How Does a Refractometer Work?

RefractometerNew Cole-Parmer® Digital Refractometer

What is a refractometer?

A refractometer is an instrument used for measuring concentrations of aqueous solutions. It requires only a few drops of liquid and is used as lab equipment throughout the food, agricultural, chemical, and manufacturing industries. A refractometer can be found in a mechanic’s toolbox, a winery, and even a gemologist’s desk. Home beer brewers and aquarium keepers also use it. Refractometers can test everything from battery acid to the authenticity of emeralds. This handy device is most often used in the food and beverage industry.

How does a refractometer work?

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.

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. The reverse would happen with a high concentration solution.

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The % Brix Scale

The % Brix Scale shows the concentration percentage of the soluble solids content of a sample (water solution). The soluble solids content is the total of all the solids dissolved in the water, beginning with sugar, salts, protein, acids, etc.; the measurement reading value is the value of the sum total of those. The Brix scale is calibrated to the number of grams of cane sugar contained in 100 g of water. When measuring a sugar solution, the % Brix reading should perfectly match the actual concentration.

Sample fluid

% Brix

Cutting oils

0 to 8

Oranges

4 to 13

Carbonated beverages

5 to 15

Apples

11 to 18

Grapes and wines

14 to 19

Concentrated juices

42 to 68

Condensed milk

52 to 68

Jams and jellies

60 to 70

Common Refractive Indexes: Refractive index readings are temperature-dependent.

Sample fluid

Temperature

Refractive index

Methanol

25°C

1.326

Acetone

25°C

1.357

Ethanol

25°C

1.359

Acetic acid

25°C

1.370

Benzene

25°C

1.498

Paraffin oil

20°C

1.412

Palm oil

20°C

1.456

Olive oil

20°C

1.471

Methyl salicylate

25°C

1.522

Methyl iodide

25°C

1.740

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