j
Background:
Refractometry is a method to measure the refractive index of particular
substance. It depends on the physical property that is light refraction theory but the
refractometer is an instrument used to determine the concentration of particular
substance in a solution "liquid sample" by measuring refractive index of sample.
Principle:
Light refraction is defined as the bending of light when it passes from one media
to another either toward or away from normal line between these media ;for example,
when the light is passing from a lower refractive substance (air) to higher refractive
substance (water), the refractive light toward normal line. When the light is passing
from a higher refractive substance (water) to low refractive substance (air), the
refractive light is away from normal line.
Normal ray
Incident ray
Angle of incidence
Angle of refraction
Refraction ray
Angel of incidence: is the angel between normal ray
and incident ray.
Angel of refraction: is the angel between normal ray
and refracted ray.
Angel of refraction is related to index value called index of refraction. Each
compound has its specific index of refraction. The refractive index is commonly
determined as part of the characterization of liquid samples.
Refractive index of substance (n) = Velocity of light in a vacuum
Velocity of light in a material
1
Angel of refraction is depends on three variables:
1. Composition of the media: "Concentration of substance", therefore we can use
refractometer to determine concentration of compounds. As concentration
increase the angel of refraction increase and then the value of refractive index
increase.
2. Temperature of testing environment: Affect on the angel of refraction (the
index of refraction of sample decreases when the temperature increases for a
sample).
3. Wavelength of light.
Using Refractometer to determine sample concentration:
First we determine the index of refraction of a liquid sample, and then by
plotting standard curve of refractive index against concentration of a serial of known
concentration samples, we can determine the concentration of unknown sample.
The components of the refractometer instrument:
Eyepiece
Dial
(to adjust sharp light
and dark, no gradient)
On/Off
It has three positions:
On/Off/Show the scale
Two prisms: measuring and
illuminating prisms (to put the
sample between theses prisms).
Circulating water
(to resist any change in temp.)
Light source
2
Thermometer
Hand wheel (knob)
(adjust the place of
borderline onto the
center of the cross
lines).
How to use the refractometer:
1. Open the prism and add 2 to 3 drops of sample solution.
Add drops of sample
2. Close the prism and adjust the lamp then turn on the lamp using the switch on the
left side.
3. Look through the eyepiece, this view will appear. (if you do not see a light and a dark
region, adjust the hand wheel on the right side of the instrument until the view appears).
Lighter region on the top
Dark region on the bottom
3
4. Use dial to sharpen the borderline between the light and the dark regions as shown.
5. Readjust the hand wheel on the right hand side to place the borderline exactly on
the center of the cross lines as below figure.
6. To read the index of refraction, press the switch (on the left hand side of the
Refractometer) until you see the scale through the eyepiece.
The example here has a refractive index of 1.4606
7. The upper scale indicates the index of refraction.
8. You have to read four values; for example, 1.4606 not 1.460
9. Record the refractive index in your lab notebook.
Materials:
 Distill water
 Glucose solutions (4%, 8%, 12%, 16%)
 Fructose solutions (4%, 8%, 12%, 16%)
4
Instrument:
Refractometer
Methods:
To measure the different concentration of sugar using refractometer instrument see
the following steps.
1. Calibrate a refractometer using drop of distill water on the sample prism then read
the scale at 0 (dark area is visible inside the eyepiece on the scale then turn the
dial until the shadow falls on zero).
2. Calibrate a refractometer as the temperature change.
3. Open the cover of refratometer and dry using soft tissue.
4. Place a drop or two drops of sugar (fresh) on the sample prism, close the cover
and then read the scale (the line at the under of the light area).
5. Dry the rectangular area with soft tissue and do it after each reading.
Applications:
 Identify the purity of a sample by comparing the refractive index of a sample to
the literature value. The closer refractive index to literature value that means the
sample is pure.
 Measure the concentration of saline (NaCl) in brine solution this known as
salinity.
 Measure the percent sucrose in beverage and food for quality control this known
as Brix.
 Measure the plasma proteins and specific gravity of urine.
 Measure the coolant freezing point.
References:



http://www2.ups.edu/faculty/hanson/labtechniques/refractometry/intro.htm
http://en.wikipedia.org/wiki/Brix
Elements of Physics, Fifth Edition. George Shortly and Dudley Williams. Prentice
Hall Publishers; 197.
5
Result Sheet
Name: ---------------
Compound name: -----------
Refractive index
Concetration g%
A) Draw a standard curve between the refractive index against concetration of
glucose or fractose.
B) Calculate the intrinsic refractive index.
6
Paste the curve here.
7