Determination of the Acid Number According to ASTM D664 and the Base Number in Fresh Motor Oil

Refined petroleum products are evaluated using the acid number (AN) and base number (BN) which are vital in their quality control and validation. This Application Note describes how to carry out a potentiometric determination of the AN in compliance with ASTM D644 using a reduced solvent volume of 60 mL. It also describes how to determine the BN using a fully automated OMNIS system. A dSolvtrode was used for this indication.


New motor oil.


4 – 7 g of the sample is accurately weighed into a sample beaker to which the device automatically adds 60 mL solvent. The stirred sample is then titrated with the corresponding titrant until the equivalence point is passed.

Following the completion of the titration, the electrode and buret tip are rinsed first with the solvent mixture and then with IPA and carbon dioxide-free water.  Membrane rehydration is carried out by placing the electrode for 3 to 5 minutes in distilled water. The electrode should be rinsed out with IPA prior to the next measurement.





New motor oil sample.

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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