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

Importance of the topic

The acid number (AN) is a critical indicator of oil quality and its tendency to induce corrosion. Regular monitoring of AN values guides quality assurance for new oils and tracks degradation in used oils, helping to prevent equipment damage and ensure compliance with manufacturer specifications.

Objectives and study overview

This overview examines traditional titration techniques and near-infrared spectroscopy (NIRS) for determining AN. It compares potentiometric titration (ASTM D664), thermometric titration (ASTM D8045), and NIRS calibration (ASTM E1655), highlighting Metrohm’s instrumentation solutions for each method.

Methodology and instrumentation

• Potentiometric titration (ASTM D664): Oil is dissolved in toluene, isopropanol, and water and titrated with KOH in isopropanol. A non-aqueous combined pH electrode detects multiple equivalence points, with the last inflection used to calculate AN.
• Thermometric titration (ASTM D8045): The sample is dissolved in anhydrous isopropanol mixed with xylenes and paraformaldehyde catalyst. KOH titrant produces a measurable temperature change detected by a thermistor, delivering a single endpoint and faster analysis.
• Near-infrared spectroscopy (ASTM E1655): NIR spectra capture overtone and combination bands linked to AN by Partial Least Squares models. Vision Air 2.0 software supports calibration and routine analysis. Samples are measured in temperature-controlled cuvettes or disposable vials, with optional flow cells for automation.

Key results and discussion

Titration methods provide high confidence and can resolve variations in acid strength, though slight method-dependent differences in AN values may occur. Thermometric titration offers improved throughput and solubility for certain oil types. NIRS achieves comparable accuracy without chemical preparation, enabling rapid, multi-parameter analysis.

Benefits and practical applications

• Titration delivers precise AN values and insight into acid composition.
• NIRS enables high-throughput, minimal-preparation testing ideal for QC, R&D, and production labs.
• Metrohm instruments accommodate diverse oil types and workflows, from manual titrations to automated spectroscopic measurements.

Future trends and applications

Emerging developments include expanded NIR calibrations for additional quality metrics, inline process monitoring, enhanced chemometric algorithms for greater robustness, and integration with predictive maintenance systems to minimize downtime and optimize operations.

Conclusion

Both titration and NIRS methods offer robust solutions for AN determination. Titration ensures analytical rigor, while NIRS maximizes throughput and operational simplicity. Metrohm’s comprehensive portfolio allows laboratories to select and integrate the optimal technique to meet quality and productivity objectives.