Base number (BN), commonly referred to as total base number (TBN), is an important quality parameter for lubricating oils. 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).
This application demonstrates automated determination of base number using potentiometric hydrochloric acid titration with a Hiranuma COM Series automatic titrator.
The method follows the principles of ASTM D4739, Standard Test Method for Base Number Determination by Potentiometric Hydrochloric Acid Titration, and JIS K 2501.
Principle of the Method
The lubricating-oil sample is dissolved in a mixed nonaqueous solvent and titrated with standardized hydrochloric acid in 2-propanol.
A glass electrode and reference electrode monitor the titration. When a clear inflection point is obtained, it can be used to determine the endpoint. For samples without a sufficiently distinct inflection point, the endpoint can be determined using a pH value established with the appropriate buffer solution.
Instrument Configuration
Automatic titrator: Hiranuma COM Series
Glass electrode: GE-101B
Reference electrode: RE-201Z
Temperature sensor: TE-403 thermistor electrode
Reagents
Titrant:
0.1 mol/L hydrochloric acid in 2-propanol
Titration solvent:
500 mL toluene + 495 mL 2-propanol + 5 mL water
Buffer solution:
Prepared according to the applicable JIS K 2501 procedure.
Measurement Procedure
- Determine the endpoint pH using the specified buffer solution.
- Accurately weigh approximately 5 g of lubricating-oil sample into a suitable titration vessel. Adjust sample size when necessary according to the expected base number.
- Add 125 mL of titration solvent and stir until the sample is dissolved or uniformly dispersed.
- Immerse the electrodes in the sample solution.
- Titrate with standardized alcoholic hydrochloric acid to the predetermined endpoint.
- Perform a blank determination using the same solvent and procedure.
- Calculate the base number automatically and report the result as mg KOH/g.
Example Results
| Measurement | Sample (g) | Titrant (mL) | Base Number (mg KOH/g) |
|---|---|---|---|
| 1 | 5.0066 | 1.786 | 1.978 |
| 2 | 5.0291 | 1.770 | 1.951 |
| 3 | 5.0086 | 1.747 | 1.934 |
Average: 1.95 mg KOH/g
SD: 0.022 mg KOH/g
RSD: 1.14%
The blank determination averaged 0.018 mL.
These results demonstrate good repeatability for automated base-number determination in lubricating oil.
Hydrochloric Acid vs. Perchloric Acid Methods
JM Science now has three complementary base-number applications:
ASTM D4739 — potentiometric hydrochloric acid titration
ASTM D2896 — direct potentiometric perchloric acid titration
ASTM D2896 — perchloric acid back titration
The appropriate method depends on the sample, lubricant formulation and applicable laboratory or product specification.
Electrode and Buret Care
Nonaqueous titration requires appropriate electrode maintenance. Hiranuma recommends periodically conditioning the glass electrode in water because extended exposure to nonaqueous solvents can reduce electrode response.
Crystallization around the reference-electrode junction should also be removed when necessary. If the titrator will remain unused for an extended period, the buret flow path should be cleaned appropriately.
Automated Lubricant QC with Hiranuma
The Hiranuma COM Series automates titrant delivery, endpoint determination, calculation and data handling for routine lubricant testing.
For laboratories processing larger numbers of samples, the COM-A19 can also be configured for automated sample handling, providing a path from individual measurements to higher-throughput lubricant QC.
