How does a refractometer work?

Refractometers are optical instruments used to determine the percentage of soluble solids in a liquid solution. To do this, the refractometer makes use of the principle of total refraction of light (caused by the type and concentration of substances dissolved in a liquid solution, for example sugar), which takes place in the boundary layer between the prism and the sample. The refractive index of the prism determines the upper limit of the measurement range, since this must always be greater than that of the sample. The refractometer therefore measures the density of liquids, the denser a liquid, the greater the refraction.

Refractometry is the optical method of determining the speed of propagation of light in a medium/compound/substance/body, which is directly related to the density of this medium/compound/substance/body. To use this principle, the refraction of light is used, (which is a fundamental physical property of any substance), and the measurement scale of this principle is called the refractive index. Refractometers are the instruments that use this principle of refraction. either the refractive one, (using several prisms), or the critical angle one, (using only one prism), and its primary measurement scale is the refractive index, from which the different specific scales are constructed


Refractometers were invented by Dr. Ernst Abbe, a German/Austrian scientist in the early 20th century. The Abbe refractometer is the “grandfather” of all modern refractometers, thanks to its simple operation and reliability, it still has a place in laboratories. There are two types of refractometers based on refractive index detection; transparent systems and reflection systems. Portable refractometers and Abbe refractometers use the transparent systems, while digital refractometers use the reflection systems.

How do they work?

The refractometer is used to measure the refractive index of liquids and translucent solids allowing:

  • Identify a substance.
  • Check its degree of purity.
  • Analyze the percentage of solute dissolved in a given solution.
  • Offer other qualitative analyses.

Its operation is based on using the refraction of light, which is a physical property of any substance and is related to some physical properties such as density. From this, and according to its application, different scales are built. The most used scale is Degrees Brix (Proportion of sucrose or salts in a solution). There are other scales, such as: % salt, gs, g/dl, nD, % w/w, % vol, % water, mash sacch, M-10, MDT, among others.

What are the parts of a refractometer?

  • Main Prism: It is where the samples are placed, it allows to measure different types of solutions, for example: drinks, sugar, honey, salt water, alcohol, jams, grapes, etc. The prism has a sheet that allows to cover the substances that are deposited here.
  • Adjustment Screw: Used to adjust the calibration of the device.
  • Mirror Tube: Reflects light up through the refractometer.
  • Eyepiece: Focusable lens through which the scale is observed.
  • Scale: Located inside, it indicates the measurement scale and varies according to its application and characteristics.
  • Measurement procedure.
  • Place 1 or 2 drops of the sample on the prism.
  • Close the sheet that prevents the entry of light, gently.
  • The sample should spread over the surface of the prism.
  • Look at the scale through the lens.
  • The reading of the scale is on the border line.
  • Dry and clean the prism sample with tissue paper and water.

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