Iodine Value of Lubricating Oils - Engineering chemistry Notes by Mohan Dangi

Iodine Value of Lubricating Oils

📚 Table of Contents

• 1. Introduction & Definition
• 2. Theory & Significance
• 3. Apparatus & Reagents
• 4. Test Procedure
• 5. Calculation Method
• 6. Applications & Significance
• 7. Exam Key Points

1. Introduction & Definition

Iodine Value: The grams of iodine (I₂) absorbed by 100 grams of oil, indicating the degree of unsaturation (double bonds) in fatty constituents.

Unsaturated double bonds react with iodine monochloride; the unreacted iodine liberated is titrated to determine the iodine value.

2. Theory & Significance

Iodine value measures unsaturation. Higher values indicate more double bonds—better low-temperature fluidity but greater oxidation susceptibility and gum formation.

• Low iodine value (<50): mostly saturated—good stability.
• High iodine value (>120): highly unsaturated—poor oxidative stability.

3. Apparatus & Reagents

Apparatus

• Conical flasks (250 mL)
• Burette (25 mL), Class A
• Pipettes (5–10 mL)
• Dark place or amber bottles (for standing)

Reagents

• Wij’s reagent (iodine monochloride in glacial acetic acid)
• 10% potassium iodide (KI) solution
• Standard 0.05 N sodium thiosulfate (Na₂S₂O₃)
• Starch indicator solution

4. Test Procedure

1. Weigh 1 g oil into two flasks (sample & blank).
2. Add 25 mL CCl₄ and 25 mL Wij’s reagent; swirl to dissolve.
3. Close flasks, keep in dark for 1 hour to allow reaction.
4. Add 20 mL 10% KI to each; shake gently.
5. Titrate liberated iodine with 0.05 N thiosulfate to pale yellow (V₁ for sample, V₂ for blank).
6. Add starch indicator; titrate to colorless endpoint.

5. Calculation Method

Iodine Value = \(\{(V_2 - V_1) \times N \times 126.9}{W}\)
where:
\(V_1\) = sample titration volume (mL)
\(V_2\) = blank titration volume (mL)
\(N\) = normality of thiosulfate (0.05 N)
\(W\) = weight of oil sample (g)
126.9 = molecular weight of iodine

6. Applications & Significance

• Indicates oil unsaturation—balance between cold flow and oxidation stability.
• Helps predict gum/sludge formation on aging.
• Quality control of base oils and additive packages.

7. Exam Key Points

• Define iodine value in g I₂/100 g oil.
• List reagents: Wij’s reagent, KI, 0.05 N thiosulfate, starch.
• Describe dark standing period (1 hour).
• Explain titration steps and starch endpoint.
• Memorise calculation formula and constant (126.9).
• Discuss trade-offs: low unsaturation vs. high stability.