What is a Refractometer?

Woman using refractometer

What is a Refractometer?

A refractometer is a simple instrument used for measuring concentrations of aqueous solutions. It requires only a few drops of liquid, and is used throughout the food, agricultural, chemical, and manufacturing industries.

How a Refractometer Works

When light enters a liquid it changes direction; this is called refraction. Refractometers measure the degree to which the light changes direction, called the angle of refraction. A refractometer takes the refraction angles and correlates them to refractive index (nD) values that have been established. Using these values, you can determine the concentrations of solutions. For example, solutions have different refractive indexes depending on their concentration in water.

prism in a refractometer

The prism in the refractometer has a greater refractive index than the solution. Measurements are read at the point where the prism and solution meet. With a low concentration solution, the refractive index of the prism is much greater than that of the sample, creating a large refraction angle and a low reading ("A" on diagram). The reverse would happen with a high concentration solution ("B" on diagram).

Brix Scale and Common Brix %

The Brix scale is calibrated to the number of grams of cane sugar contained in 100 mL of water. Therefore, the Brix % reading equals actual sugar Concentration.

Sample fluid Brix %
Cutting oils
Oranges
Carbonated beverages
Apples
0 to 8
4 to 13
5 to 15
11 to 18
Grapes and wines
Concentrated juices
Condensed milk
Jams and jellies
14 to 19
42 to 68
52 to 68
60 to 70

Common Refractive Indexes

Refractive index readings are temperature-dependent.

Sample fluid Temperature Refractive
index
Methanol
Acetone
Ethanol
Acetic acid
Benzene
25°C
25°C
25°C
25°C
25°C
1.326
1.357
1.359
1.370
1.498
Paraffin oil
Palm oil
Olive oil
20°C
20°C
20°C
1.412
1.456
1.471
Methyl salicylate
Methyl iodide
25°C
25°C
1.522
1.740