Determination of the total acid number (TAN) using thermometric titration

Significance of the Topic

Total acid number (TAN) measurement is a critical quality control parameter in the petroleum industry. By quantifying the amount of acidic species in oils and fuels, it provides insight into oil degradation, corrosion potential, and overall product integrity. Conventional titration methods face challenges in colored or water-free samples, making a robust, fast alternative highly valuable.

Objectives and Study Overview

This study introduces a thermometric titration approach for determining TAN in petroleum products. The aim was to develop a rapid, maintenance-free method that overcomes limitations of manual indicator and potentiometric titrations, ensuring high reproducibility and minimal sample preparation.

Methodology

A nonaqueous titration system was employed using a solvent mixture of toluene and isopropanol (1:1), to which the oil sample is dissolved. Paraformaldehyde serves as a catalytic indicator. A 0.1 mol/L solution of potassium hydroxide (or tetrabutylammonium hydroxide) in isopropanol is delivered at a constant rate. Temperature changes from exothermic or endothermic reactions are tracked in real time, and the endpoint is identified by the inflection in the temperature–volume curve using the second derivative.

Instrumentation Used

• 859 Titrotherm thermometric titrator
• 815 Robotic Sample Processor XL for automated sample handling
• 800 Dosinos dosing units and 772 Pump Unit for precise reagent delivery
• tiamo™ 2.2 software for data acquisition and evaluation
• Thermoprobe sensor (response time 0.3 s, resolution 1×10⁻⁵ K)

Main Results and Discussion

Validation with a certified TAN reference standard (2.93 mg KOH/g) yielded an average value of 2.87 mg KOH/g. The close agreement demonstrates accuracy comparable to ASTM D 664 potentiometric titration. The thermometric method delivered results in under one minute, showing superior reproducibility by avoiding electrode fouling and dehydration in nonaqueous media.

Benefits and Practical Applications of the Method

• Rapid analysis time (~1 min per determination)
• Maintenance-free temperature sensor eliminates rehydration and fouling issues
• High reproducibility and accuracy in highly colored or water-free samples
• Minimal sample preparation and automated processing capacity

Future Trends and Opportunities

Thermometric titration could be extended to other nonaqueous assays in petrochemicals and lubricant analysis. Integration with advanced robotics and real-time process monitoring systems may enable online TAN determination. Further miniaturization and multiplexed sensor arrays could broaden applications in field testing and high-throughput laboratories.

Conclusion

The thermometric titration approach offers a robust, fast, and accurate alternative for TAN determination in petroleum products. It addresses key drawbacks of traditional methods and aligns with industry demands for high throughput and reliable quality control.

Reference

• ASTM D 664: Standard Test Method for Acid Number of Petroleum Products
• ASTM D 974: Standard Test Method for Acid and Base Number by Color-Indicator Titration
• IP 1777: International Standard for Total Acid Number Determination
• Paragon Scientific TAN030 Certified Reference Material