Ensuring Accuracy of Acid Number Crude Oil and Refined Petroleum Products by ASTM D8045

Acid number (AN) is an important parameter in the valuation and quality control of refined petroleum products, including crude oil. Directly influencing how the oil is processed, just a small change in acidity can lead to corrosion within the refining process and considerable adjustments in barrel pricing. Until now, it has been difficult and ineffective to ensure the accuracy of acid number in these samples.

This article shows how these challenges are resolved with Metrohm titration and ASTM D8045.

Balancing Risk with Opportunity

Discounted opportunity crude oils could radically enhance refinery operating margins and bring a considerable increase in profitability to the overall refining industry. Refineries can reduce their raw material costs and drive up their margins on refined products by combining opportunity crude with well-defined samples. In spite of their potential economic benefits, these opportunity crude oils are discounted because of the risk associated with processing petroleum products that contain high levels of naphthenic acid.

Every year, increasing amounts of opportunity crude oils are introduced in the market, highlighting the need for a better approach to reduce the transactional risk between sellers and buyers.

Even if a refinery has no interest in rolling the dice with opportunity crude oils, its own operation is constantly at risk from damaging corrosion. A federal corrosion report sponsored by NACE remains the landmark report, which estimates the yearly cost of corrosion in U.S. refineries at $3.7 billion. The gap analysis of the report projects a direct profit loss that ranges from $2 to $12 billion owing to disruption in refinery operations caused by corrosion-related shutdowns.1

Additionally, it is the responsibility of refinery and terminal management to balance the potential revenue with the cost of corrosion control and refinery infrastructure risk when opting to accept certain petroleum products or opportunity crude oils.

The Need for a Better Acid Number Test

Monitoring acid number in petroleum products and crude oils continues to be the benchmark for adjusting refinery protocol to accommodate corrosive petroleum products and assessing the risk of processing opportunity crudes. Written to address acidity in lubricant products, conventional potentiometric techniques need huge amounts (up to 120 mL) of solvent and detailed electrode care procedures.

Also, poor solubility of crude products particularly bitumen, paraffinic and asphaltic materials leads to electrode fouling and wrong acid number results through potentiometric methods. In the year 2008, major petroleum industry leaders appealed for a new test method from ASTM. Being a member of ASTM, Metrohm collaborated with the petroleum industry to develop the D8045 method – a thermometric titration standard that deals with these problems while enhancing reagent expenditure and analysis time.

ASTM D8045 was developed using the Metrohm 859 Acidity Analyzer.

Figure 1. ASTM D8045 was developed using the Metrohm 859 Acidity Analyzer.

What is Thermometric Titration?

Thermometric titration does not measure a potential difference, but instead it uses a sensitive thermistor instrument to detect the enthalpy change in an exothermic or endothermic reaction initiated by the continuous dosing of a titrant. Just like standard titration methods, the endpoint is indicated at the point of surplus titrant as soon as the titrant neutralizes the targeted unknown sample species.

Although enthalpy changes down to ±0.001 °C can be detected as a result of fast response time and high resolution of the sensor, there are certain applications that benefit from the use of a catalytic indicator to make the titration endpoint apparent. Thermometric titration is better than conventional titration methods as it uses a maintenance-free sensor that requires no reference solution, no calibration and minimal total sample volume. Thanks to solvent resistant construction, the sensor is not affected by harsh solvents or difficult samples.

Algorithms that are unique from other types of titration (i.e., conductometric and potentiometric) are required for accurate evaluation of thermometric titrations. Compared to potentiometric, up to 30-times more data is obtained for thermometric measurements. Noise is filtered out by Metrohm instrument algorithms to create a smooth curve with a single, well-defined endpoint, as indicated in Figure 2.

Acid Number: Thermometric vs. Potentiometric

Performing acid number analysis with thermometric titration provides a number of benefits. Thermometric acid number by ASTM D8045 uses a solvent mixture of 2-propanol and xylene in a ratio designed to sufficiently disperse complex crude oils. This successfully eliminates solubility challenges.

This optimized solvent mixture enables a 75% reduction in solvent employed for acid number analysis, providing an immediate improvement in cost per analysis. An average lab running 20 samples per day can save as much as $30,000 per year in solvent expense alone. Besides material savings, thermometric titration also ensures laboratory efficiency gains over potentiometric titration.

Potentiometric titrations will need more than 3X the time for the same sample, while thermometric titration time averages around 60 seconds or less. Further, potentiometric method D664 requires regeneration time and electrode cleaning while thermometric titration through D8045 does not require any additional sensor regeneration. Table 1 shows a full method comparison.

Typical thermometric acid number titration curve

Figure 2. Typical thermometric acid number titration curve demonstrating a single, well-defined endpoint.

Table 1. Comparison of acid number techniques.

Parameter ASTM D664
ASTM D8045
Titrant 0.1 mol/L KOH in IPA 0.1 mol/L KOH in IPA
Solvent Toluene/IPA/water Xylene/IPA
Solvent volume 120 mL 30 mL
Titration time ~220 s ~60 s
Conditioning of electrode 3-5 min None
Sensor maintenance Solvent wash, rehydration, IPA dip, refill with electrolyte, store in LiCl in ethanol Solvent wash is sufficient
Sample size (expected AN of 0.05–1.0 mg KOH/g) 20 ± 2 g ~10 g

Improve Commerce and Prevent Corrosion

According to major petroleum industry leaders, thermometric acid number by ASTM D8045 provides the highest accuracy for analysis of acid number in refinery fractions and crude oils.2 The advantages to this accuracy are actualized in terms of corrosion prevention and improved commerce. In refinery operations, quick decisions based on accurate acid number need to be made in order to prevent corrosive damage to valuable infrastructure. For instance, the catalytic cracking units integrate expensive catalyst, high temperature and pressure to transform heavy vacuum gas oil fractions into revenue generating gasoline products.

Moreover, quick acid number assessment using the ASTM D8045 standard enables the adjustment of costly acid inhibitor chemicals and catalyst to prevent sudden damage from the presence of corrosive acids. One example is a shutdown in this process, where quick acid number results can save millions of dollars for a refinery.

Since the prices of crude and petroleum products are largely dependent on acid number, ASTM D8045 is the only internationally approved standard that can help assess acidity in both these product types. This thermometric method solves acidity accuracy and sample solubility and offers better agreement between sellers and buyers of petroleum products and is a proposed standard to add to these agreements.

Future of Thermometric Titration

Now, the industry can control its most valuable testing parameter for the first time ever. Metrohm’s collaboration with the ASTM to develop the method D8045 as well as the acceptance of thermometric titration as an international standard presents new opportunities for the adoption of other standards where faster analysis times, reduced solvent toxicity, and increased accuracy are urgently required.

Laboratories examining total base number by current approved standard, ASTM D2896, have to use higher amounts of toxic solvents such as chlorobenzene. Application research has also been completed to document how thermometric titration can be exploited to bring faster titration times and reduced solvent toxicity and consumption to total base number analyses.3 Industry requirements within ASTM will be the driving force behind the implementation of thermometric titration in the determination of total base number.


[1] NACE report (2002). Retrieved from https://www.nace.org/Publications/Cost-of-Corrosion-Study/ on July 13th, 2015.

[2] B. McGarvey, B. Thakkar, C. McGarvey, L. Tucker, L. Carey, Controlling Corrosion: A New Method to Measuring Acid Number in Crude Oil and Refi nery Distillation Fractions, Sarnia Technology Applications and Research, Metrohm USA (2016).

[3] L. Carey; Thermometric Determination of TBN in Petroleum Products; Metrohm USA Application Work TI US 08/2014 (2014).

This information has been sourced, reviewed and adapted from materials provided by Metrohm AG.

For more information on this source, please visit Metrohm AG.

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