Fast determination of acid and base number by thermometric titration
Want to learn more? Check out our blog post on this topic.
Acid number (AN) and base number (BN) are critical parameters in the quality control of petroleum products as they are often stipulated by product specifications. Traditionally both parameters can be determined by potentiometric or photometric titration according to various standards such as ASTM D664 (Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration), ASTM D2896 (Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration), or ASTM D974 (Standard Test Method for Acid and Base Number by Color-Indicator Titration). However, there is a rapid and reliable alternative titration method – thermometric titration.
Why determine the acid and base number?
The acid number is an indication for the amount of acids present in petroleum products. Weak acids present in crude oils (e.g. naphthenic acid) can be linked to corrosion of refinery equipment. For petroleum products, aging can lead to the buildup of acids, which increases the risk of corrosion to pipes and holding tanks.
To prevent such an 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. This temperature change during a titration can be measured with a highly sensitive thermistor (Figure 1) in order to determine the endpoint of the titration.
Figure 1. Metrohm’s maintenance-free Thermoprobe used for fast and reliable indication of thermometric titration endpoints.
If you would like to read more about the basic principles of thermometric titration, check out our previous blog post «Thermometric titration – the missing piece of the puzzle».
TET: the best choice for AN and BN determination
If you’ve performed a potentiometric titration of the acid and base number, you probably know 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 in order 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 of 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 Thermoprobe requires no conditioning or additional cleaning steps – only a rinse with solvent
- The Thermoprobe is maintenance-free – no electrolyte refilling necessary, just store it dry
There are 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 120 mL saves additional costs and reduces waste
- Faster titrations: half the time of potentiometric titrations, saving about 2 minutes per analysis
- Robust sensor: the Thermoprobe is completely maintenance-free and needs no conditioning, further reducing analysis time.
For a comprehensive comparison between the AN determination according to ASTM D8045 (thermometric titration) and ASTM D664 (potentiometric titration), check out Table 1 below. While the titrant and solvent mixtures differ if you perform a base number determination, the values for solvent volume, titration time, electrode conditioning, and sensor maintenance reflect the comparison between thermometric base number determination and potentiometric determination according to ASTM D2896 very well. Discussions for an ASTM standard on thermometric BN determinations are currently ongoing within the respective committee.
Table 1. Comparison between ASTM D664 and ASTM D8045 concerning various parameters.
Since you are titrating faster, using less solvent, and do not have to perform complicated sensor maintenance, you can save quite a bit of money by switching to thermometric titration. How much? Try out our cost savings calculator!