Basic chemicals are added to petroleum products to prevent corrosion as they neutralize acidic components that form during the use and aging of these products. The base number (BN) gives an indication regarding the amount of these basic additives present, and it can be used as a measure for the degradation of the petroleum product.

The use of a pH electrode suitable for non-aqueous titrations ensures the reliable determination of the equivalence point. A flexible sleeve diaphragm facilitates its cleaning especially after use in heavily contaminated samples, such as used engine oils. Using the correct electrode greatly increases the precision and reliability of the results.

This Application Note describes the potentiometric determination of the base number according to ISO 3771, ASTM D2896, and IP 276 using the Metrohm Solvotrode easyClean and a fully automated OMNIS system.

This application is demonstrated on lubricating oil and fresh motor oil. 

Care should be taken to use a representative, well-homogenized sample portion. Otherwise, no sample preparation is necessary.

Sample Robot and OMNIS Titrator Advanced equipped with a Solvotrode easyClean for the determination of the base number of lubricating oil and motor oil.
Figure 1. Sample Robot and OMNIS Titrator Advanced equipped with a Solvotrode easyClean for the determination of the base number of lubricating oil and motor oil.

The analysis is carried out on an automated system consisting of an OMNIS Sample Robot S and an OMNIS Advanced Titrator equipped with a Solvotrode easyClean.

Before the samples are measured, a blank determination is performed using the same amount of solvent as for the sample titration.

The sample is dissolved in a solvent mixture consisting of toluene, glacial acetic acid, and acetone. For the most efficient release of the basic constituents, these three solvents are added subsequently with a waiting time between the additions. Then the solution is titrated with standardized perchloric acid in glacial acetic acid until after the equivalence point is reached.

After each sample determination, the electrode needs to be rinsed with solvent solution, then isopropyl alcohol (IPA) followed by water. To rehydrate the glass membrane of the electrode, the glass membrane is placed into deionized water.

For the tested samples, well-defined titration curves are obtained with acceptable results and low standard deviations. Results are summarized in Table 1. An example titration curve is displayed in Figure 2.

Table 1. Results for the acid number determination according to ASTM D2896 on a fully automated OMNIS system.
BN in mg KOH/g sample lubricating oil (n = 3) motor oil (n = 26)
Mean 0.397 7.2325
SD(abs) 0.0001 0.0727
SD(rel) 0.03 1.01
Figure 2. Example titration curve of the base number determination in fresh motor oil.

Fully automated determination of the base number of petroleum products according to ASTM D2896, ISO 3771, and IP 276 can be achieved using a Metrohm OMNIS system. Using an OMNIS Sample Robot allows the fully automated titration of up to four samples simultaneously, increasing throughput. The OMNIS platform offers the opportunity to customize your system according to your needs and expand it for other required titration applications.


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