Acid Number Determination in Lubricating Oil
Acid number (AN), commonly called total acid number (TAN), is an important quality parameter for lubricating oils and petroleum products. It represents the amount of potassium hydroxide required to neutralize acidic components in one gram of sample and is reported as mg KOH/g.
Acid number can be used to evaluate lubricant quality and monitor the development of acidic compounds during service.
This application demonstrates automated acid-number determination using a Hiranuma COM Series automatic potentiometric titrator, following the principles of ASTM D664, Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration.
Principle of the Method
The lubricating-oil sample is dissolved in a mixed nonaqueous solvent and titrated with standardized potassium hydroxide in 2-propanol.
A glass electrode and reference electrode continuously monitor the titration. A clear potentiometric inflection point can be used as the endpoint. For samples that do not produce a sufficiently sharp inflection point, an endpoint based on the appropriate buffer response can be used according to the applicable method.
The Hiranuma titrator automatically controls titrant addition, detects the endpoint and calculates acid number.
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 potassium hydroxide in 2-propanol
Titration solvent:
500 mL toluene + 495 mL 2-propanol + 5 mL water
Buffer solution:
Prepared according to the applicable method requirements.
Measurement Procedure
- Determine the appropriate endpoint response using the specified buffer solution when required.
- Accurately weigh approximately 5 g of lubricating-oil sample into a suitable titration vessel. Adjust the sample amount according to the expected acid 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 automatically with standardized alcoholic potassium hydroxide.
- Perform a blank determination using the same solvent and procedure.
- The titrator calculates acid number and reports the result as mg KOH/g.
Example Results
| Measurement | Sample (g) | Titrant (mL) | Acid Number (mg KOH/g) |
|---|---|---|---|
| 1 | 4.9901 | 1.923 | 2.090 |
| 2 | 4.9989 | 1.890 | 2.049 |
| 3 | 5.0012 | 1.892 | 2.050 |
Average: 2.06 mg KOH/g
SD: 0.02 mg KOH/g
RSD: 1.13%
Average blank: 0.075 mL
The results demonstrate good repeatability for automated acid-number determination in lubricating oil.
Electrode and Buret Maintenance
Nonaqueous titration requires appropriate electrode maintenance. Extended exposure of a glass electrode to nonaqueous solvents can reduce response speed and electrode potential.
Hiranuma recommends periodically conditioning the electrodes in purified water. Crystallization around the reference-electrode junction should also be removed when necessary.
Alcoholic potassium hydroxide can crystallize in the buret flow path. If the instrument will remain unused for an extended period, remove the titrant and clean the flow path appropriately.
Automated TAN Analysis with Hiranuma
The Hiranuma COM Series automates titrant delivery, potentiometric endpoint detection, calculations and result handling for routine petroleum and lubricant QC.
For laboratories analyzing larger numbers of samples, the COM-A19 can be combined with automated sample handling, providing a higher-throughput approach to routine TAN analysis.
