Neutralisation Number (Acid Value) of Lubricating Oils
📚 Table of Contents
1. Introduction and Definition
Neutralisation Number: Also known as acid value. It is the number of milligrams of potassium hydroxide (KOH) required to neutralise all free fatty acids in 1 gram of oil sample.
This test quantifies the acidity of lubricating oils, indicating oxidation, degradation, or contamination. An acid value below 0.1 mg KOH/g is desirable; values above 0.1 mg KOH/g signal sludge formation and corrosion risk.
2. Theory and Importance
During oil service, oxidation produces organic acids. The acid value measures these acids’ concentration. Higher acid values correlate with increased corrosivity, deposit formation, and loss of lubricity.
Acid number = total acid content; indicates oil health and remaining service life.
3. Apparatus and Reagents
3.1 Apparatus
- Burette (25 mL), Class A
- Conical flask (100 mL)
- Pipette (5 mL) or analytical balance for sample weighing
- Magnetic stirrer or glass stirring rod
3.2 Reagents
- Neutral solvent: a 1:1 mixture of ethyl methyl ketone and toluene
- Phenolphthalein indicator: 5 drops
- Standard KOH solution, 0.1 N (standardised against oxalic acid)
4. Titration Procedure
- Weigh exactly 3–4 g of oil sample (W) into the conical flask.
- Add 50 mL of neutral solvent to dissolve the sample.
- Add 5–6 drops of phenolphthalein indicator; solution remains colorless.
- Perform a blank titration: titrate solvent + indicator with 0.1 N KOH to faint, permanent pink endpoint; record volume V₂.
- Shake flask thoroughly; titrate oil solution with 0.1 N KOH until permanent light pink color persists for 30 seconds; record volume V₁.
- Repeat duplicate titration for concordance (±0.1 mL). Calculate average volumes.
5. Calculation Method
Acid Value (mg KOH/g):
\[ \text{Acid Value} = \frac{(V_1 - V_2) \times N \times 56.1}{W} \]
where:
\(V_1\) = KOH volume for sample (mL)
\(V_2\) = KOH volume for blank (mL)
\(N\) = Normality of KOH (0.1 N)
\(W\) = Weight of oil sample (g)
56.1 = Molecular weight of KOH
\[ \text{Acid Value} = \frac{(V_1 - V_2) \times N \times 56.1}{W} \]
where:
\(V_1\) = KOH volume for sample (mL)
\(V_2\) = KOH volume for blank (mL)
\(N\) = Normality of KOH (0.1 N)
\(W\) = Weight of oil sample (g)
56.1 = Molecular weight of KOH
6. Significance and Applications
- Oxidation Monitoring: Increasing acid value indicates oxidative breakdown and sludge formation.
- Corrosion Risk: High acid content corrodes metal surfaces and gaskets.
- Oil Replacement: Guides change intervals to prevent equipment damage.
- Quality Control: Ensures fresh oil meets specification (<0.1 mg KOH/g).
7. Exam Key Points
- Define neutralisation number (acid value) in mg KOH/g.
- List apparatus and reagents: solvent, indicator, 0.1 N KOH.
- Describe blank and sample titration steps.
- Memorise calculation formula with molecular weight of KOH = 56.1.
- Explain significance: oxidation, corrosion, oil replacement decisions.
0 Comments