ANALYTICAL cHEMISTRY - IV

MSCM302

Experiment 1

Aim of the Experiment

Determination of the iodine value of oils.

Principle

The vegetable oil sample is reacted with an excess of iodine mono-chloride (ICl) solution (Wij’s reagent) under controlled conditions, the halogen adds quantitatively to double bonds in the unsaturated fatty acids. The un-reacted iodine is determined by titration with Na₂S₂O₃.5H₂O solution. The iodine number is defined as the grams of halogens expressed as iodine, reacting with 100 gm of oil.
The iodine values of some oils are:

Cod liver oil: 145-180;

Palm oil: 50-55;

Butter: 26-40;

Olive oil: 80-88;

Castor oil: 82-90;

Coconut oil: 7-10;

Corn oil: 109-133;

Cottonseed oil: 100-117;

Sunflower oil: 118-144;

Peanut oil: 84-106

Schematic reaction showing halogenation at the unsaturation in the oil

Apparatus required

Iodine flask, Burette, Pipette, Funnel, Beaker, etc.

Chemicals required

Carbontetrachloride (CCl₄), Starch solution (as an indicator), 30% KI solution, Wij’s reagent, 0.1 N Na₂S₂O₃.5H₂O solution, oil samples, etc.

Preparation of Reagents

Wij’s reagent:

Dissolve 6.5 g of iodine in 500 mL glacial acetic acid (warming in water both may be required). Pipette out 10 mL of this solution in 250 mL iodine flask. Add 5 mL 30% KI solution and 50 mL distilled water. Titrate it against 0.1 N Na₂S₂O₃.5H₂O solution adding starch as indicator. This gives the exact normality of iodine solution.

Now, take 100 mL of iodine solution in iodine flask and pass chlorine gas for about 30 sec to 1 min (color of the iodine solution changes).

Pipette out 10 mL of this solution and add 5 mL 30% KI and 100 mL distilled water to it. Titrate against 0.1 N Na₂S₂O₃.5H₂O solution from burette.

Calculation of halogen ratio:

I₂/Cl₂ ratio = 2A/[3B − 2A]

Where A = Burette reading before passing Cl₂ gas

B = Burette reading after passing Cl₂ gas

I₂/Cl₂ ratio = 2(10.3)/[3(27.9) − 2(10.3)] = 0.33

30% KI solution:

Dissolve 30 g of KI in 100 mL distilled water.

0.1 N Na₂S₂O₃ .5H₂O solution:

1000 mL 1 N Na₂S₂O₃.5H₂O = 248.19 g

250 mL 0.1 N Na₂S₂O₃.5H₂O = X g.

X = ____________ g.

Procedure

In an iodine flask, take 1.0 g oil sample and to it add 25 ml of Wij’s reagent and 20 ml carbontetrachloride (CCl₄). Keep it in dark for 30 minutes.
Similarly, for blank take 25 ml of Wij’s reagent in iodine flask add 20 ml CCl₄ to it and keep in dark for 30 minutes.
After 30 minutes, Add 10 ml 30% KI to each flask and 100 ml water. Titrate it against 0.1 N Na₂S₂O₃ .5H₂O solution using starch as indicator.

To determine the iodine value of oil sample by Wij's solution.mp4

Observation

Burette readings are,

For blank - _____ mL

For ground nut oil - _____ mL

For corn oil - _____ mL

For castor oil - _____ mL

For used corn oil - _____ mL

Calculation

Result

Iodine values for the respective oils are,

Ground nut oil __________ g of I₂/100 g of oil
Corn oil __________ g of I₂/100 g of oil
Castor oil __________ g of I₂/100 g of oil
Used ground nut oil __________ g of I₂/100 g of oil

Reference Material

G H Jeffery, J Bassett, J Mendham and R C Denney, Vogel's Textbook of Quantitative Chemical Analysis, 5th Edition
S. Suzanne Nielsen, Food analysis, 4th Edition

Questions

What is relevance of iodine value of oil with respect to the quality of an oil?
What other reagents can be used instead of Wij's reagent for the determination of iodine value of an oil?
Why there is a need of keeping the solution in dark after addition of Wij's reagent?

Developed by

Dr. Viraj Bhanvadia,

Assistant Professor, Chemistry,

viraj.bhanvadia@gsfcuniversity.ac.in

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