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2. Refractive Index and Brix(%)
2-1. What is refractometer?
When a straw is placed into a glass filled with water, the straw appears to bend. Whe n a straw is
placed into a glass filled with sugar water, the straw appears to bend much more sharply than in the
case of water alone.
This phenomenon is known as the refraction of light. The refractometer is an instrument that
measures the Refractive Index by utilizing this principle (the Refractive Index increases in proportion
to the concentration of the solution), and was developed by Dr. Ernst Abbe at the end of the 19th
century.
2-2. What is the Refractive Index?
If the Refractive Index of air under atmospheric pressure is 1,
then when light enters medium χ, the ratio of the sine of the
incident angle α measured against the phase boundary to the
sine of the refracting angle β is called the Refractive Index of
the mediumχ.
The Refractive Index varies with the wavelength of light and
temperature and is represented as follows:
For example, Refractive Index of water at 20˚C under the D-line is:
=1.33299(Generally expressed as nD = 1.33299.)
m
The Refractive Index is based upon the supposition that the Refractive Index in a vacuum is 1
and is called the absolute Refractive Index. Generally, however, this index is seldom used.
2-3. Brix(%) scale
The PRM-
2000α is programmed with the Brix(%) scale, based on the Refractive Index of water (nD =
1.33299) as the reference (0%). The Brix(%) scale represents the weight of sucrose expressed by
percentage (sucrose weight in grams contained in 100 grams of sucrose solution).
Therefore, this scale corresponds with the sucrose concentration. However, mos t samples are
comprised of ingredients other than sugar. The total concentration of these ingredients is
represented by the Brix(%), so this makes the Brix(%) scale a practical tool for measuring
concentrations.
m
The Brix(%) scale is recommended by ICUMSA (International Committee of Uniform Method
of Sugar Analysis) and is expressed in % mas (Sucrose) in international units (SI unit).
2-4. Temperature correction
The Refractive Index of a substance varies with temperature. Thus, when using a refractometer to
measure the Refractive Index of a liquid, the measurement value will vary with the sample
temperature.The Brix or concentration value displayed by this product is automatically compensated
by the internal software based on the measured temperature and the same value is always displayed
as if measurements were made at 20˚C (in case of a sample temperature of 5 to 90˚C).
Air
Medium χ
n=1
α
β
nχ=
sin α
sin β
n
: Represents the Refractive Index
t
: Temperature (˚C)
D
: Sodium D-line (589nm)
