Iodine value
Iodine number or “Iodine absorption value” expresses the percentage of iodine that is absorbed by the wax when treated with a suitable solution of iodine. It indicates the amount of unsaturated fat or wax acids, unsaturated esters, unsaturated hydrocarbons, and unsaturated alcohols inclusive of sterols and resins.[1]
Methods
Several variations of iodine value have been developed, although Hanus method, Wijs method and Hubl method are still widely used. The iodine number is determinated with a “carrier” for the iodine, in which case the solution may be iodine monochloride in glacial acetic acid (Wijs); iodine monobromide in glacial acetic acid (Hanus); or iodine-mercuric chloride in alcohol (Hubl).[1]
Hanus Method
Transfer an accurately weighed quantity of sample, as determinated from the accompanying table, into a 250 mL iodine flask, dissolve it in 10 mL of chloroform, add 25.0 mL of iodobromide 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 N sodium thiosulfate VS until the blue color is discharged. Perform a blank test at the same time with the same quantities of the same reagents and in the same manner. Calculate the Iodine Value from formula:
[126.9(VB-VS)N] 10W
In which 126.9 is the atomic weight form iodine; VB and VS are the volumes, in mL, of 0.1 N sodium thiosulfate VS consumed by the blank test and the actual test, respectively; N is the exact normality of the sodium thiosulfate VS; and W is the weight, in g, of the substance taken for the test. [NOTE-If more than half of the iodobrimide TS is by the portion of the substance taken, repeat the determination, using a smaller portion of the substance under examination.][2]
| Iodine Value expected | Weight in g, ±0.001 |
|---|---|
| <1 | 3.000 |
| 5-20 | 1.000 |
| 21-50 | 0.400 |
| 51-100 | 0.200 |
| 101-150 | 0.130 |
| 151-200 | 0.100 |
WIJS METHOD
Dissolve 16.2 g iodine monochloride (ICl) in a 1 L volumetric flask with glacial acetic acid. Store the Wijs solution in amber bottle sealed with paraffin until it is used. [NOTE- Wijs solution is sensitive to temperature considerably affect titter of the Wijs solution. Therefore it is essential that blanks and samples are titrated at the same time.] Dissolve the sample in 15 mL CCl4, add 25 mL Wijs solution and keep in the dark for approx. 1 hour to complete the reaction. Add deionized water and titrate excess iodine with sodiumthiosulphate. [3]
HubI method:
Substances: (a) Iodine (25 g) is dissolved in 500 cc of 95 per cent ethanol; (b) m..reuric chloride (30 g) is dissolved in 500 cc of 95% ethanol. The reagent is made up by taking an equal volume of (a) and (b) when needed.
To determine iodine number by the Hubl method, weigh out an amount of Sample which will absorb 0.3-0.4 gram of iodine; place this in a clean dry flask and add 10 cc of chloroform. When the sample has completely dissolved, add 25-30 cc of the mixed iodine solution, and stopper the flask carefully. Fill the gutter around the stopper with potassium iodide solution to guard against loss 'of iodine. Shake flask gently and set aside for 3 hours or more in a dink closet. Run blank in a similar dry flask using the same amounts of chloroform, iodine solution, and potassium iodide. Add 100 cc distilled water to each flask and 20 cc of potassium iodide solution. Titrate the excess of iodine at once by means of the standard thiosulfate solution. Attempts have been made to modify the existing methods so as not only to shorten the time of the determination to a matter of minutes but to secure the optimum halogen absorption under all circumstances, and the most reliable iodine numbers. [1]