Determination of Acid Number (AN) with Titration and NIR Spectroscopy

Importance of the topic

Monitoring the acid number (AN) of oils is essential for ensuring product quality, preventing equipment corrosion, and optimizing maintenance schedules. Regular determination of AN allows manufacturers and end users to track oil degradation, validate compliance with specifications, and avoid costly downtime.

Objectives and study overview

This white paper compares three methods for AN determination: potentiometric titration (ASTM D664), thermometric titration (ASTM D8045), and near-infrared spectroscopy (NIRS) (ASTM E1655). The aim is to evaluate their performance, throughput, and suitability for routine quality control and research applications.

Methodology and Instrumentation

Potentiometric titration

• Principle: Non-aqueous titration of oil in toluene/isopropanol/water with KOH, endpoint detected by a combined pH electrode.
• Advantage: Differentiates strong and weak acid contributions via multiple equivalence points.

Thermometric titration

• Principle: Reaction enthalpy measured by precise thermistor; paraformaldehyde acts as catalyst to amplify thermal signal during KOH titration in isopropanol/xylenes.
• Advantage: Faster analysis and improved solubility for high-viscosity or crude oils.

Near-Infrared Spectroscopy (NIRS)

• Principle: Overtones and combination bands in the NIR region correlate with AN through chemometric models.
• Model development: Calibration against titration reference data using Partial Least Squares algorithms in Vision Air 2.0 software.
• Instrumentation: XDS RapidLiquid Analyzer with flow cell or cuvettes; temperature stabilization for spectral reproducibility.

Main Results and Discussion

Potentiometric and thermometric titrations yield highly accurate AN values, with thermometric titration offering shorter analysis times. NIRS provides comparable accuracy without sample preparation, delivering results within one minute. Differences between titration techniques are minor but may reflect varied detection of acid species.

Benefits and Practical Applications

• Potentiometric titration: Ideal for in-depth acid speciation and R&D environments.
• Thermometric titration: Suited for high-throughput QC labs requiring rapid turnaround.
• NIRS: Best for large sample volumes, minimal consumables, and multi-parameter testing in production control.

Future Trends and Applications

Advances in portable NIR instrumentation, enhanced chemometric algorithms, and integration with laboratory information management systems will further streamline AN analysis. Emerging methods combining spectroscopy with machine learning may offer real-time monitoring in industrial processes.

Conclusion

Each AN determination technique presents distinct advantages: titration methods deliver high confidence and detailed acid profiling, while NIRS offers unmatched speed and operational simplicity. Implementing the appropriate method depends on throughput demands, accuracy requirements, and laboratory resources.

References

ASTM D664, ASTM D8045, ASTM E1655 by ASTM International; Metrohm White Paper WP-053EN, December 2019