Acidity in crude oils and petroleum products by thermometric titration according to ASTM D8045

Importance of the Topic

Accurate determination of the acid number in crude oils and related products is critical for evaluating feedstock quality, setting market prices, and preventing unexpected downtime in refinery processes caused by corrosion and fouling. Monitoring Total Acid Number (TAN) supports optimized maintenance scheduling and reduced consumption of downstream treatment chemicals.

Objectives and Study Overview

This application bulletin presents a fast and reliable approach for measuring the acid number of crude oil, process oils, bitumen, and high-wax samples using catalytic thermometric titration in accordance with ASTM D8045. It aims to demonstrate enhanced analysis speed, reduced reagent usage, and lower sensor maintenance compared to conventional potentiometric methods.

Methodology and Instrumentation

The technique employs a thermometric titrator fitted with a Thermoprobe temperature sensor and an intensive stirring rod. Key reagents include 0.1 M KOH in isopropanol as titrant and paraformaldehyde as a catalytic indicator. Samples are dissolved in a 1:3 mixture of isopropanol and xylene, then titrated at a constant addition rate. Endpoint detection relies on an exothermic temperature inflection rather than potential measurements.

Main Results and Discussion

Benzoic acid standards establish the titrant titer, while blank determinations use linear regression of multiple solvent-only runs. Sample aliquots of varying mass yield precise TAN values by correlating titrant consumption to sample weight. Thermometric traces exhibit distinct temperature inflections at the endpoint, even for weak acids.

Benefits and Practical Applications

• Faster throughput and lower reagent consumption compared to ASTM D664 potentiometric titration.
• Minimal sensor maintenance due to the absence of glass electrodes.
• Improved refinery uptime and optimized usage of corrosion inhibitors.

Future Trends and Opportunities

Automation of paraformaldehyde dosing via suspension pumps or Dosino devices can streamline workflows. Inline thermometric sensors and miniaturized titration cells may enable real-time acidity monitoring in process streams. Standardized thermometric titration kits could facilitate broader adoption in QC laboratories.

Conclusion

Catalytic thermometric titration per ASTM D8045 offers a robust, efficient, and precise method for assessing acidity in crude oils and petroleum products. Its rapid analysis, low reagent use, and minimal maintenance deliver cost savings and enhanced process control.

Reference

• ASTM D8045: Standard Test Method for Acid Number of Crude Oils and Petroleum Products by Catalytic Thermometric Titration
• Metrohm Application Note AN-H-131: Determination of Titer and Method Blank for Thermometric Titrations
• Metrohm Application Note AN-T-095: Automated Mixing of a Suspension and a Solvent Using a 50 mL Dosing Unit