Refractive Index for Quality Test of Essential Oil
We have discussed in detail about the Acid Value and the Optical Rotation testings in the previous articles. Today, we will explain another analytical method that is commonly used to determine the quality of essential oil: Refractive Index.
What is the refractive index?
Refractive index is a fixed value of when light rays pass through a substance and are refracted . Each substance has a different refraction index. An easy way to describe refraction is when a pencil is placed in a glass of water. When viewed from the side, the pencil parts that are in the outside and the inside of water look separated or bended due to the light refraction .
Figure 1. Example of refraction 
This refraction occurs when light passes through the interface between two different substances. Some case when extreme refraction happens, light does not enter the interface between the two substances, but is reflected like a mirror. The angle where light comes from when this phenomenon occurs is called critical angle .
How to measure the refractive index?
The instrument commonly used to measure the refractive index is refractometer. This tool consists of prism that has a high refractive index (usually glass or artificial sapphire) and a sensor. An overview of how refractometer work is as follows. Light (1) moves from prism (2) to the sample (3) causing a reflection light (when the angle of incidence exceeds a certain value). The reflected light will then be detected by the CCD (or CMOS) sensor (4) .
Figure 2. Principle of refractometer
The lower the refractive index of the measured sample, the smaller the critical angle, thus the larger illuminated surface of the sensor. The refractive index value is shown on the line which is the limiting ratio of the bright and dark parts of the sensor.
Why we should measure refractive index?
Measurement of the refractive index in a solution can provide information about the density of the solution. This value can provide information about how much dissolved substance in solution. Therefore, refractometer can be used to determine the concentration of the solutions, such as oil-based solutions, sugar solution, heavy chemical products, etc. The measured value is generally given in degrees Brix .
The advantage of this test in determining the quality of essential oil is that the sample used is very small and the testing time is very fast. Analysis can be done with one or two drops of oil and the result is ready within minute. However, this test is a qualitative test and does not provide information on the purity percentage .
Figure 3. Refractometer