It is a device which is used to measure the optical rotation produced by an optically active substance. By measuring the angle e the specific rotation of an optically active substance can be determined.
Two types of polarimeters are generally used in the laboratory now a days:
(a) Laurent’s Half Shade Polarimeter
(b) Biquartz Polarimeter
6.13.1 Laurent’s Half Shade Polarimeter
Construction: It consists of a monochromatic source S which is placed at focal point of a convex lens L. Just after the convex lens there is a Nicol Prism P which acts as a polariser. H is a half shade device which divides the field of polarised light emerging out of the Nicol P into two halves generally of unequal brightness. T is a glass tube in which optically active solution is filled. The light after passing through T is allowed to fall on the analyzing Nicol A which can be rotated about the axis of the tube. The rotation of analyser can be measured with the help of a scale C. Laurent’s half shade polarimeter is shown in Figure 6.22.
Working: In order to understand the need of a half shade device, let us suppose that half shade device is not present. The position of the analyzer is so adjusted that the field of view is dark when tube is empty. The position of the analyzer is noted on circular scale. Now the tube is filled with optically active solution and it is set in its proper position. The optically active solution rotates the plane of polarization of the light emerging out of the polariser P by some angle. So the light is transmitted by analyzer A and the field of view of telescope becomes bright. Now the analyzer is rotated by a finite angle so that the field of view of telescope again become dark. This will happen only when the analyzer is rotated by the same angle by which plane of polarization of light is rotated by optically active solution.
The position of analyzer is again noted. The difference of the two readings will give you angle of rotation of plane of polarization (8).
A difficulty is faced in the above procedure that when analyzer is rotated for the total darkness, then it is attained gradually and hence it is difficult to find the exact position correctly for which complete darkness is obtained.
To overcome above difficulty half shade device is introduced between polariser P and
glass tube T.
Half Shade Device
It consist of two semicircular plates ACB and ADC. One half ACB is made of glass while other half is made of quartz. Both the halves are cemented together. The quartz is cut parallel to the optic axis. Thickness of the quartz is selected in such a way that it introduces a path difference of ’A/2 between ordinary and extraordinary ray. The thickness of the glass is selected in such a way that it absorbs the same amount of light as is absorbed by quartz half.
Let us consider that the vibration of polarisation is along OP. On passing through the glass half the vibrations remain along OP. But on passing through quartz half these vibrations will split into 0- and £-components. The £-components are parallel to the optic axis while O- component is perpendicular to optic axis. The O-component travels faster in quartz and hence an emergence 0-component will be along OD instead of along OC. Thus components OA and OO will combine to form a resultant vibration along OQ which makes same angle with optic axis as OP. Now if the Principal plane of the analyzing Nicol is parallel to OP then the light will pass through glass half unobstructed. Hence glass half will be brighter than quartz half or we can say that glass half will be bright and the quartz half will be dark. Similarly if principal plane of analyzing Nicol is parallel to OQ then quartz half will be bright and glass half will be dark.
When the principal plane of analyzer is along AOB then both halves will be equally bright. On the other hand if the principal plane of analyzer is along DOC. then both the halves will be equally dark.
Thus it is clear that if the analyzing Nicol is slightly disturbed from DOC then one half becomes brighter than the other. Hence by using half shade device, one can measure angle of rotation more accurately.
Determination of Specific Rotation
In order to determine specific rotation of an optically active substance (say sugar) the polarimeter tube T is first filled with pure water and analyzer is adjusted for equal darkness (Both the halves should be equally dark) point. The position of the analyzer is noted with the help of scale. Now the polarimeter tube is filled with sugar solution of known concentration and again the analyser is adjusted in such a way that again equally dark point is achieved. The position of the analyzer is again noted. The difference of the two readings will give you angle of rotation θ. Hence specific rotation S is determined by using the relation.
[S]t λ =θ /LC
The above procedure may be repeated for different concentration.
6.13.2 Biquartz’s Polarimeter
It is a polarimeter which can be used to determine specific rotation more accurately. The arrangement in biquartz polarimeter is same as in Laurent’s half shade polarimeter except half shade device and source of light i.e., half shade device is replaced by biquartz plate and monochromatic source is replaced by white light.
Biquartz plate consists of two semicircular halves AYB and AY’B. One half AYB is dextrorotatory while other half is made of laevorotatory quartz as shown in Figure 6.2 4. Both the halves are cut with their refracting faces perpendicular to the optic axis. The thickness of each half is approximately 3.75 mm so that each half rotates the plane of polarization of yellow light by 90°. One rotates in clockwise while other in anticlockwise. Since white light is falling normally on biquartz plate and quartz crystal is cut with the optic axis perpendicular to the refracting face so light does not split into two components and only rotatory dispersion occurs. Red colour is rotated by minimum angle while violet colour by maximum angle. OR, OY, OV, and OR’, OY’, OV’ are the vibrations of red, yellow and violet colours in two halves of the Biquartz plate as shown in Figure 6.24.
If the principal plane of analyzing nicol is along AOB then colour of both the halves will
be greyish violet coiled tint of passage. If analyzing Nicol is rotated slightly on either side of
this position then one half becomes red and other bluish. Thus we can measure the angle of
rotation and hence specific rotation more accurately.
Determination of Specific Rotation
In order to determine specific rotation of an optically active solution (say sugar solution) polarimeter tube is first filled with pure water and analyzer is adjusted in such a way position that we get tint of passage. The position of the analyser is noted. Now the polarimeter is filled with sugar solution of known concentration and again the analyzer is adjusted in a position that tint of passage is achieved. The position of analyzer is again noted. The difference of the two readings will give you angle of rotation <j>. Putting this value in equation
S = θ /L x C , one get value of specific rotation