Name of student:......................................................
11 POLARIMETRY
Task: Measuring of density using densimeter, angle of rotation (α) and calculation of
concentration of prepared saccharose solution.
Polarimetry is a sensitive, non-destructive technique for measuring the optical activity exhibited
by inorganic and organic compounds. A compound is considered to be optically active if plane
polarized light is rotated when passing through it. The amount of optical rotation is determined by
the molecular structure and concentration of chiral molecules in the substance.
The term specific rotation
has been established in order to be able to compare the rotation
of individual substances. The specific rotation is given by the following equation:
[° m2 kg-1]
[1]
where: t is the temperature (°C)
α is the measured angle of rotation of the polarized light plane (°),
w is the concentration of the dissolved compound (the mass fraction of the dissolved
substance) (%),
ρ is the density of the solution (kg.m-3-)
d is the length of the sample tube (m)
The specific rotation is a typical property of an optically active substance at a given temperature
(t; usually t = 20 °C) and a given wavelength (λ) of the used radiation. According to Equation [1],
the specific rotation equals to the angle of rotation of the polarized light plane which was induced
by a 10 cm thick layer that contains 1 g of an optically active compound in 1 cm 3 of a solution.
In this laboratory exercise you will work with solutions of saccharose. The specific rotation of
an aqueous solution of saccharose is
° m2 kg-1.
From equation (1) we can calculate the concentration of the dissolved substance (W %):
[%]
[2]
A polarimeter is a device used for the measurement of optical rotation and its scheme is
depicted in Figure 1.
Figure 1 Components of a polarimeter
The main components of the polarimeter are polarizer (Nicol prism), analyzer (Nicol prism)
and the sample tube. The polarizer is used to obtain polarized light. The analyzer is used to
measure the angle of rotation. The sample tube should be filled with the sample in a way that
after the filling a convex surface of the liquid is formed on the tube thread. This surface is
then “cut off” with a lens. This procedure should be performed fast in order to avoid the
formation of gas bubbles in the sample tube. After screwing the tube thread the filling of the
tube is checked, the tube is wiped and inserted into the polarimeter.
The polarimeter used in this exercise has two scales. On the left side is the circular scale which is
divided into 360° (Figure 2). One division of the circular scale corresponds to α = 1°.
On the right side is the nonius scale which has a division from 0 to 10. One division of the
nonius scale corresponds to 0.05°. When reading the measured value look first between which
divisions of the circular scale is the zero value of the nonius scale .Read the smaller value of
the two values; this will be the integer value of α. According to Figure 3 this value is 2°. The
tenths and hundredths of α will be read from the nonius scale. This should be done by finding
a division on the nonius scales which is exactly opposite to a division on the circular scale.
According to Figure 2 this value is 0.85°. Finally, add these two values and you have the final
value of α, following Figure 2 α = 2° + 0.85° = 2.85°
Figure 2 Scales of a polarimeter
What about measuring with a polarimeter? The first step is to set up the zero value on the
nonius scale exactly opposite to the zero value on the circular scale. This should be done by
using the polarimeter screw which is under the ocular. The second step is to insert the sample
tube (filled with the sample) into the polarimeter. The field of vision observed in the ocular is
depicted in Figure 3a. This position is denoted as the first extreme position. Then, turn the
polarimeter screw left till you won t observe a change in the field of vision which is depicted
in Figure 3c. This position is denoted as the second extreme position. The last step is to find the
medium position by which we read the value of α.
Turn the polarimeter screw slowly backwards till you won t observe the field of vision which is
depicted in Figure 3b.
Figure 3 Different fields of vision observed during a measurement.
Equipment:
polarimeter, pipettes, densimeter, filter paper
Chemicals:
saccharose solution, distilled water
Experimental procedure:
1 Wash the polarimeter sample tube with a small amount of the measured solution and
fill the sample tube with the solution. Close and wipe the sample tube and check that
the solution contains no bubbles.
2 Measure five times the angle of rotation of the prepared solution and write down the values
in Table 1.
3 Pour out the solution into the storage bottle.
4 When finished measurements determine the angle of rotation without the sample tube
inserted (α0). The value of α0 should be close to zero. Write down the value of α0 inTable 1.
5 Write down the room temperature.
6 Write down the length of the sample tube (d).
7 When finished all measurements disamntle the sample tube, carefully wash all parts of the tube
with distilled water.
Processing of the measured data:
- Calculate the following data and write them down in Table 1:
 The average values of angles of rotation (αavrage) calculated from the measured values.
 The corrected values of angles of rotation calculated according to the Equation [3].

[°]
[3]
 The values of concentrations of the dissolved substance (W) according to the Equation [2].
 Determine the densities of the solutions by means of a densimeter.
The procedure of determination of density using the densimeter is following:
1 Wash the graduated cylinder with distilled water. Fill the graduated cylinder with distilled
water. The height of the water level should be 7 – 10 cm below the rim (Figure 4).
Figure 4 Measuring with a densimeter
2 Immerse slowly and carefully the densimeter into the cylinder. Read the value of density. It is
the value on the scale of the densimeter which is equal to the level of the measured liquid.
3 Pour out the distilled water into a beaker and wash the graduated cylinder with
a small amount of the measured solution. Fill the graduated cylinder with the solution and
insert the clean and dry densimeter. Read the value of density. When finished the measurement
pour the solution into the storage bottle.
4 Write down the measured values in Table 1.
Table 1 Meaasured and calculated values
t=
ºC
d
α0 = ..................°
m
° m kg
2
solution
water
1
Calculations:
-1
α1 – 5 [°]
αaverage
α
ρ [kg m-3]
W [%]
Conclusions: