Expt. 3.pdf

-1EXPERIMENT NO. -3 POLARIZATION OF LIGHT OBJECT: To find the specific rotation of cane- sugar solution by a polarimeter at room temperature, using half-shade / Bi-quartz device. APPARATUS USED: A polarimeter, cane sugar, a physical balance, weight box, a measuring cylinder, a beaker and a source of light (Sodium lamp). [Note: Use monochromatic source for half-shade device and white light source for bi-quartz device.] INTRODUCTION: The study of optical activity of liquids began in the early 19th century with Biot and other scientists. They found that solutions of sugar and certain other naturally occurring chemicals would rotate a beam of polarized light passing through the solution. They called such substances optically active, a term which is still used. The instrument used to demonstrate or to measure this rotation is known as polarimeter. FUNCTIONING OF THE POLARIMETER: Schematic representation of the functioning of the polarimeter.

Fig.1 (a): When the sample tube is empty, the planes of polarization of the polarizing and the analyzing prisms are same i.e plane of polarization does not rotate (α=0) while passing through empty space in air medium.

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Fig.1 (b): When the sample tube has a solution of a chiral (optically active) substance, the plane of polarization of the emergent polarized light changes. One now needs to rotate the analyzer prism for its plane of polarization to coincide with the plane of the emergent light. This corresponds to the maximum intensity of the transmitted light. The angle of rotation of analyser to pass plane polarized light is denoted by α (shown by a curved arrow in the above Fig. 1(b)) THEORY: The ability to rotate light is termed optical activity and substances that exhibit this property are called optically active substances. Optically active organic molecules have a spiral structure like a right-handed or left-handed screw. It is this spiral nature of the molecule, which rotates the plane of polarization of light passing through it. Right-handed molecules will rotate the plane of polarization clockwise as viewed in the direction of the beam, while left-handed molecules rotate the plane in a counter clockwise direction. If right-handed and left-handed species of a given molecule occurred with equal abundance, then there would be no net effect on the polarization of light passing through. Though, naturally occurring biological molecules of a given species are always either purely righthanded or purely left-handed. However, these enantiomer compounds rotate light by exactly the same amount but in the opposite direction. The degree to which a substance rotates light may be used to determine a) the identity of the substance, b) the enantiomer purity of the substance or c) the concentration of a known substance in a solution. In order to observe rotation, the light which is passed through the solution must be plane polarized. Ordinary light has waves which are oriented in all directions as shown in fig. 2 (a). Plane polarized light is made up of waves which are oriented parallel to a defined plane as shown in fig. 2 (b).

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Fig. 2 (a)

Fig. 2 (b)

Fig. 3 This is because, when light interacts with matter, two basic phenomena occur, namely, absorption and the decrease in the velocity of light. Absorption is the decrease in the intensity of light because a part of the incident light is absorbed by the material. The decrease in velocity of light is due to refractive index of the material, because the velocity of light is smaller in the material than in the vacuum. FORMULA USED: The specific rotation of cane-sugar solution is given by S = θ / l.c = θ.V / l.m Where θ l m V c

= rotation produced in degrees, = length of the tube in decimetre, = mass of sugar in gms. Dissolved in water, = volume of sugar solution in ml. = concentration of cane-sugar solution = m/V

-4OBSERVATIONS: Length of the polarimeter tube

= ............dm.

Room temperature

= ............oC

Mass of the sugar taken ‘m’

= ............gm.

Volume of the solution ‘V’

= ............ml.

Concentration of the solution c

= ............gm/ml.

PROCEDURE: 1. The least count of vernier attached to the instrument is determined. 2. The eye piece attached to the analyzer is focused on the half-shade. 3. The polarimeter tube is then carefully cleaned and filled with water (taking care that no air bubble is enclosed in it). The tube is now placed at its proper place inside the polarimeter. 4. The source of light is switched on. Looking through the telescope, the analyzer is rotated till the two half of the field of view are equally illuminated (in half-shade arrangement)). This position of the analyzer is noted by reading the main and vernier scales. 5. A second position of equal illumination of the two half of the field of view (in half-shade), nearly 1800 apart of the first position is also noted. 6. A solution of cane sugar of known strength is prepared (say by dissolving 5 gm of sugar in water till solution is 50 c.c.). The solution is kept in a beaker. 7. The water of the polarimeter tube is then replaced by the sugar solution and the analyzer is reset for equal brightness. Both the positions (1st and 180º apart) are found out by taking the readings of the main and vernier scales. 8. The mean of the differences of the corresponding positions of the analyzer (for water and solution) gives the angle of rotation produced by the solution.

-59. The experiment

is

repeated

with sugar

solution

of

different

concentrations as below: a) 5 gm sugar in 50 ml of water b) 5 gm sugar in 75 ml of water c) 5 gm sugar in 100 ml of water 10. The length of the polarimeter tube is measured in decimetres. Specific rotation is calculated for each strength of the solution separately and its mean value is evaluated. 11. Plot the graph between concentration and the angle of rotation. It will be a straight line.

RAY DIAGRAM: (Must be on first left page by pencil)

Note: bi-quartz device is inbuilt in polarimeter

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OBSERVATION FOR THE ANGLE OF ROTATION:Value of one division of the main scale (MS) = x Total no. of division on the vernier scale (VS) = y Least count of instrument (LC) = x/y Analyzer reading with pure wate Ist position (a) =MS+VSxLC

Analyzer reading with solution

o

180 apart positio S. No.

Conc. in gm /cc. c=m/V

(b) =MS+VSxLC

Ist position (c) =MS+VSxLC

Angle of rotation (θ)

o

180 apart position (d)=MS+VSxLC

Ist position

180o apart

e=a~c f=b~d 1

c1 =

2

c2 =

3

c3 =

Mean θ =(e + f)/2

-7CALCULATIONS: (Must be on left page by pen) S1 = θ1/ l.c1 = ........ ; S2 = θ2/ l. c2 = ........ ; S3 = θ3/ l. c3 = ........ ; S=

(S1 + S2 + S3) / 3 = ........ Degree/dm/gm/ml

RESULTS: The specific rotation for cane- sugar solution at .........0C is ...........0ml/gm/dm (degree per dm per unit concentration.)

CALCULATION FOR MAXIMUM PROBABLE ERROR: (Should be on left page by pen) S = θ/ l.c = θ.V/ l.m Taking log both side and differentiate ∆S/S = ∆ θ / θ + ∆V / V+ ∆ l / l + ∆ m/ m

[Note: ∆ θ = 0.1º, ∆ V =0.1ml , ∆ l = 0.1cm, ∆ m = 0.001gm] SOURCES OF ERROR AND PRECAUTIONS: (i) End faces of the polarimeter tube should be cleaned and fitted air tight. (ii) Water and the solution must be dust free. (iii) There should be no air bubble in the part of light through the polarimeter tube. (iv) The tube should be well rinsed before filling it with the solution every time.

-8Viva- Voce: Q1. How does polarized light differ from the ordinary light? Q2. What do you understand by polarized light and the plane of polarization? Q3. What are the factors on which the optical activity of a substance depends? Q4. How does the optical activity depend on the state of the substance? Q5. Does specific rotation depend on the wavelength of light used and temperature of the solution?