Determination of low-level total acid number in mineral oils and biodiesel and low-level free fatty acid content in edible fats and oils

Significance of the topic

The accurate determination of free fatty acid content in edible fats and oils and the total acid number in mineral oils and biodiesel is critical for quality control, engine reliability and compliance with fuel standards. Low concentrations of acidic species can lead to corrosion, deposits and performance loss, driving the need for sensitive and robust analytical methods.

Objectives and Study Overview

This work presents a thermometric titration approach for low-level detection of free fatty acids (FFA) in solid palm oil products and total acid number (TAN) in biodiesel derived from mixed tallow and vegetable oil. The study aims to overcome limitations of manual phenolphthalein titrations and potentiometric methods by using a catalytically enhanced, non-aqueous acid-base titration with temperature-based endpoint recognition.

Methodology

Samples are weighed and dissolved in a hydrocarbon/alcohol mixture (typically toluene/2-propanol for fats or neat 2-propanol for biodiesel). Paraformaldehyde is added as a catalytic endpoint indicator. A 0.01 mol/L KOH solution in 2-propanol is delivered at a constant rate while the temperature change of the reaction mixture is recorded. The endpoint is identified by a characteristic break in the titration curve when excess hydroxyl ions trigger endothermic depolymerization of paraformaldehyde. Titration times range from 10 to 60 seconds.

Instrumentation Used

• Metrohm 859 Titrotherm thermometric titration interface
• Metrohm 6.9011.020 Thermoprobe sensor (response time 0.3 s; resolution 1×10⁻⁵ K)
• Metrohm 800 Dosino dosing unit
• Metrohm 802 Stirrer and 804 Titration Stand
• Glass titration vessels (Metrohm 6.1415.220)
• Metrohm Titrotherm software 2.0

Main Results and Discussion

Thermometric titration delivered precise quantification of FFA and TAN at low levels with excellent reproducibility:

• FFA in solid palm oil products (expressed as % w/w oleic acid): P-1 0.062±0.001 (n=7), P-2 0.040±0.001 (n=6), P-3 0.042±0.001 (n=7), P-4 0.038±0.001 (n=7).
• TAN in biodiesel samples (mg KOH/g): B-1 0.064±0.001 (n=5), B-2 0.062±0.001 (n=5), B-3 0.073±0.001 (n=5).

The method proved rapid and insensitive to sample color or turbidity. Unlike glass-membrane electrodes, the thermoprobe requires no regeneration or recalibration, ensuring consistent performance in non-aqueous media.

Benefits and Practical Applications

• Fast titration cycles enhance laboratory throughput.
• High sensitivity and precision at low acid concentrations.
• No indicator or electrode maintenance needed.
• Applicable to colored and turbid samples without interference.
• Fully automatable with sample processors for routine QA/QC.

Future Trends and Potential Applications

Advancements may include integration of online thermometric titration for real-time process monitoring, miniaturization of sensors for field analysis, expansion to additional matrices such as lubrication fluids or bio-oil blends, and coupling with laboratory information management systems for automated data handling.

Conclusion

Thermometric titration with catalytically indicated endpoints offers a robust, rapid and precise alternative to conventional manual and potentiometric titrations for low-level acid determinations in mineral oils, biodiesel and edible fats. Its ease of use and compatibility with automation make it a valuable tool for modern analytical laboratories.

Reference

1. M.J.D. Carneiro, M.A. Feres Júnior and O.E.S. Godinho. Determination of the acidity of oils using paraformaldehyde as a thermometric endpoint indicator. J. Braz. Chem. Soc. 13(5):692–694 (2002).
2. Metrohm Application Note AN-H-072. Determination of low levels of free fatty acids in edible oils and fats.
3. S. Mahajan, S.K. Konar and D.G.B. Boocock. Determining the acid number of biodiesel. JAOCS 83(6):567–570 (2006).