Determination of the total acid number in petroleum products

Importance of the topic

The total acid number (TAN) is a crucial indicator of acidity in petroleum products and oils. It reflects corrosive potential, oxidative degradation and additive depletion. Accurate TAN measurement ensures product quality and compliance with international standards such as ASTM, IP and DIN.

Goals and overview of the bulletin

This application bulletin presents standardized procedures for the potentiometric, photometric and thermometric determination of TAN in petroleum products. Each approach follows a recognized ASTM method: D664, D974 and D8045. The bulletin aims to provide guidelines for sample preparation, reagent selection, titration parameters and data interpretation to achieve reliable and reproducible results.

Methodology and used instrumentation

Potentiometric titration (ASTM D664):

• Semi-automated titrator with differential endpoint detection (DET mode)
• Solvotrode easyClean pH electrode with lithium chloride electrolyte
• Thermoprobe, stirrer and 10 mL burette
• Titrant: 0.1 mol/L KOH in isopropanol; solvent: toluene/IPA/water mixture

Photometric titration (ASTM D974):

• Titrator with metered endpoint detection (MET mode)
• Optrode 574/610 nm photometric sensor, stirrer and 10 mL burette
• Indicators: phenolphthalein and p-naphtolbenzein
• Titrant and solvent system similar to potentiometric method

Thermometric titration (ASTM D8045):

• Thermometric titrator with Thermoprobe sensor and 10/50 mL burettes
• Solvent: isopropanol/xylene mixture; titrant: 0.1 mol/L KOH in IPA
• Catalyst: paraformaldehyde; standardization reagent: benzoic acid

Key steps common to all methods include sample weighing based on expected TAN, addition of solvent and catalysts or indicators, titration to the defined endpoint, sensor maintenance and blank correction calculations.

Main results and discussion

Each titration technique delivers precise TAN values across a wide acidity range from 0.05 to over 250 mg KOH/g. Potentiometric titration offers direct equivalence point detection and automated drift correction, suitable for transparent and colored samples. Photometric titration provides a color-change endpoint for fresh or light-colored oils, while thermometric titration exploits heat flow changes and catalysis by paraformaldehyde for robust measurements in complex matrices. Calibration using standard reagents ensures accuracy and linearity, and method blanks correct for solvent and catalyst contributions. Comparative analysis demonstrates consistency between methods, with method selection driven by sample color, acid strength distribution and laboratory instrumentation.

Benefits and practical applications

• Compliance with international standards (ASTM, EN, ISO)
• Wide dynamic range covering trace to high acidity
• Automation capability for high throughput and routine QA/QC
• Minimal sample preparation, suitable for new and used oils
• Adaptable to different sample colors and matrices
• Detailed sensor maintenance protocols enhance reliability

Future trends and opportunities

Advancements may include micro-scale titrations to reduce solvent use, inline TAN monitoring in production pipelines, integration with machine learning for endpoint detection, development of universal solvent systems for all methods and digitalization of titration data for regulatory reporting and predictive maintenance.

Conclusion

This bulletin compiles three complementary titration methods for comprehensive TAN determination in petroleum products. By following defined sample protocols, titration parameters and maintenance routines, laboratories can achieve accurate, reproducible and standard-compliant acidity measurements across diverse oil matrices.

References

• ASTM D664 Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration
• ASTM D974 Standard Test Method for Acid and Base Number by Color-Indicator Titration
• ASTM D8045 Standard Test Method for Acid Number of Crude Oils and Petroleum Products by Catalytic Thermometric Titration
• BS DIN EN 12634 Petroleum Products and Lubricants – Determination of Acid Number, Non-Aqueous Potentiometric Titration Method
• DIN ISO 6618 Petroleum Products and Lubricants – Determination of Acid or Base Number – Color-Indicator Titration Method