Operational points
The acid value of oils and fats is a key index for evaluating the content of free fatty acids in oils, reflecting the degree of hydrolysis and rancidity of oils. Potentiometric titration determines the endpoint by monitoring changes in solution potential during titration, providing more objective results than traditional indicator methods, especially for dark or cloudy oil samples. This method mainly refers to the general procedure for potentiometric titration for the determination of acid value of oils and fats in food and related industrial standards.
Method principle
The grease sample was dissolved in a suitable organic solvent and titrated using a known concentration of potassium hydroxide or sodium hydroxide standard. The hydrogen ions in the solution react with hydroxide ions, causing changes in the solution potential. When the free fatty acids are completely neutralized, the potential jump occurs, and the instrument automatically recognizes this jump point as the titration endpoint. The acid value of the oil is calculated based on the volume and concentration of the standard alkali liquid consumed. The formula for calculating acid valence (AV) is:
AV = (V × C × 56.1) / m
Among them, V is the volume of potassium hydroxide standard solution (mL) consumed by titration, C is the concentration of the standard solution (mol/L), m is the mass of the grease sample (g), and 56.1 is the molar mass of potassium hydroxide (g/mol).
Preparation before operation
Preparation is fundamental to ensuring accuracy and repeatability of assays. It mainly involves instrument status confirmation, reagent preparation and sample processing.
Instrument Calibration and Setup:Turn on the potentiometric titrator and warm it up to stability. Calibrate the pH/potential electrode according to the manufacturer's guidelines. Set appropriate titration parameters such as end jump threshold, titration speed (fast and slow is recommended), and stirring speed (ensure homogeneous mixing but no foaming).
Reagent preparation:Prepare and calibrate a standard titration solution for potassium hydroxide (or sodium hydroxide) ethanol. The common concentration is 0.05 mol/L or 0.1 mol/L. The solvent is usually a mixture of ether and ethanol or isopropyl alcohol, which needs to be neutralized to neutral before use. All reagents should be analytically pure.
Sample Processing:Weigh the appropriate amount of grease sample accurately (mass m, usually so that the titration is consumed within the expected range). The sample is completely dissolved in a pre-neutralized solvent to ensure a homogeneous solution. For solid fats, moderate warmth can help dissolve, but the temperature should not be too high.
Measurement process
Following standardized operating procedures is key to reliable data.
Titration initiation:Place the beaker containing the sample solution on the titration table and adjust the stirrer position to ensure that the electrode is submerged below the liquid level and does not collide with the stirrer. Start the titration procedure.
Endpoint judgment:The instrument will automatically record the potential-volume curve. The titration endpoint is the point with the largest rate of change in potential on the curve (breakthrough point). Check whether the automatically determined endpoint is reasonable and manually correct it if necessary.
Blank test:Under the same conditions, blank titration was performed using an equal amount of solvent (without sample). Record the volume of standard solution consumed. The final volume of the sample is to be deducted from the blank value.
Influencing factors
Multiple factors can interfere with the determination and need to be controlled during operation.
| Solvent selection | It is necessary to be able to fully dissolve fats and fatty acids, and isopropyl alcohol-toluene or ethanol-ether mixtures are commonly used. |
| Electrode maintenance | Regularly clean and activate the composite pH electrode to avoid sluggish response due to grease contamination. |
| CO2 interference | The lye may absorb CO₂ from the air, so it is recommended to use a freshly calibrated solution and reduce exposure time. |
| Sample representativeness | The grease sample should be mixed evenly to prevent sampling errors caused by local rancidity. |
| Temperature control | The titration process avoids violent temperature fluctuations, which affect the electrode potential and reaction rate. |
Analysis of results
Each assay should be performed at least parallel doubles to calculate relative deviation. If the deviation exceeds the range specified by the standard method, it needs to be re-measured. The accuracy of the method can be verified by determining a certified reference material or by a spike recovery experiment. It is necessary to participate in the comparison between laboratories on a daily basis to ensure that the testing capacity continues to meet the requirements.
Conclusion
The determination of oil acid value by potentiometric titration method has the characteristics of high degree of automation, accurate endpoint judgment and wide adaptability. Strict control of sample preparation, reagent quality, instrument parameter setting and environmental factors is the core of obtaining stable and reliable test results. Operators should have a deep understanding of the methodological principles and strictly follow the relevant standard operating procedures.
References
National Food Safety Standard Determination of Acid Value in Food, GB 5009.229-2016.
Determination of acid and acidity of animal and vegetable oils, ISO 660:2020.
American Society of Oleochemists Official Methods, Cd 3d-63.