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What is Refractometry?

You’ve lost the key to your locker in the swimming pool. You spot it lying on the bottom of the shallow part of the pool, reach in to take it – and your hand misses. The refraction of light at the boundary of two different materials can be explained as easily as that. If the swimming pool was filled with salt water, the image of the key would have been shifted even more.

Refraktometrie Messprinzip

Light moves at different speeds in materials of different densities. In a vacuum, it reaches 299 792 458 m/s, however in water “only” 225 000 000 m/s. If a ray of light with a defined wavelength strikes a boundary between one medium to another at a fixed angle, the angle of the ray will change according to the refractive indices of the media. Snell’s law describes this phenomenon:

n1∙ sin δ1 = n2∙ sin δ2
where δ1 is angle α and δ2 is angle β.

Under constant conditions with known material properties, the formula can be manipulated to calculate the refractive index of an unknown second medium. The angle of incidence and angle of refraction can be measured, the refractive index of one of the materials (the prism of the refractometer) is known, and so, after adjusting the formula, the refractive index of the unknown material is a matter of simple mathematics.

Measurement of the refractive index depends on the temperature and wavelength of the light. Determination of the refractive index can provide information on the purity of a substance, but not its exact composition. The refractive index of water at 20 0C is 1.33 nD. Ice has a refractive index of 1.31 nD. Adding sugar to pure water changes the refractive index, depending on the amount added. Adding salt changes the refractive index as well, but in relation to the concentration.

This means that if pure water at 20 0C does not have a refractive index of 1.33, it has been “polluted” with some other material.

As a rule, determini ng the refractive index of a substance is a quick and reliablecheck of its purity.
Sun flower oil diluted with cheaper oil can be detected just as easily as the sugar content of marmalade during the production process.

Another example: cyclohexane at 20 0C has the same refractive index as a 52.9 % sugar solution. This shows that no statements on the composition or possible admixture of a substance can be made without knowing exactly what it is.
Temperature is one of the greatest factors which can influence the refractive index. Each substance reacts differently and specifically to temperature.

40 % Brix Sugar Solution

TemperaturBrechungsindex
20,0 ºC1,39986 nD
20,1 ºC1,39985 nD
21,0 ºC1,39971 nD
ΔnD = 0,00015 / ºC

Paraffine Oil

TemperaturBrechungsindex
20,0 ºC1,48001 nD
20,1 ºC1,47997 nD
21,0 ºC1,47965 nD
ΔnD = 0,00036 / ºC

A temperature corrected scale in a refractometer must always be specific to a substance, and can never be considered to be universal.

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