Determination of Acid Value in Lubricating Oil - Potentiometric Titration Method

The acid value of lubricating oil is an important indicator to determine the amount of free acidic substance (mainly organic acid) in the oil, and the unit is mg KOH/g of oil sample. The higher the acid value, the more acidic free fatty acids (FFAs) in the oil sample, which means the oil may be oxidized, contaminated, or contain water, and may cause metal corrosion. Therefore, the determination of acid value is widely applied in oil quality control, oil change determination, and monitoring oil aging.

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Recommended Method

The recommended standard is ASTM D664 (potentiometric titration method). This method is especially recommended for the determination of acid value of dark-colored oil, as the potentiometric method can more accurately determine the endpoint of titration.

Instruments and Reagents

Procedure (Potentiometric Titration Method)

1. Weigh the Sample

Accurately weigh 1.0 g (±0.001 g) of the lubricating oil sample into a dry titration flask.

2. Dissolve the Sample

Add 50 mL of the toluene-isopropanol mixed solvent to the titration flask, and shake thoroughly to ensure the oil sample is completely dissolved.

3. Add Indicator (optional)

If using the colorimetric method, add 2-3 drops of phenolphthalein indicator until the solution turns light pink.

4. Prepare Electrodes

Immerse the glass electrode and reference electrode into the solution and record the starting potential.

5. Titration Process

Automatically titrate with the 0.1 M KOH standard solution, recording the potential changes in real time.

The endpoint is reached when a sharp increase of approximately 55 mV is observed in the potential (or when the color changes from pink to colorless).

6. Blank Calibration

Perform a blank titration using the same volume of solvent, and record the volume consumed for subsequent calculations.

7. Record Endpoint Volume

Record the volume of KOH consumed to reach the endpoint (mL).

Calculation Formula

The acid value (mg KOH / g) can be calculated using the following formula:

V: Titration volume of the sample (mL)

V₀: Blank titration volume (mL)

C_KOH: Concentration of KOH standard solution (mol/L), typically 0.1 mol/L

56.1: Molar mass of KOH (g/mol) converted to milligrams

m: Mass of the oil sample (g)

If using 0.1 M KOH, the formula can be simplified as:

Since 0.1 mol/L × 56.1 = 5.61

Result Interpretation and Application

Explanation: An increase in acid value often indicates oxidation degradation, metal wear products, or water contamination in the oil. Regular monitoring of acid value can help detect issues early, allowing for timely actions such as oil replacement or antioxidant addition to prevent oil failure.

Common Considerations

Sample Drying: If the oil sample contains free water, it may cause the acid value to be too high. It is recommended to dry the sample at 105°C for 30 minutes before testing.

Electrode Calibration: The glass electrode needs to be calibrated in a pH = 7.00 buffer solution, and the reference electrode needs to use fresh TEABr solution.

Blank Calibration: Blank titration is required to eliminate the weak acidity of the solvent.

Temperature Control: Titration should be performed at 25 ± 2°C, and the temperature change will affect the accuracy of the potential jump.

Repeatability: At least two titrations should be performed on the same oil sample, and the average should be taken. If the difference exceeds 5%, the sample should be re-weighed or the instrument should be checked.

Conclusion

The determination of acid value in lubricating oil is an important indicator to ensure oil quality and stable operation of equipment. By regularly monitoring the acid value of lubricating oil, users can detect the early signs of oil aging or contamination, take timely measures to change the oil or add antioxidants, and extend the service life of equipment, and also ensure the smooth progress of production.