In: Chemistry
What does the symbol nD 20 mean?
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 (or n20
D ), 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.