Base Number Determination in Lubricating Oil
Base number (BN), also commonly referred to as total base number (TBN), is an important quality parameter for lubricating oils. It indicates the reserve alkalinity available to neutralize acidic compounds that can form during lubricant service.
Base number is expressed as milligrams of potassium hydroxide equivalent per gram of sample (mg KOH/g).
For petroleum products and lubricants, potentiometric titration with perchloric acid provides a reliable method for determining base number. This application demonstrates automated analysis using a Hiranuma COM Series automatic titrator and is based on the principles of ASTM D2896, Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration.
Principle of the Method
The lubricating-oil sample is dissolved in a nonaqueous titration solvent and titrated potentiometrically with standardized perchloric acid in glacial acetic acid.
A glass electrode and reference electrode monitor the change in potential during titration. When a distinct inflection point is obtained, the automatic titrator determines the endpoint and calculates the base number.
For samples that do not produce a sufficiently clear endpoint by direct titration, a perchloric acid back-titration method may be appropriate.
Instrument Configuration
Automatic titrator: Hiranuma COM Series
Glass electrode: GE-101B
Reference electrode: RE-201Z
For this nonaqueous application, the normal aqueous reference-electrode filling solution is replaced with saturated sodium perchlorate in glacial acetic acid.
Reagents
Titrant:
0.1 mol/L perchloric acid in glacial acetic acid
Titration solvent:
Glacial acetic acid/chlorobenzene mixture
Toluene may be used as an alternative solvent where appropriate.
Reference electrode filling solution:
Saturated sodium perchlorate in glacial acetic acid
Measurement Procedure
- Accurately weigh the appropriate amount of lubricating-oil sample into a titration vessel. Sample size may be adjusted according to the expected base number.
- Add the titration solvent and stir until the sample is dissolved or uniformly dispersed.
- Immerse the glass and reference electrodes in the solution.
- Titrate automatically with standardized perchloric acid in glacial acetic acid.
- Perform a blank determination using the same solvent system.
- The titrator automatically detects the potentiometric endpoint and calculates the base number in mg KOH/g.
Example Results
| Measurement | Sample (g) | Titrant (mL) | Base Number (mg KOH/g) |
|---|---|---|---|
| 1 | 1.0126 | 2.227 | 12.236 |
| 2 | 0.9114 | 1.998 | 12.181 |
| 3 | 1.0073 | 2.218 | 12.250 |
Average: 12.2 mg KOH/g
SD: 0.0366 mg KOH/g
RSD: 0.30%
The low relative standard deviation demonstrates excellent repeatability for automated base-number determination.
Important Considerations for Nonaqueous Titration
Electrode condition is particularly important in nonaqueous titration. Extended exposure of a glass electrode to nonaqueous solvents can reduce response speed, so appropriate cleaning and conditioning should be included in routine laboratory procedures.
Temperature should also be controlled as closely as practical. Glacial acetic acid has appreciable thermal expansion, so titrant standardization and sample analysis should preferably be performed under similar laboratory temperature conditions.
Automated Base Number Analysis with Hiranuma
The Hiranuma COM Series provides automatic titrant delivery, potentiometric endpoint detection, calculation and data handling for routine lubricant QC.
For laboratories processing larger numbers of oil samples, the COM-A19 can also be configured with automated sample handling to reduce operator involvement and improve consistency between analyses.
