Method A: (Iodine Monochloride Method or Wijs Method)
Place an accurately weighed quantity of the substance being examined 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 15°C and 25°C for 30 minutes. Place 15 ml 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 0.1M Sodium Thiosulphate using Starch solution, added towards the end of the titration, as indicator. Note the number of ml required (a). Repeat the operation omitting the substance being examined and note the number of ml required (b).
Calculate the iodine value from the expression
Iodine value = 1.269 x (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.
Preparation of Reagents:
- Iodine Monochloride solution: Dissolve 8 g of Iodine Trichloride in about 200 ml of glacial acetic acid and separately dissolve 9 g of iodine in 300 ml of dichloromethane. Mix the two solutions and dilute to 1000 ml with glacial acetic acid. Store the solution in stopperred, light-resistant glass container at a temperature not exceeding 15o
- Potassium Iodide solution: Dissolve 16.6 g of Potassium Iodide in sufficient water and dilute to 100 ml with water.
- Method B: (Iodine Monobromide Method or Hanus Method)
Unless otherwise specified, weigh accurately the quantity of the substance being examined, stated in Table 1, 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 of water and titrate with 0.1M Sodium Thiosulphate using Starch solution, added towards the end of the titration, as indicator (red brown colour to blue violet end point). Note the number of ml required (a). Repeat the operation omitting the substance being examined 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 Table 1.
Table 1
| Presumed Iodine Value | Quantity of the substance (g) |
| Less than 20 | 1.0 |
| 20 to 60 | 0.25 to 0.5 |
| 61 to 100 | 0.15 to 0.25 |
| More than 100 | 0.10 to 0.15 |
Preparation of Reagents:
- Iodine Monobromide solution: Dissolve 20 g of Iodine bromide in sufficient glacial acetic acid produce to 1000 ml.
- Method C: (Pyridine Bromide Method)
Place an accurately weighed quantity of the substance being examined 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.
Preparation of Reagents:
- Pyridine Bromide solution: Dissolve 8 g of Pyridine and 5.4 ml of concentrated Sulphuric acid in 20 ml of glacial acetic acid, keeping the mixture cool. Add 2.5 ml of Bromine dissolved in 20 ml of glacial acetic acid and dilute with glacial acetic acid to 1000 ml.