TBN determination in lubricants

Importance of the Topic

The total base number (TBN) is a critical parameter for assessing the alkaline reserve in engine and marine lubricants, which neutralizes acidic byproducts and prevents corrosion in powertrain systems. Accurate and timely determination of TBN supports preventive maintenance of engines, extends lubricant service life, and reduces operational costs.

Objectives and Study Overview

This study evaluates near-infrared spectroscopy (NIRS) as a rapid, reagent-free alternative to the conventional potentiometric titration (ASTM D2896) for quantifying TBN in lubricants. A feasibility assessment was conducted on 60 lubricant samples, including marine cylinder and engine oils, to develop and validate a predictive calibration model.

Methodology and Instrumentation

• Instrument: Metrohm DS2500 Liquid Analyzer with 2.5 mm flow cell for transmission measurements from 400 to 2500 nm.
• Software: Vision Air Complete for data acquisition, model development, and routine analysis.
• Samples: 23 marine cylinder lubricants and 37 engine lubricants.
• Procedure: Automated sample handling via flow cell; spectral data recorded in under one minute per sample.

Main Results and Discussion

A multivariate calibration model was built correlating NIR spectra to reference TBN values obtained by titration. Key figures of merit include:

• Coefficient of determination (R2): 0.998.
• Standard error of calibration (SEC): 1.1 mg KOH/g.
• Standard error of cross-validation (SECV): 1.2 mg KOH/g.

The high R2 and low errors demonstrate that NIRS provides accurate and precise TBN predictions suitable for routine quality control. Analysis time was reduced from 5–10 minutes for titration to under one minute without sample preparation.

Benefits and Practical Application of the Method

• Elimination of toxic reagents and chemical waste, enhancing operator safety and environmental compliance.
• Rapid, scalable analysis enabling high-throughput screening in laboratory and production settings.
• Minimal sample handling and automation potential supports inline or at-line monitoring.
• Cost savings from reduced consumable use and faster decision-making on lubricant replacement.

Future Trends and Potential Applications

• Integration of NIRS with process control systems for real-time lubricant health monitoring.
• Expansion to additional lubricant properties, such as oxidation and viscosity prediction.
• Application of advanced chemometric and machine learning algorithms to enhance model robustness.
• Development of portable NIR devices for field-based maintenance checks.

Conclusion

The Metrohm DS2500 Liquid Analyzer combined with NIR spectroscopy offers a rapid, accurate, and eco-friendly alternative for TBN determination in lubricants. The validated model achieves high correlation with reference titration methods, streamlining quality control workflows and reducing reliance on hazardous chemicals.