ATAGO 3603, Instruction Manual

Page: 7 / 26

7
2. Refractive Index and Brix
The In-line Refractometer CM-BASE
α is a refractometer that detects the Refractive Index of
a sample and outputs the Brix(%) value on the display.
(1) What is refractometer?
When a straw is placed into a glass filled with water, the straw appears to bend. When 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) What is the Refractive Index?
If the Refractive Index of air under atmospheric
pressure is 1, then when light enters medium x, the
ratio of the sine of the incident angle <alpha>
measured against the phase boundary to the sine of
the refracting angle <beta> is called the Refractive
Index of the medium x.
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.
(3) Brix(%) scale
The CM-
BASEα 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. For
samples such as water-soluble cutting oil, it is possible to create a conversion table in
advance to show the relation between the sample’s concentration % and Brix %, and
convert the Brix measurement value into its concentration. However, it is very common to
create a concentration standard using Brix values, and monitor with Brix values in
everyday operations.
C
The relationship between Brix(%) and the Refractive Index (nD) is outlined on
page 24.
(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 CM-BASE
α always detects the prism temperature. The value of the measurement is
automatically corrected for temperature by a built-in processor, so that the displayed value is
equal to the value measured at 20˚C (provided that the sample temperature is within the range of
10 to 50˚C).
n : Represents the Refractive Index
t
: Temperature (˚C)
D : D-line of natrium (589nm)