Iodine value

The Iodine Value (IV), also known as the Iodine Number. It is a fundamental analytical parameter in the analysis of fats and oils. It is a direct measure of the degree of unsaturation (number of double bonds) in a lipid sample.

Definition

The Iodine Value is defined as the number of grams of iodine absorbed by 100 grams of a fat or oil. Higher Iodine Value means greater number of double bonds which shows higher degree of unsaturation.

Principle

The determination is based on the electrophilic addition reaction of halogens across the carbon-carbon double bonds (-C=C-) of unsaturated fatty acids. Since pure iodine reacts very slowly with double bonds, “Iodine Reagents” (like Iodine Monochloride) are used. The reaction follows these steps:

• An excess amount of halogenating reagent is added to the fat sample.
• The halogen adds to the double bonds.
• The unreacted (excess) halogen is then determined by back titration using sodium thiosulfate.
• The difference between the initial amount added and the amount remaining gives the amount of iodine consumed by the fat.

Theory (Chemical Reactions)

Most commonly, Wijs Method is used, where the reagent is Iodine Monochloride (ICl). Because Chlorine is more electronegative than Iodine, the ICl molecule is polarized (I^δ+-Cl^δ-).

Step 1: Electrophilic Addition Reaction– The ICl acts as an electrophile. The ICl reacts with the unsaturated fat. The pi (π) electrons of the fatty acid double bond attack the positively polarized Iodine atom. The Chloride ion (Cl^–) then attacks the carbon from the opposite side (Anti-addition).

R-CH=CH-R’ + ICl → R-CH(I)-CH(Cl)-R’

Step 2: Release of Free Iodine– The excess (unreacted) ICl is treated with Potassium Iodide (KI) to stop the reaction. It converts the remaining, unreacted ICl into molecular Iodine (I₂), which is stable enough to be titrated. Free iodine is not very soluble in water. The excess KI reacts with I₂ to form the Tri-iodide complex (I₃^–), which is water-soluble and reacts easily with the titrant.

ICl + KI → KCl + I₂ ; I₂ + I^– ⇌ I₃

^–Step 3: Titration- The liberated iodine is titrated against a standard 0.1N Sodium Thiosulfate (Na₂S₂O₃) solution using Starch as an indicator. Sodium tetrathionate (Na₂S₄O₆) is formed by the oxidation of sodium thiosulfate with iodine. Starch is added only towards the end of the titration when the solution becomes pale straw yellow. It is done to prevent the formation of an irreversible starch-iodine complex. The endpoint is Blue to Colorless.

I₂ + 2Na₂S₂O₃ → 2NaI + Na₂S₄O₆ (Sodium tetrathionate)

Laboratory Procedure (Wijs Method)

Requirements:

• Iodine Flask: A flask with a glass stopper and a “cup” top to prevent the escape of iodine vapors.
• Wijs Reagent: Iodine monochloride in glacial acetic acid (acts as a solvent).
• Solvent for oil: Chloroform or Carbon tetrachloride (CCl₄).
• Titrant: 0.1N Sodium Thiosulfate.
• Indicator: Freshly prepared Starch solution.
• Sample: Oil or fat (e.g., Castor oil, Sunflower oil).

Steps:

1. Preparation of Test: Weigh accurately about 0.2-0.5g of the oil sample into a 500 mL iodine flask. Dissolve it in 15 mL of chloroform.
2. Addition of Reagent: Pipette 25 mL of Wijs reagent into the flask. Stopper it, swirl, and keep it in the dark for 30 minutes (to prevent photo-catalytic substitution reactions).
3. Preparation of Blank: Prepare a “Blank” flask containing everything except the oil sample and treat it identically. Blank titration corrects for iodine loss due to evaporation or impurities in reagents.
4. Stopping the Reaction: After 30 mins, add 20 mL of 15% KI solution and 100 mL of distilled water to both flasks.
5. Titration: Titrate the liberated iodine with 0.1N Sodium Thiosulfate until the yellow color fades. Add 1 mL starch indicator (solution turns blue) and continue titrating until the blue color disappears (Endpoint: Blue to Colorless).

Summary of Color Changes:

1. Initial mixture (Oil + Wijs): Dark brown/Orange.
2. After KI addition: Dark brown (due to liberated I₂).
3. Titrating with Thiosulfate: Brown to Pale Straw Yellow.
4. After adding Starch: Deep Blue/Violet.
5. Endpoint: Blue to Colorless.

Formula and Calculation

The Iodine Value is calculated using the following formula:

Significance of Iodine Value

• Identification of Oils: Each oil has a characteristic IV range (e.g., Coconut oil: 7–10, Linseed oil: 170–200).
• Detection of Adulteration: If the IV of a sample deviate from standard values, it indicates potential adulteration. High IV oils (like Sunflower oil) mixed into low IV oils (like Coconut oil) can be easily detected.
• Predicting Stability: Oils with higher iodine values are more prone to oxidative rancidity (oils become rancid faster) because they have more reactive double bonds.
• Classification of Oils:

Non-drying oils:  IV < 100, e.g.  Olive oil, Peanut oil

Semi-drying oils:  IV= 100 – 130, e.g. Cottonseed oil, Sesame oil

Drying oils:   IV > 130, e.g. Linseed oil, Soyabean oil

Iodine Value of Common Pharmaceutical Oils

The Iodine Value is used to check the purity of oils used as vehicles for injections or in topical formulations.

Type of Lipid	Oil Sample	Average Iodine Value	Pharmaceutical Significance
Non-drying	Coconut Oil	7 – 10	Highly stable; used in ointments.
Non-drying	Olive Oil	79 – 88	Used as a vehicle and for nutrition.
Non-drying	Castor Oil	82 – 90	Used as a laxative and in cosmetics.
Non-drying	Arachis Oil	84 – 100	Standard solvent for IM injections.
Semi-drying	Sesame Oil	103 – 116	Common vehicle for oily injections.
Drying	Sunflower Oil	118 – 141	High in Vitamin E; dietary supplement.

Master Revision Table: Analytical Constants

Parameter	What it Measures	Principal Reagent	Key Sample/Limit (IP)
Acid Value	Rancidity (Free Fatty Acids)	0.1M KOH	Generally, < 2.0 for purity.
Saponification Value	Chain length / Molecular Weight	Alcoholic KOH	Coconut Oil: 250–264 (Highest)
Ester Value	Bound Fatty Acids	(Derived)	Beeswax: 70–80
Acetyl Value	Free Hydroxyl (—OH) Groups	Acetic Anhydride	Castor Oil: more than 143
Iodine Value	Double Bonds (-C=C-), Degree of Unsaturation	Wijs Reagent (ICl)	Unsaturated oils, Castor Oil: 82–90 Linseed Oil: >170