.
The refractive index (n) of a
substance with reference to air is the ratio of the sin of the angle of
incidence to the sin of the angle of refraction of a beam of light passing
from air into the substance. It varies with the wavelength of the light
used in its measurement.
Unless otherwise specified in the
individual monograph, the refractive index, n ~o
is measured at 20° ± 0.5° with reference to the wavelength of the D-line
of sodium (A = 589.3 nm). The temperature should be carefully adjusted and
maintained since the refractive index varies significantly with
temperature.
Related: Calibration of Refractometer
Related: Calibration of Refractometer
The Abbe refractometer is convenient
for most measurements of refractive index but other refractometers of
equal or greater accuracy may be used. Commercial refractometers are
normally constructed for use with white light but are calibrated to
give the refractive index in terms of the D line of sodium.
The apparatus is provided with a water jacket to control
the temperature of measurements. The manufacturer's instructions relating to a suitable light
source should be followed subject to the directions given in the
Pharmacopoeia. To achieve accuracy, the apparatus should be calibrated
against distilled water which has a refractive index of
1.3325 at 25° or against the reference liquids given in the following
table.
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