Application Solution for Oil and Fat Detection Using Abbe Refractometer

When light passes from air into different substances, the ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant. However, the numerical value of this constant varies depending on the substance and is a reflection of the substance's inherent properties. Each homogeneous substance has its own characteristic refractive index. For solutions of the same substance, the magnitude of the refractive index is also related to its concentration; generally, the refractive index is directly proportional to the concentration.

Therefore, by utilizing this property of refractive index and measuring the refractive index of oils and fats, one can determine the type, purity, and concentration of the oil or fat. Natural oils and fats are mixtures of mixed triglycerides. Oils and fats from different sources contain different triglycerides, and each triglyceride has its own unique refractive index. Hence, different oils and fats have different refractive indices. For each natural oil or fat, its refractive index is a physical constant. When the quality of the oil or fat changes, such as through adulteration or rancidity, its refractive index will also change. Oils and fats with higher relative density also have higher refractive indices. When the acidity of the oil increases, meaning its density decreases, its refractive index lowers. Thus, measuring the refractive index of oils and fats is an important means of identifying their composition and quality.

2. Experimental Preparation

2.1 Test Equipment: Digital Abbe Refractometer
Digital Abbe Refractometer

2.2 Experimental Reagents:

1. Soybean Oil

2. Peanut Oil

3. Corn Oil

4. Rapeseed Oil

5. Cottonseed Oil

2.3 Reference Standard
《Method for Determination of Refractive Index of Animal and Vegetable Fats and Oils GB/T5527-2010》

3. Experimental Procedure

3.1 Sample Testing

1. Connect the ZY-252 Digital Abbe Refractometer to a constant temperature water bath. When the temperature is controlled at 20°C, calibrate the refractometer using distilled water to a refractive index of 1.3330.

2. Separate the two prisms. Using a dropper, add a small amount of sample (1-3 drops) onto the surface of the fixed prism and immediately close the prisms. Wait for a few minutes to allow the sample to reach the prism temperature.

3. Adjust the screw handwheel of the prism until the field of view is divided into light and dark parts. Rotate the compensator knob (dispersion handwheel) to eliminate the chromatic dispersion and make the light-dark boundary line sharp. Continue adjusting the screw handwheel to align the light-dark boundary line with the crosshair.

4. Click the test button. The instrument will automatically display the refractive index of the sample. Read immediately again. At least two readings should be taken for each sample, and their arithmetic mean should be calculated.

5. Clean and completely dry both prisms. Perform a second determination of the above sample. The average of the two determinations is taken as the refractive index of this sample.

4.1 Experimental Results

4.2 Discussion

Refractive index is one of the main parameters reflecting the purity of edible oils. For example, the refractive index of fatty acids increases with molecular weight and degree of unsaturation. Therefore, short-chain saturated fatty acid esters have lower refractive indices, while oils like linseed oil, which are rich in unsaturated acids, have higher refractive indices. Refractive index is an essential parameter for quality control and mandatory inspection of edible oils. In the quality inspection industry, refractive index detection methods can also be used to quickly identify counterfeit products. For instance, industrial and commercial systems use the WYA-3S Digital Abbe Refractometer to rapidly test for sunflower oil content in peanut oil.