Specific Rotation of Sugar Solution | Polarimeter Experiment Viva

Last updated on Tuesday, October 3rd, 2023

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Polarimeter Experiment’s Aim

Table of Contents

In this experiment, we will find the specific rotation of sugar solution by using Laurent’s Half Shade Polarimeter.

Apparatus Used

Polarimeter, sodium lamp, sugar, beakers, graduated jar, weight box, and balance.

Specific Rotation of Sugar Solution Formula Used

The angle of rotation produced by any optically active substance depends on some factors;

the thickness of the substance
the concentration of the solution (gm/cc)
temperature and,
the wavelength of light used

THIS EXPERIMENT POST IS ALSO SUGGESTED AS A REFERENCE POINT FOR FURTHER STUDY

Screenshot date 27/03/2023.

The optical activity of a substance is measured by its specific rotation (or specific rotatory power).

The specific rotation of sugar solution is determined by the formula;

S = θ / LC

where ‘θ’ is the rotation produced in degree, L is the length of the tube in the decimeter (1 decimeter = 10 cm) and C is the concentration of the active substance in gm/cc in the solution. The unit of specific rotation is degree/(decimeter) x (gm/cm³ ).

If length of the solution (tube) is in cm then the formula will be;

S = 10θ / LC

also, C = mass of the substance (M) / volume (V).

So,

specific rotation = (10 X θ X V) / (L X M)

Laurent’s Half-Shade Polarimeter

In this experiment, we are using Laurent’s half shade polarimeter to measure the angle of a plane-polarized light beam. In this experimental arrangement, we use a monochromatic extended light source (sodium lamp with its D Line, wavelength 589 nm). A convex lens to make this sodium light parallel for falling on the Nicol prism which is called polariser.

When light passes through the Nicol prism it becomes plane-polarized light. Further, this plane-polarized light passes through Laurent’s half shade plate and enters into the tube that contains a sugar solution.

Read Also: Photoelectric effect experiment viva

Now this plane-polarized light transmitted through another Nicol prism called an analyzer. This analyzer is rotatable, we rotate it so plane-polarized light propagating through liquid reach at our eye. We note down this rotation in degrees with the help of a vernier.

Procedure

Make a sugar solution first, weigh sugar crystal in a watch glass and dissolve it in 100 c.c. distilled water. From it you will calculate the concentration of the substance C. For example, you are taking 40 gm of cane sugar that is dissolved in water to make 100 c.c. of solution. The concentration C will be 40/100 =0.40 g/c.c.
Clean the glass tube from both sides and first filled it with the distilled water. It should be free from dust and bubbles.
Now, put the tube in the frame and switch on the sodium lamp. Look through the analyzer, an eyepiece connected with the rotating scale.
You will see two equal portions of dark and bright, that can be interchanged after rotating the analyzer.
Now rotate the analyzer first in the clockwise direction and then in anticlockwise. You have to rotate it until the circular field of view becomes bright.
Note down these readings
Similarly, you can take second and third readings for both the directions in the absence of sugar solution.
Find the mean of two directions reading separately.
Now, take out the polarimeter tube and fill it with the sugar solution that you prepared in the beginning.
Put it in the same frame and fix properly.
Now repeat the same procedure as you did with distilled water case.
Note down the first reading of the analyzer in the clockwise and anticlockwise direction. Similarly for the other set of readings.
The difference between the water and sugar solution readings gives the value of specific rotation.
Repeat the experiment with different concentrations of sugar solution.
Note down the room temperature and length of the tube used in this experiment.

Observation of Polarimeter Experiment

Room temperature = 25⁰ C
Weight of sugar = 24 gm
The volume of water taken = 100ml
The least count of the analyzer = 1
Length of the polarimeter tube = 11.5 cm (1.15 dm)[ remember if you have to use length in cm then the formulation will be different for specific rotation, as mentioned above].

Table For Specific Rotation of Sugar Solution (Angle of Rotation)

For Water

Sr. No.	Analyzer’s Reading		Mean = (θ + θ’)/2
Clockwise rotation (θ)	Anticlockwise rotation (θ’)		Mean = (θ + θ’)/2
1	70	290	180
2	130	222	176
Mean X			= 178

For 1st Sugar Solution

Sr. No.	Analyzer’s Reading		Mean = (θ₂ + θ₂‘)/2
Clockwise rotation (θ₂)	Anticlockwise rotation (θ₂‘)		Mean = (θ₂ + θ₂‘)/2
1	50	275	162.5
2	112	215	163.5
Mean Y			=163

So, rotation produced by the sugar solution = X-Y = 15⁰ [15 degrees is the difference, its interpretation is important.]

Then the specific rotation of the sugar solution, from the formula is;

S = 10θ / LC

also, C = mass of the substance (M) / volume (V).

So,

specific rotation = ( θ X V) / (L X M)

specific rotation= (15) X 100 / (1.15 X 24)

The observed specific rotation = 54.35⁰cm³/ g dm

This result is at room temperature (25⁰C) and wavelength (589nm).

The standard result of the specific rotation of the sugar solution = +66.54⁰ cm³/ g dm

Percentage error: (Std. value -Obs. Value) / Std. value X 100

= 66.54 – 54.35 / 66.54

= 18.31 %

For the other substances the specific rotation value;

sucrose + 66.54
glucose + 52.74
fructose – 93.78
maltose + 137.5
lactose + 55.3
dextrose + 194.8

Specific Rotation of Sugar Solution | Polarimeter Experiment Viva Questions:

Q1. Are light waves of transverse nature?

Ans. Yes, polarization experiments support this belief.

Q2. Do you believe electromagnetic waves are light waves?

Ans. Yes, light waves are of electromagnetic wave nature, having the electric and magnetic field components.

Q3. Which component plays the role of light?

Ans. Electric field component.

Q4. Can you observe polarization through sound waves?

Ans. No, they have longitudinal no transverse components.

#Q5. What is the difference between the polarized and ordinary light?

Ans. Polarized light has a transverse component (E vector) in a specific direction, while in ordinary light it vibrates in all directions.

#Q6. What do you mean by the plane of polarization?

Ans. An imaginary plane that is parallel to the electric vector components or vibrations is known as a plane of vibration. A second imaginary plane that is perpendicular to it is known as the plane of polarization.

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#Q7. What are polaroids?

Ans. Polaroids are extremely thin layers of certain materials (e.g., herapathite, iodoquinosulphate) with the crystal axes parallel to one another. they can render ordinary light into polarized light. The polaroids behave like a polarizer and analyzer.

#Q8. What is Nicol prism?

Ans. Nicol prism is made of calcite crystal in such a way, when unpolarized ordinary light enters through it, prism gives an intense plane-polarized light. This was designed by William Nichol in 1828.

#Q9. What are the uses of Nicol Prism?

Ans. It acts as a polarizer (through which plane-polarized light we obtain) and an analyzer (to detect the plane-polarized light).

#Q10. If two prisms are placed co-axially, can you see the polarized light from the second one?

Ans. Yes, we can see. First prism produce and second one analyze the polarized light. If they are placed co-axially polarized light will be seen through the analyzer.

#Q11. The Nicol prisms are placed in the crossed position, will you be able to see the light?

Ans. No.

#Q12. What do you mean by optically active substances?

Ans. Substances that rotate the plane of polarization are said to be optically active substances and the phenomenon known as the optical activity or rotatory polarization.

#Q13. Mention the name of some optically active substances?

Ans. These are sugar crystals, sugar solution, turpentine, sodium chlorate, tarter acids in aqueous solutions, solution of quinine sulfate, etc.

#Q14. Which substances are called dextro-rotatory?

Ans. Those substances which rotate the plane of polarization in the clockwise direction when the observer faces the source of light. Known also as right-handed substances.

#Q15.Which substances are called Laevo-rotatory?

Ans. Those substances which rotate the plane of polarization in the anticlockwise direction when the observer faces the source of light. Also called left-handed substances.

#Q16. Cane sugar is ‘sucrose’ or ‘fructose’?

Ans, Cane sugar is sucrose, fruit sugar is fructose.

#Q17. What do you understand from the specific rotation of sugar solution?

Ans. The specific rotation is a measure of the rotation of the said plane of polarization of incident light by a tube of material 1 decimeter long and of concentration 1 g/cc.

#Q18.What is the relation between the angle of the plane of polarization and the thickness of the optically active material?

Ans. Directly proportional to each other, (θ ∝ l).

#Q19.What is the relation between the angle of the plane of polarization (θ) and the wavelength of the monochromatic source?

Ans. The θ is Inversely proportional to the square of the wavelength (λ).

#Q20.What is the relation between the angle of the plane of polarization (θ) and the concentration of the optically active material?

Ans. The θ is directly proportional to the concentration (c).

#Q21 What are those factors on which specific rotation depends?

Ans.

thickness of the substance
concentration of the solution
temperature
wavelength of light used

#Q22. one decimeter is …

Ans. 1 decimeter = 10 cm

#Q23. What is polarimeter?

Ans. It is an instrument used to measure the rotation produced by a substance. When used for finding the optical rotation of sugar, it is called saccharimeter.

#Q24. 20 gm of cane sugar is dissolved in water to make 50 c.c. of solution. A 20cm. length of the solution causes +53⁰ optical rotations. Calculate the specific rotation?

Ans.

$large texttt{The specific rotation is given by the formula} \ S=frac{10 theta}{l c}\ texttt{here given,}\ theta = 53^{0}, c = frac{20}{50}=0.40 g/c.c., l = 20 cm.\ therefore S= frac{10 times 53 ^{0}}{0.40 times 20}\ S= 66.25 ^{0} frac {cm^{3}}{g.dm}$

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Specific Rotation of Sugar Solution | Polarimeter Experiment Viva

Specific Rotation of Sugar Solution | Polarimeter Experiment Viva

Polarimeter Experiment’s Aim

Apparatus Used

Specific Rotation of Sugar Solution Formula Used

Laurent’s Half-Shade Polarimeter

Read Also: Photoelectric effect experiment viva

Procedure

Observation of Polarimeter Experiment