Saponification Number of Lubricating Oils - Engineering chemistry Notes by Mohan Dangi

Saponification Number (Value) of Lubricating Oils

📚 Table of Contents

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

1. Introduction & Definition

Saponification Number: The milligrams of potassium hydroxide (KOH) required to saponify (hydrolyze) the esters and triglycerides in 1 gram of oil.

This value distinguishes between vegetable/animal oils (high saponification) and mineral oils (no saponifiable esters). It also indicates extent of adulteration and additive content.

2. Theory & Importance

Saponification involves breaking ester bonds to yield glycerol and soap. The saponification number reflects total ester content. A higher number indicates more ester groups (natural oils or ester-based additives).

Impurities or dilution with non-saponifiable hydrocarbons lowers the saponification number, signaling adulteration.

3. Apparatus & Reagents

3.1 Apparatus

• Reflux flask (250 mL) with condenser
• Analytical balance (±0.1 mg)
• Burette (25 mL), Class A
• Fume hood for safe KOH handling

3.2 Reagents

• 0.1 N alcoholic KOH solution (50 mL)
• Neutral solvent: ethyl methyl ketone
• Standard 0.1 N HCl titrant
• Phenolphthalein indicator (8–10 drops)

4. Titration Procedure

1. Weigh 2–3 g of oil sample (W) into the reflux flask.
2. Add 50 mL 0.1 N alcoholic KOH and 50 mL solvent; fit condenser and reflux for 30–45 minutes.
3. Cool the mixture; add 8–10 drops of phenolphthalein indicator.
4. Perform blank titration: titrate solvent+KOH with 0.1 N HCl until pink persists (V₂).
5. Titrate sample solution with 0.1 N HCl to pink endpoint; record volume V₁.
6. Repeat for concordant readings (±0.1 mL).

5. Calculation Method

Saponification Number (mg KOH/g):
(V₂ – V₁) × N × 56.1 / W
where:
V₁ = HCl volume for sample (mL)
V₂ = HCl volume for blank (mL)
N = Normality of HCl (0.1 N)
W = Weight of oil sample (g)
56.1 = Molar mass of KOH

6. Significance & Applications

• Quality Control: Ensures correct ester/additive levels in formulated oils.
• Adulteration Detection: Drops if mineral oils are added to vegetable/animal oils.
• Additive Verification: Confirms presence and concentration of ester-based additives.
• Lubricant Performance: Ester-rich oils provide improved lubricity and film strength.

7. Exam Key Points

• Define saponification number in mg KOH/g.
• List apparatus: reflux flask, balance, burette, condenser.
• Describe reflux time (30–45 min) and titration steps.
• Memorise calculation formula and constants.
• Explain significance for adulteration and additive content.