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1.
The concept of refractive index
When light enters another medium from one medium, the light changes its direction due to different propagation speeds, which causes refraction
.
No matter how the incident angle changes, the ratio of the sine of the incident angle to the sine of the refraction angle is always equal to the ratio of the propagation speed of light in the two media
.
This ratio is called the refractive index
The refractive index is a characteristic constant of a substance
.
For a solution of the same substance, the refractive index depends on the wavelength of the light, the temperature and the concentration of the solution
It represents a refractive index of the light to be marked and the temperature used for the measurement, a predetermined standard as a vegetable oil, oil of refractive index was measured at a temperature of 20 ℃ as the standard, with the result n- D 20 is or n- 20 is represented
.
Second, the significance of refractive index measurement
The refractive index is a commonly used process control index in food production.
By measuring the refractive index of liquid food, the composition of the food can be identified, the concentration of the food can be determined, and the purity and quality of the food can be judged
.
For example, because the refractive index of the sucrose solution increases with the increase in concentration, the concentration of the sugar solution and the sugar content of foods such as beverages and canned sugar can be determined by measuring the refractive index, and it can also be used to determine the fruit juice, honey and other foods with sugar as the main component.
Various oils and fats have their own fatty acid composition, and each fatty acid has its own specific refractive index
.
When the number of carbon atoms is the same, the refractive index of unsaturated fatty acids is much larger than that of saturated fatty acids ; the greater the molecular weight of unsaturated fatty acids, the greater the refractive index; the higher the acidity of the oil has a lower refractive index
The refractive index of liquid food has a certain range under normal circumstances.
For example, the refractive index of normal cow whey is between 1.
34199 and 1.
34275.
When the quality of these liquid foods changes due to adulteration, concentration changes, or variety changes, etc.
, The refractive index often changes
.
Therefore, measuring the refractive index can preliminarily judge whether certain foods are normal