Quality Control of Lubricants

Importance of the Topic

The acid number is a critical quality parameter for lubricants, reflecting the concentration of acidic constituents that can promote corrosion and material degradation. Traditional titrimetric methods (ASTM D664) require significant reagent use, sample preparation, and instrument cleaning, leading to high operational costs and extended analysis times. Rapid, reagent-free alternatives are therefore highly desirable for routine quality control in laboratories and production environments.

Objectives and Study Overview

This application note evaluates the performance of a visible to near-infrared (Vis-NIR) spectroscopic method for the unassisted, automated determination of the acid number in lubricants. The primary goal was to develop and validate a prediction model offering comparable precision to wet chemical titration while reducing analysis time, reagent consumption, and overall running costs.

Methodology and Instrumentation

The study employed a Metrohm XDS RapidLiquid Analyzer (400–2500 nm) equipped with a 5.0 mm flow cell and an 815 Robotic USB Sample Processor (141-position rack). Samples were measured in transmission mode at controlled temperature (30 °C) without any chemical pretreatment. Chemometric model development and data acquisition were conducted using Metrohm tiamo and Vision Air Complete software. Calibration and cross-validation were performed using partial least squares regression to correlate Vis-NIR spectra with reference acid number values obtained by titration.

Key Results and Discussion

The calibration model achieved a coefficient of determination (R2) of 0.950, with a standard error of calibration (SEC) of 0.344 mg KOH/g and a standard error of cross-validation (SECV) of 0.395 mg KOH/g. Spectral data exhibited distinct absorption features in the NIR region attributable to overtone and combination bands of acidic functional groups. Correlation diagrams confirmed strong agreement between predicted and primary method values, indicating reliable routine performance.

Benefits and Practical Applications

Vis-NIR spectroscopy offers multiple advantages over conventional titration:

• No sample preparation or chemical reagents required
• Analysis time under one minute per sample
• Reduced operator exposure and maintenance effort
• Lower per-analysis cost and higher throughput

This approach is particularly suited for high-throughput QC laboratories, inline process monitoring, and sustainability-focused operations.

Future Trends and Potential Applications

Advancements in miniaturized NIR sensors, enhanced chemometric algorithms, and cloud-based data analytics are expected to further improve method robustness and accessibility. Integration with process analytical technology (PAT) platforms and real-time quality assurance frameworks will enable continuous monitoring of lubricant quality across manufacturing and service operations.

Conclusion

The Vis-NIR method demonstrated here provides a fast, cost-efficient, and environmentally friendly alternative to ASTM D664 titration for acid number determination in lubricants. With comparable precision and significantly reduced consumable costs, this approach supports modern quality control requirements and sustainability goals.

References

• No external literature was cited in the original document.