Base Number Determination by Back Titration
Base number (BN), often referred to as total base number (TBN), is an important quality parameter for lubricating oils and petroleum products. It represents the alkaline reserve available to neutralize acidic compounds and is expressed as milligrams of potassium hydroxide equivalent per gram of sample (mg KOH/g).
Potentiometric titration with perchloric acid is widely used for base-number determination. When a sufficiently distinct endpoint is difficult to obtain by direct perchloric acid titration, back titration provides an alternative approach.
This application demonstrates automated base-number analysis using a Hiranuma COM Series automatic titrator, based on the back-titration approach described in ASTM D2896.
Principle of the Method
The lubricating-oil sample is dissolved in a nonaqueous titration solvent. A known excess of standardized perchloric acid in glacial acetic acid is added to react with the basic components in the sample.
After the reaction period, the remaining perchloric acid is back-titrated with standardized sodium acetate in glacial acetic acid.
A blank determination is performed using the same procedure. The difference between the blank and sample titration volumes is used to calculate the base number.
Instrument Configuration
Automatic titrator: Hiranuma COM Series
Electrode: GE-101B glass electrode
Reference electrode: RE-201Z
For this nonaqueous titration, the reference-electrode filling solution is replaced with saturated sodium perchlorate in glacial acetic acid.
The method requires controlled addition of perchloric acid followed by automated back titration with sodium acetate.
Reagents
Back titrant:
0.1 mol/L sodium acetate in glacial acetic acid
Perchloric acid solution:
0.1 mol/L perchloric acid in glacial acetic acid
Titration solvent:
Glacial acetic acid/chlorobenzene mixture
Toluene may be used instead of chlorobenzene where appropriate.
Reference-electrode filling solution:
Saturated sodium perchlorate in glacial acetic acid.
Measurement Procedure
- Accurately weigh approximately 1 g of lubricating-oil sample into a suitable titration vessel. Adjust the sample amount when necessary according to the expected base number.
- Add 60 mL of titration solvent and stir to dissolve or uniformly disperse the sample.
- Immerse the electrodes in the solution.
- Add 4 mL of standardized 0.1 mol/L perchloric acid in glacial acetic acid.
- Stir for approximately two minutes to allow reaction with the basic components.
- Back-titrate the remaining perchloric acid with standardized 0.1 mol/L sodium acetate in glacial acetic acid.
- Perform a blank determination using the same procedure.
- Calculate the base number from the difference between the blank and sample titration volumes.
Example Results
| Measurement | Sample (g) | Titrant (mL) | Base Number (mg KOH/g) |
|---|---|---|---|
| 1 | 1.0307 | 1.719 | 12.375 |
| 2 | 1.0015 | 1.779 | 12.398 |
| 3 | 1.0108 | 1.763 | 12.373 |
Average: 12.4 mg KOH/g
SD: 0.0138 mg KOH/g
RSD: 0.11%
The excellent repeatability demonstrates the suitability of automated back titration for routine lubricant base-number analysis.
When Is Back Titration Useful?
Direct perchloric acid titration is appropriate when the lubricant produces a clear potentiometric endpoint.
For samples where the endpoint is weak or difficult to identify, the back-titration technique provides another approach: a known excess of perchloric acid is allowed to react with the sample, and the remaining acid is quantitatively determined with sodium acetate.
This complements the direct ASTM D2896 perchloric acid method we just rewrote.
Important Considerations
Because this is a nonaqueous titration, electrode maintenance is important. Prolonged exposure of the glass electrode to nonaqueous solvents can reduce response speed. The original Hiranuma method recommends periodic electrode conditioning and cleaning of the reference-electrode liquid junction.
Temperature should also remain reasonably consistent between titrant standardization and sample analysis because glacial acetic acid undergoes appreciable thermal expansion.
Automated Lubricant QC with Hiranuma
The Hiranuma COM Series automates reagent delivery, potentiometric endpoint detection, calculations and result handling, reducing operator-dependent variability in routine oil and lubricant testing.
For laboratories processing larger sample volumes, automated sample handling can further increase throughput and consistency.
