Refractometry is the analytical method of measuring substances'

refractive index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

(one of their fundamental physical properties) in order to, for example, assess their composition or purity. A

refractometer A refractometer is a laboratory or field device for the measurement of an index of refraction ( refractometry). The index of refraction is calculated from the observed refraction angle using Snell's law. For mixtures, the index of refraction the ...

is the instrument used to measure refractive index ("RI"). Although

s are best known for measuring liquids, they are also used to measure gases and solids; such as glass and gemstones. The RI of a substance is strongly influenced by temperature and the

wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...

of light used to measure it, therefore, care must be taken to control or compensate for temperature differences and wavelength. RI measurements are usually reported at a reference temperature of 20 degrees Celsius, which is equal to 68 degrees

Fahrenheit The Fahrenheit scale () is a temperature scale based on one proposed in 1724 by the physicist Daniel Gabriel Fahrenheit (1686–1736). It uses the degree Fahrenheit (symbol: °F) as the unit. Several accounts of how he originally defined hi ...

, and considered to be room temperature. A reference

of 589.3 nm (the

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

D line) is most often used. Though RI is a dimensionless quantity, it is typically reported as nD20 (or n), where the "n" represents refractive index, the "D" denotes the wavelength, and the 20 denotes the reference temperature. Therefore, the refractive index of water at 20 degrees Celsius, taken at the Sodium D Line, would be reported as 1.3330 nD20. Refractometers are frequently used by grape growers and

kiwifruit Kiwifruit (often shortened to kiwi in North American, British and continental European English) or Chinese gooseberry is the edible berry of several species of woody vines in the genus '' Actinidia''. The most common cultivar group of kiwi ...

growers for Brix testing of sucrose levels in their fruit. Refractometry is also used in the gelatin industry. To convert the RI of a gelatin sol (reported in Brix) to a gelatin concentration, one need only multiply by eight-tenths (0.8). A sol with a 10.0 RI would therefore be 8% gelatin by weight. This is known to be a reliable conversion for gelatin sols as low as 1% up to over 50%.

Types of refractometers

There are four main types of fluid refractometers:

traditional handheld refractometer A traditional handheld refractometer is an analog instrument for measuring a liquid's refractive index. It works on the ''critical angle'' principle by which lenses and prisms project a shadow line onto a small glass reticle inside the instrume ...

digital handheld refractometer A digital handheld refractometer is an instrument for measuring the refractive index of materials. Principle of operation Most operate on the same general ''critical angle'' principle as a traditional handheld refractometer. The difference is t ...

Abbe refractometer An Abbe refractometer is a bench-top device for the high-precision measurement of an Refractive index, index of refraction. Details Ernst Abbe (1840–1905), working for Carl Zeiss AG in Jena, Germany in the late 19th century, was the first ...

s, and

inline process refractometer Inline process refractometers are a type of refractometer designed for the continuous measurement of a fluid flowing through a pipe or inside a tank. First patented by Carl A. Vossberg Jr.br>US2807976A- Refractometer US2549402A, these refractom ...

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Refractometry - Gemstone Buzz
methods for measuring refractive index. Measurement

Types of refractometers

Further reading

External links