Acid Number of Lubricants: Definition, Formula, Determination & Importance | Engineering Chemistry Notes

What is Acid Number of Lubricants?

Acid Number, also known as Acid Value, is an important chemical property used to evaluate the condition and quality of lubricating oils. It indicates the amount of acidic substances present in a lubricant. During prolonged service, lubricating oil may undergo oxidation and thermal degradation. These processes can produce acidic degradation products that may affect lubricant performance and increase the possibility of corrosion of metallic machine components.

In simple terms, Acid Number is the number of milligrams of potassium hydroxide (KOH) required to neutralize the acidic constituents present in one gram of lubricant. It is generally expressed in mg KOH/g of oil.

1. Definition of Acid Number

Acid Number is defined as the number of milligrams of potassium hydroxide required to neutralize the acidic constituents present in one gram of a lubricant.

It is commonly expressed as mg KOH/g. A higher Acid Number generally indicates a greater amount of acidic material in the lubricant. In used lubricating oil, an increasing Acid Number can be an indication of oxidation, degradation or contamination.

Acid Number = Milligrams of KOH required to neutralize acidic constituents present in 1 gram of lubricant

2. Importance of Acid Number in Lubricants

The primary function of a lubricant is to reduce friction and wear between moving machine components. However, lubricating oil can undergo chemical changes during operation. Exposure to oxygen, high temperature, moisture and contaminants may cause oxidation and degradation of the oil.

Oxidation and degradation can result in the formation of acidic compounds. Acid Number provides an indication of the overall acidic character of the lubricant and is therefore useful for lubricant condition monitoring.

Major Importance

• It indicates the presence of acidic constituents in lubricating oil.
• It helps monitor oxidation and lubricant degradation.
• It can assist in assessing corrosion-related concerns.
• It is useful for comparing fresh and used lubricating oils.
• It can support lubricant replacement decisions.
• It is useful in industrial preventive maintenance and oil-condition monitoring.

3. Causes of Increase in Acid Number

High Temperature

High operating temperatures can accelerate oxidation and degradation reactions in lubricating oil. Continuous exposure to elevated temperature may therefore increase acidic degradation products.

Oxidation

Lubricating oil can react with atmospheric oxygen during service. Oxidation may produce organic acids and other degradation products, resulting in an increase in Acid Number.

Moisture and Water Contamination

Water contamination may promote chemical reactions and corrosion-related processes. Moisture can therefore negatively affect lubricant stability and machine protection.

Long Service Period

When lubricating oil is used for an extended period without appropriate replacement, degradation products may accumulate. As a result, the Acid Number of used oil may become higher than that of fresh oil.

Contamination

Dust, moisture, chemicals and other contaminants can affect lubricant stability and contribute to oil degradation.

4. Principle of Acid Number Determination

The determination of Acid Number is based on the principle of acid-base neutralization. Acidic constituents present in the lubricant are neutralized using a standard alcoholic potassium hydroxide solution.

The amount of KOH consumed during titration is used to calculate the Acid Number.

R–COOH + KOH → R–COOK + H₂O

Here, R–COOH represents an acidic compound. Potassium hydroxide reacts with the acidic compound to form a potassium salt and water.

5. Apparatus and Chemicals Required

• Lubricating oil sample
• Standard alcoholic KOH solution
• Suitable solvent
• Phenolphthalein or another suitable indicator
• Burette
• Pipette
• Conical flask
• Measuring cylinder
• Analytical balance
• Distilled water and other laboratory accessories

6. Procedure for Determination of Acid Number

A known mass of lubricant is taken and dissolved in a suitable solvent. The resulting solution is titrated against a standardized alcoholic KOH solution. A suitable indicator is used to identify the endpoint.

Step 1: Weighing the Sample

A known quantity of lubricant is accurately weighed. Let its mass be represented by W grams. Accurate measurement of sample mass is important for obtaining a reliable result.

Step 2: Dissolving the Sample

The lubricant sample is dissolved in an appropriate solvent so that the acidic constituents can participate effectively in the titration reaction.

Step 3: Addition of Indicator

A suitable indicator is added to the sample solution. The indicator helps in identifying the titration endpoint.

Step 4: Titration with KOH

The sample solution is titrated with a standardized alcoholic KOH solution. KOH is added gradually from the burette while the solution is continuously mixed.

Step 5: Endpoint Detection

The titration is continued until the required permanent indicator colour change is obtained. A blank titration may also be performed to account for the contribution of solvent and reagents.

7. Formula for Acid Number

The Acid Number is calculated from the volume and normality of KOH consumed and the mass of lubricant used for the test.

Acid Number = (V × N × 56.1) / W

• V = Volume of KOH solution used in mL
• N = Normality of KOH solution
• 56.1 = Equivalent mass of KOH
• W = Mass of lubricant sample in grams

When blank correction is required, the blank volume is subtracted from the sample titration volume:

Acid Number = ((V − V_b) × N × 56.1) / W

• V = KOH volume used for the sample
• V_b = KOH volume used for the blank
• N = Normality of KOH
• W = Mass of lubricant sample

8. Numerical Example on Acid Number

Suppose the mass of a lubricant sample is 5 grams. During titration, 2.5 mL of 0.1 N KOH solution is consumed. If blank correction is ignored, calculate the Acid Number.

Given:

• W = 5 g
• V = 2.5 mL
• N = 0.1 N

Formula:

Acid Number = (V × N × 56.1) / W

Substitution:

Acid Number = (2.5 × 0.1 × 56.1) / 5

Acid Number = 2.805 mg KOH/g

Therefore, the Acid Number of the lubricant sample is approximately 2.805 mg KOH/g.

9. Relationship Between Acid Number and Lubricant Quality

The acceptable Acid Number range depends on the lubricant formulation, application, operating conditions and applicable specification or standard. Therefore, a single universal limit should not be applied to every lubricant.

10. Acid Number and Corrosion

An increase in acidic constituents can increase the possibility of corrosion of metallic machine components, particularly when moisture, oxygen and elevated temperature are also present.

For this reason, Acid Number analysis can be useful in lubricant condition monitoring and preventive maintenance. A continuous increase in the Acid Number of used oil should be investigated together with other lubricant-condition parameters.

11. Advantages of Acid Number Determination

• It provides an indication of acidic constituents in lubricating oil.
• It helps monitor oxidation and lubricant degradation.
• It is useful for comparison of fresh and used oil.
• It can support corrosion-risk assessment.
• It assists maintenance and lubricant replacement decisions.
• It can be determined using a laboratory titration procedure.

12. Limitations of Acid Number

• It represents the overall acidic character rather than identifying every individual acid.
• Different lubricant formulations may have different acceptable Acid Number ranges.
• Accurate sample preparation and titration are required.
• Endpoint detection may introduce experimental error.
• Acid Number should preferably be interpreted together with other lubricant-condition parameters.

13. Difference Between Acid Number and Base Number

14. How to Write Acid Number in Engineering Chemistry Exam?

For an Engineering Chemistry examination answer, Acid Number should be presented systematically. Start with the definition, followed by the unit, principle, reaction, apparatus, procedure and formula. For numerical problems, clearly write the given data, formula, substitution and final answer with units.

Recommended Exam Answer Structure

1. Definition of Acid Number
2. Unit
3. Principle
4. Neutralization Reaction
5. Apparatus and Chemicals
6. Determination Procedure
7. Formula
8. Numerical Calculation
9. Importance
10. Conclusion

15. Important Points for RGPV Examination

• Acid Number is also known as Acid Value.
• It is generally expressed in mg KOH/g of oil.
• It is based on the amount of KOH required to neutralize acidic constituents.
• The determination is based on acid-base neutralization and titration.
• An increase in Acid Number can indicate lubricant degradation.
• High acidic content can increase corrosion concerns.
• Sample mass, KOH volume and KOH normality are important for calculations.
• It is useful for monitoring the condition of used lubricating oil.

Conclusion

Acid Number of Lubricants is an important chemical property that indicates the quantity of acidic constituents present in lubricating oil. It is generally expressed as the number of milligrams of KOH required to neutralize the acidic constituents present in one gram of lubricant.

The determination of Acid Number is based on acid-base neutralization and titration. Long service, oxidation, high temperature and contamination may increase the concentration of acidic degradation products. Therefore, Acid Number is useful for lubricant condition monitoring, corrosion assessment and preventive maintenance. For Engineering Chemistry examinations, the definition, principle, determination procedure, formula and numerical problems related to Acid Number are important.