Quality Control of Lubricants

Significance of the Topic

The Acid Number (AN) of lubricants is a key quality parameter that indicates the concentration of acidic components formed during oil oxidation and degradation. Accurate and timely determination of AN is critical for monitoring lubricant performance, predicting maintenance intervals, and ensuring equipment protection in industrial applications.

Study Objectives and Overview

This study evaluates the feasibility of using visible and near-infrared (Vis-NIR) spectroscopy, specifically the XDS RapidLiquid Analyzer, to provide a fast, chemical-free alternative to the conventional titrimetric method (ASTM D664) for determining the Acid Number of lubricants.

Methodology and Instrumentation

Vis-NIR spectra were collected in transmission mode over 400–2500 nm at 30 °C using an XDS RapidLiquid Analyzer with a 5.0 mm flow cell and an 815 Robotic USB Sample Processor. A total of 141 samples were measured without sample preparation or reagents. Data acquisition and multivariate model development were performed using Vision Air Complete and tiamo software.

Used Instrumentation

• XDS RapidLiquid Analyzer (Metrohm 2.921.1410)
• 815 Robotic USB Sample Processor XL (Metrohm 2.815.0010)
• 800 Dosino (Metrohm 2.800.0020)
• 5.0 mm quartz flow cell (Hellma)
• Vision Air Complete spectroscopy software (Metrohm 6.6072.208)
• tiamo 3.0 software (Metrohm 6.6056.301)

Key Results and Discussion

Multivariate regression models correlating Vis-NIR predictions to titrimetric AN values yielded:

• Coefficient of determination (R2): 0.950
• Standard error of calibration (SEC): 0.344 mg KOH/g
• Standard error of cross-validation (SECV): 0.395 mg KOH/g

The high R2 and low errors demonstrate that Vis-NIR spectroscopy can reliably predict AN within practical precision limits. Correlation diagrams confirmed agreement between predicted and reference values across the analyzed concentration range.

Benefits and Practical Applications

Compared to traditional titration, the Vis-NIR approach offers:

• Analysis time under one minute per sample versus ten minutes
• Elimination of chemical reagents and waste disposal
• Reduced per-analysis cost (approximately $1.50 vs. $10 in consumables)
• Lower annual operating costs ($7,125 vs. $31,875 for 10 analyses per day)
• Minimal maintenance and no cleaning between measurements

This method is ideal for high-throughput QC laboratories and on-site monitoring where rapid decision making is required.

Future Trends and Opportunities

Advancements expected in the field include:

• Integration with inline process control systems for real-time monitoring
• Expansion of Vis-NIR models to additional lubricant quality indices (e.g., oxidation, water content)
• Application of advanced machine learning and artificial intelligence for improved prediction accuracy
• Development of portable NIR instruments for in-field lubricant analysis

Conclusion

The study confirms that Vis-NIR spectroscopy using the XDS RapidLiquid Analyzer is a valid, cost-effective, and rapid alternative to wet chemical titration for determining the Acid Number of lubricants. Its implementation can streamline quality control workflows, reduce environmental impact, and lower operating expenses.

References

• ASTM International. ASTM D664-14: Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration. West Conshohocken, PA; 2014.
• ASTM International. ASTM E1655-05: Standard Practice for Infrared Multivariate Quantitative Analysis. West Conshohocken, PA; 2005.