Acid number (AN) and base number (BN) which are critical parameters in controlling the quality of petroleum products because they are often determined by product specifications. Traditionally, both parameters can be measured by potentiometric or photometric titration, which is one of the laboratory equipment, based on various standards such as  TBN & TAN ASTM D664 (Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration), TBN & TAN ASTM D2896 (Standard Test Method Determined for the base number of potentiometric (potentiometric) products. titration), or ASTM D974 (Standard Test Method for Acid and Base Number by Color Indicator Titration). However, there is a fast and reliable alternative titration method – thermometric titration.

Why do we determine the number of acid and base?
The acid number indicates the amount of acids in petroleum products. Weak acids in crude oil (eg naphthenic acid) can be associated with corrosion of refinery equipment. For petroleum products, aging can lead to the accumulation of acids, which increases the risk of corrosion of pipes and storage tanks.
To prevent acid build-up, basic additives are added to refined petroleum products such as lubricating oil. These basic additives neutralize weak acids and can prevent corrosion. The amount of basic additives can be determined using the base number.

What is thermometric titration?
Thermal titration (TET) is based on the principle of enthalpy change. Every chemical reaction is accompanied by a change in enthalpy, which in turn causes a change in temperature. This temperature change during the titration can be measured with a very sensitive thermistor (Figure 1) in order to determine the endpoint of the titration.

If you’ve done potentiometric titrations of acid and base numbers, you probably know that not all samples are soluble in solvent mixtures. Even if they are soluble, several cleaning steps (including electrode conditioning after each titration) are necessary to achieve good reproducibility.

While photometric titration provides an alternative indicator method for non-colored samples, the problem of solubility remains. AN thermometric titration according to ASTM D8045 provides an ideal solution to all these issues.

Xylene/IPA solution (1/3) provides better solubility of many samples, especially crude oils.
The endpoint symbol is not affected by color swatches
The Thermoprobe does not require any additional conditioning or cleaning steps – just a solvent rinse
The Thermoprobe is maintenance free – no need to refill the electrolyte, just keep it dry
Compared to potentiometric titration according to ASTM D664 or ASTM D2896, there are additional advantages.


Less solvent is used: 30 ml instead of 60 ml or 120 ml saves additional costs and reduces waste.
Faster titration: half the time of potentiometric titration, saving about 2 minutes per analysis
Robust sensor: The Thermoprobe is completely maintenance-free and requires no ventilation, further reducing analysis time.
For a comprehensive comparison between the determination of AN according to ASTM D8045 (thermometric titration) and ASTM D664 (potentiometric titration), check Table 1 below. While titrant and solvent mixtures are different if you perform the base numbering, the values ​​for solvent volume, titration time, electrode conditioning, and sensor maintenance reflect well the comparison between thermometric base numbering and potentiometric determinations in accordance with ASTM D2896. . The discussion on an ASTM standard on the determination of thermometric BN is currently ongoing in the relevant committee.

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