Fast determination of acid and base number by thermometric titration
26. maj 2026
Artikel
Del artikel
Acid number (AN) and base number (BN) are critical parameters for the quality control of petroleum products as they are often stipulated by product specifications. These parameters can be determined by potentiometric or photometric titration according to various standards including ASTM D664, ASTM D2896, and ASTM D974. However, potentiometric titrations can be time consuming. Also, cleaning the potentiometric sensor is not only tedious but also causes reproducibility issues. Luckily a rapid and reliable alternative titration method exists – thermometric titration. Moreover, a standard is available for the acid number determination with thermometric titration (ASTM D8045).
This article covers the following topics:
Why determine the acid and base number?
The acid number indicates the amount of acids present in petroleum products. Weak acids (e.g., naphthenic acid) in crude oils can be linked to corrosion of refinery equipment. Aging of petroleum products can lead to a buildup of acids, increasing the corrosion risk to motors and engines.
To prevent acidic buildup, basic additives are added to refined petroleum products, such as lubricating oil. These basic additives neutralize the weak acids and can prevent corrosion. The amount of basic additives can be characterized using the base number.
What is thermometric titration?
Thermometric titration (TET) is based on the principle of enthalpy change. Each chemical reaction is associated with a change in enthalpy that in turn causes a temperature change. The temperature change during a titration can be measured with a highly sensitive thermistor (Figure 1) to determine the endpoint of the titration.
If you would like to read more about the basic principles of thermometric titration, check out our blog post «Thermometric titration – the missing piece of the puzzle».
TET: the best choice for AN and BN determination
Regarding potentiometric titration of the acid and base number, note that not all samples are soluble in the solvent mixture. Even if they are soluble, several cleaning steps (including conditioning of the electrode after each titration) are necessary to achieve good reproducibility.
While photometric titration provides an alternative indication method for samples which are not colored, the solubility issue remains. Thermometric titration of the AN according to ASTM D8045 provides the ideal solution to all these issues.
- The xylene:IPA (3:1) solution allows better solubility of many samples, especially crude oils
- Endpoint indication is not affected by colored samples
- The dThermoprobe (Figure 1) requires no conditioning or additional cleaning steps – only a dip-rinse in solvent
- The dThermoprobe is maintenance free – no electrolyte refilling necessary, just store it dry
TET offers even more benefits if compared to the potentiometric titration according to ASTM D664 or ASTM D2896.
- Less solvent used: 30 mL instead of 60 mL or even 120 mL saves additional costs and reduces waste.
- Faster titrations: TET takes half the time of potentiometric titrations, saving about two minutes per analysis.
- Higher reproducibility: due to the easy cleaning of the dThermoprobe, better reproducibility is achieved.
Table 1 compares acid number determination according to ASTM D8045 (thermometric titration) and ASTM D664 (potentiometric titration).
| ASTM D664 (Potentiometric) | ASTM D8045 (Thermometric) | |
|---|---|---|
| Titrant | 0.1 mol/L KOH in IPA | 0.1 mol/L KOH in IPA |
| Solvent | Toluene / IPA / water | Xylene / IPA |
| Solvent volume | 125 mL | 30–35 mL |
| Titration duration | ~220 s | ~60 s |
| Electrode conditioning | 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 to <1.0 mg KOH/g) |
20 ± 2 g | ~10 g |
Discussions for an ASTM standard on thermometric BN determinations are currently ongoing within the respective committee. While the titrant and solvent mixtures differ when performing a base number determination, the values for solvent volume, titration duration, electrode conditioning, and sensor maintenance reflect the comparison between thermometric base number determination and potentiometric determination according to ASTM D2896 very well.
Since the titration is faster, uses less solvent, and does not require complicated sensor maintenance, switching to thermometric titration saves quite a bit of money.
Not convinced yet? One of our customers, Thomas Fischer from OELCHECK GmbH, Germany, talks about his positive experiences with Metrohm thermometric titration.
Thermometric titration has several advantages compared to potentiometric titration. It is much faster and more robust. A typical thermometric titration takes just about two minutes. Moreover, the electrode does not need to be regenerated between determinations.
Thomas Fischer,
Laboratory Manager at OELCHECK GmBH
How to perform the analysis
During the AN or BN determination, very weak acids or bases (respectively) are titrated, resulting in small enthalpy changes. By using a catalytic endpoint indicator, these weak acids and bases can also be determined by TET.
What is catalyzed endpoint indication?
Endpoint indication becomes difficult for thermometric titrations with small enthalpy changes, e.g., with weak acids or bases. In these situations, a catalytic endpoint indicator is used. The catalytic endpoint indicator undergoes either a strong exothermic or endothermic reaction during the titration.
Just as with an indicator which changes color when all analyte has been titrated, the catalytic endpoint indicator only starts its reaction with the titrant after all analyte has been consumed. In this way, the indication of the endpoint becomes possible.
Acid number
An appropriate amount of the sample (depending on the expected AN) is weighed into the titration vessel, then 30 mL solvent mixture (isopropanol:xylene 1:3) and 0.5 g paraformaldehyde are added. As an alternative, paraformaldehyde can also be suspended in the solvent mixture and added automatically. After dissolution of the sample, the solution is then titrated with alcoholic KOH to a single exothermic endpoint.
Here, the paraformaldehyde acts as the catalytic endpoint indicator. As soon as there is an excess of KOH available it will depolymerize in a strongly endothermic reaction. This will lead to a decrease in the temperature, which will be detected as an exothermic endpoint. This principle is illustrated in Figure 2.
For more detailed information about this application, download our free Application Bulletin.
AB-427: Acid number in petroleum products with thermometric titration
Base number
An appropriate amount of the sample (depending on the expected BN) is weighed directly into the titration vessel, then 1 mL isobutyl vinyl ether (or n-butyl vinyl ether) and 40 mL toluene are added. After dissolution of the sample, the solution is then titrated with HClO4 or TFMSA in glacial acetic acid to a single endothermic endpoint.
In this situation, the butyl vinyl ether serves as the catalytic endpoint indicator. When an excess of acidic titrant is present, it will polymerize in a strongly exothermic reaction, resulting in an endothermic endpoint (Figure 3).
For more detailed information about this application, download our free Application Bulletin.
The principle of the acid and base number determination with catalytic endpoint indication is illustrated in this short animation.
Summary
Thermometric titration provides a rapid and robust solution for the determination of the acid and base number in comparison to potentiometric or photometric titration. The method addresses the issue of sample solubility by using more suitable solvents. Furthermore, less solvent is needed and the analysis time is reduced. All this results in considerably lower costs per analysis, making TET a viable alternative for the acid and base number determination.
Additional resources
Corrosion control: Thermometric TAN analysis in oil & refinery distillation fractions
Many refiners look at discounted opportunity crudes as a means to improve their margin spread. The varieties of these cheap crude oils on the market are growing in number, but they have hidden risks for the purchaser caused by factors such as high naphthenic acid and sulfur content. Sulfur compounds and naphthenic acids are among the substances that contribute to the corrosive nature of crude oils and petroleum products. This is why the risk of corrosion is increased when processing crude oils with high naphthenic acid and sulfur content. The refiner must balance the cost benefit versus the risk and the cost of corrosion control when processing these crudes. A reliable acid number determination is a crucial part of corrosion control. Guest authors Bert Thakkar, Bryce McGarvey, and Colette McGarvey of Imperial Oil and Larry Tucker and Lori Carey of Metrohm USA were involved in the development of the new ASTM Method D8045 for acid number determination. Here, they report on the method and how it came to be.
