Refractometry
Encyclopedia
Refractometry is the method of measuring substances' refractive index
(one of their fundamental physical properties
) in order to, for example, assess their composition or purity. A refractometer
is the instrument used to measure refractive index ("RI"). Although refractometers 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
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
, and considered to be room temperature. A reference wavelength of 589.3 nm (the sodium
D line) is most often used. Though RI is a dimensionless quantity, it is typically reported as nD20, 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
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%.
s, digital handheld refractometer
s, Abbe refractometer
s, and inline process refractometer
s.
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....
(one of their fundamental physical properties
Physical property
A physical property is any property that is measurable whose value describes a physical system's state. The changes in the physical properties of a system can be used to describe its transformations ....
) in order to, for example, assess their composition or purity. A refractometer
Refractometer
A refractometer is a laboratory or field device for the measurement of an index of refraction . The index of refraction is calculated from Snell's law and can be calculated from the composition of the material using the Gladstone-Dale relation....
is the instrument used to measure refractive index ("RI"). Although refractometers 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
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
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
Celsius
Celsius is a scale and unit of measurement for temperature. It is named after the Swedish astronomer Anders Celsius , who developed a similar temperature scale two years before his death...
, which is equal to 68 degrees Fahrenheit
Fahrenheit
Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit . Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees...
, and considered to be room temperature. A reference wavelength of 589.3 nm (the sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
D line) is most often used. Though RI is a dimensionless quantity, it is typically reported as nD20, 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
Viticulture
Viticulture is the science, production and study of grapes which deals with the series of events that occur in the vineyard. When the grapes are used for winemaking, it is also known as viniculture...
and Kiwifruit
Kiwifruit
The kiwifruit, often shortened to kiwi in many parts of the world, is the edible berry of a cultivar group of the woody vine Actinidia deliciosa and hybrids between this and other species in the genus Actinidia....
growers for Brix
Brix
Degrees Brix is the sugar content of an aqueous solution.One degree Brix is 1 gram of sucrose in 100 grams of solution and represents the strength of the solution as percentage by weight . If the solution contains dissolved solids other than pure sucrose, then the °Bx is only approximate the...
testing of sucrose
Sucrose
Sucrose is the organic compound commonly known as table sugar and sometimes called saccharose. A white, odorless, crystalline powder with a sweet taste, it is best known for its role in human nutrition. The molecule is a disaccharide composed of glucose and fructose with the molecular formula...
levels in their fruit. Refractometry is also used in the gelatin
Gelatin
Gelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...
industry. To convert the RI of a gelatin sol
Sol (colloid)
A sol is a colloidal suspension of very small solid particles in a continuous liquid medium. They are quite stable and show the Tyndall effect. Examples include blood, pigmented ink, and paint....
(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 refractometerTraditional 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 instrument, which is then viewed by the user through a magnifying...
s, digital handheld refractometer
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 that light from an LED light source is focused on the...
s, Abbe refractometer
Abbe refractometer
An Abbe refractometer is a bench-top device for the high-precision measurement of an index of refraction.- Details :Ernst Abbe , working for the Zeiss Company in Jena, Germany in the late 19th century, was the first to develop a laboratory refractometer...
s, and inline process refractometer
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. These refractometers typically consist of a sensor, placed inline with the fluid flow, coupled to a control box...
s.
Further reading
External links
- Refractometry - Gemstone Buzz methods for measuring refractive index.