Refractometry is the analytical method of measuring substances'
refractive index (one of their fundamental
physical properties
A physical property is any property that is measurable, whose value describes a state of a physical system. The changes in the physical properties of a system can be used to describe its changes between momentary states. Physical properties are o ...
) 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
The degree Celsius is the unit of temperature on the Celsius scale (originally known as the centigrade scale outside Sweden), one of two temperature scales used in the International System of Units (SI), the other being the Kelvin scale. The ...
, 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 (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 growers for
Brix
Degrees Brix (symbol °Bx) is a measure of the dissolved solids in a liquid, and is commonly used to measure dissolved sugar content of an aqueous solution. One degree Brix is 1 gram of sucrose in 100 grams of solution and represents the strength ...
testing of
sucrose
Sucrose, a disaccharide, is a sugar composed of glucose and fructose subunits. It is produced naturally in plants and is the main constituent of white sugar. It has the molecular formula .
For human consumption, sucrose is extracted and refined ...
levels in their fruit. Refractometry is also used in the
gelatin
Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...
industry. To convert the RI of a
gelatin
Gelatin or gelatine (from la, gelatus meaning "stiff" or "frozen") is a translucent, colorless, flavorless food ingredient, commonly derived from collagen taken from animal body parts. It is brittle when dry and rubbery when moist. It may also ...
sol
Sol or SOL may refer to:
Astronomy
* The Sun
Currency
* SOL Project, a currency project in France
* French sol, or sou
* Argentine sol
* Bolivian sol, the currency of Bolivia from 1827 to 1864
* Peruvian sol, introduced in 1991
* Peruvian 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 refractometers,
digital handheld refractometers,
Abbe refractometers, and
inline process refractometers.
Further reading
{{cite book , title=Handbook of Laser Technology and Applications , author=Colin E. Webb, Julian D. C. Jones , url=https://books.google.com/books?id=8wPekoJyfEUC&pg=PA1737 , pages=1738–1740 , isbn=0-7503-0966-0 , year=2004 , publisher=Taylor & Francis
External links
Refractometry - Gemstone Buzzmethods for measuring refractive index.
Measurement