Understanding Your Lab Equipment: How a Refractometer Works

Do you have 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 as lab equipment throughout the food, agricultural, chemical, and manufacturing industries.

Refraction

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.

Learn more about the new Cole-Parmer digital refractometers.

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

Need to know more about refractometers? Read the entire article on refractometers.