Determination of Total Base Number of Lubricating Oils

Significance of the topic

Accurate determination of Total Base Number (TBN) in lubricating oils is essential for assessing oil alkalinity, predicting service life, and preventing acid-induced corrosion in engines. Rapid, reliable TBN measurement supports maintenance scheduling and ensures compliance with industry standards.

Objectives and Study Overview

This study presents a fully automated thermometric titration method for TBN determination in lubricating oils. The goal is to demonstrate high precision, adaptability to a broad alkali range, and suitability for routine quality control and research applications.

Methodology and Instrumentation

The method employs thermometric endpoint detection based on the exothermic reaction between oil-soluble bases and 0.1 mol/L trifluoromethanesulfonic acid in glacial acetic acid, using isobutyl vinyl ether as the indicator. Key experimental parameters include:

• Titrant delivery rate: 4 mL/min
• Data smoothing factor (DSF): 70
• Stirring speed (802 stirrer): level 12
• Single exothermic endpoint detection

Sample preparation involves dissolving 2–3 g of oil in toluene with indicator addition, followed by automated titration in an 814 Sample Processor. For high-TBN marine oils, sample mass and pre-dosing volumes are adjusted to maintain optimal endpoint detection.

Used Instrumentation

• Thermometric titrator with temperature sensor
• 814 Sample Processor with Dosino dosing units
• 802 stirrer module

Main Results and Discussion

Replicate analyses of various lubricating oils yielded TBN values ranging from 7.82 to 9.50 mg KOH/g for fresh lubricants and 4.78 mg KOH/g for used oil (RSD < 0.5%). Fresh marine cylinder oil (BN 70) exhibited 71.17 mg KOH/g. Blank titrations, required per oil type, ranged from 0.052 to 0.674 mL. Thermometric curves showed a sharp temperature rise at the endpoint, with second-derivative peaks confirming repeatable detection.

Benefits and Practical Applications

This automated thermometric titration approach offers:

• High throughput for routine QA/QC
• Minimal solvent consumption
• Rapid, distinct endpoint identification
• Flexibility across low to high TBN ranges

Future Trends and Potential Applications

Emerging developments may include inline monitoring of lubricant condition, multi-endpoint detection for complex formulations, and application to bio-based or renewable lubricant blends. Integration with advanced data analytics could further optimize blank correction and calibration.

Conclusion

The described thermometric titration method delivers a robust, precise, and fully automated solution for TBN determination in lubricating oils. Its adaptability and speed make it a valuable tool for industrial laboratories and research facilities.

References

• Greenhow E.J., Spencer L.E.: Ionic polymerisation as a means of endpoint indication in non-aqueous thermometric titrimetry. Analyst, 1973, 98, 81–89
• Riedel de Haën (Sigma-Aldrich) reagents: CF₃SO₃H cat. no.35317, isobutyl vinyl ether cat. no.278351