POLARIMETRY
Introduction
The electric fields associated with the beam of monochromatic light, vibrate in all directions perpendicular to the directions of propagation of light. Certain crystalline materials have different refractive indices for light, whose field vibrates parallel or perpendicular to the principal plane of the crystal. As a result, a Nicol prism constructed of this material transmits only light whose electric field oscillates in one plane. Optical activity concerns with the interaction of such plane polarised light with certain materials, particularly solutions of some organic compounds.
When a plane polarised light passes through a medium, it is retarded to an extent indicated by the refractive index of the medium. When the later is optically inactive, both circularly polarised components are retarded to the same extent and the beam emerges from the medium, polarised in the same plane as the incident beam. If the medium is optically active, the components are retarded to different extents and the beam emerges from the medium still plane polarised; but with the plane of polarisation inclined at an angle to the plane of polarisation of the incident beam. If the plane of polarisation is rotated clockwise, the substance is termed 'dextrorotatory', while if the plane of polarisation is rotated anticlockwise, the substance is termed 'laevorotatory'. Dextrorotation is designated (+) and laevorotation is designated (-).
Definitions
a) Optical Rotation : Optical rotation, unless otherwise specified, is measured at the wavelength of the 'D' line of Sodium (wavelength = 589.3 nm), at 25°C, on a layer 1 dm thick. It is expressed in degrees.
b) The specific optical rotation : The specific optical rotation of a liquid substance is the angle of rotation of the plane of polarisation at the wavelength of the 'D' line of Sodium, measured at 25°C unless otherwise specified, calculated with reference to a 1 dm thick layer of liquid and divided by the specific gravity of the liquid at 25°C.
The specific optical rotation of a solid substance is the angle of rotation of the plane of polarisation at the wavelength of the 'D' line of Sodium, measured at 25°C unless otherwise specified, calculated with reference to a 1 dm thick layer of a solution containing one gm of the substance per ml. The specific optical rotation of a solid is always expressed with reference to a given solvent and concentration.
Apparatus
The apparatus for measurement of rotation of a compound is known as the, Polarimeter.
It Consist of following parts
a) Light source -The source of light is usually Sodium vapour lamp, which emits monochromatic light.
b) Polariser - The polariser consists of a fixed Nicol prism at one end to convert ordinary light into plane polarised light.
c) Sample compartment -In this compartment the sample is placed in 1 dm or 2 dm tube. In the case of solids, a suitable solution is made, through which the plane polarised light is passed.
d) Analyser - The analyser consists of a movable Nicol prism, with a scale marked off in degrees. This is placed at the other end of the polarimeter.
e) Eye piece -The eye piece is placed at the analyser end.
Calibration -The polarimeter is calibrated using a solution of previously dried Sucrose and measuring the optical rotation in a 2 dm tube at 25°C for concentrations ranging from 10% w/v to 50% w/v.
The angle of rotation is as given in following table :
10.0 - 13.33°
20.0 - 26.61°
30.0. - 39.86°
40.0 - 53.06°
50.0. - 66.23°
Alternatively a quartz control plate with known optical activity can be used for calibration.
Precautions
The accuracy and precision of optical rotation measurements can be increased, if following precautions are taken
a) The instrument must be in a good condition. The optical elements must be very clean and in exact alignment.
b) Specific attention should be paid to the temperature control of the solution and the polarimeter.
c) Five consecutive readings are taken and the mean of these five readings is used for calculations to improve precision.
d) Polarimeter tube must be filled in such a way as to avoid air bubbles.
e) For tube with removable end plates fitted with gaskets and caps, the end plates must be tightened to ensure a leak proof seal between the end plate and the body of the tube.
f) For substances with low rotatory power, the end plates should be loosened and tightened again after each reading.
g) Spillage of the sample must be avoided.
h) The tube having the sample must be thoroughly cleaned. Liquid and solution of solids must be clear.
Introduction
The electric fields associated with the beam of monochromatic light, vibrate in all directions perpendicular to the directions of propagation of light. Certain crystalline materials have different refractive indices for light, whose field vibrates parallel or perpendicular to the principal plane of the crystal. As a result, a Nicol prism constructed of this material transmits only light whose electric field oscillates in one plane. Optical activity concerns with the interaction of such plane polarised light with certain materials, particularly solutions of some organic compounds.
When a plane polarised light passes through a medium, it is retarded to an extent indicated by the refractive index of the medium. When the later is optically inactive, both circularly polarised components are retarded to the same extent and the beam emerges from the medium, polarised in the same plane as the incident beam. If the medium is optically active, the components are retarded to different extents and the beam emerges from the medium still plane polarised; but with the plane of polarisation inclined at an angle to the plane of polarisation of the incident beam. If the plane of polarisation is rotated clockwise, the substance is termed 'dextrorotatory', while if the plane of polarisation is rotated anticlockwise, the substance is termed 'laevorotatory'. Dextrorotation is designated (+) and laevorotation is designated (-).
Definitions
a) Optical Rotation : Optical rotation, unless otherwise specified, is measured at the wavelength of the 'D' line of Sodium (wavelength = 589.3 nm), at 25°C, on a layer 1 dm thick. It is expressed in degrees.
b) The specific optical rotation : The specific optical rotation of a liquid substance is the angle of rotation of the plane of polarisation at the wavelength of the 'D' line of Sodium, measured at 25°C unless otherwise specified, calculated with reference to a 1 dm thick layer of liquid and divided by the specific gravity of the liquid at 25°C.
The specific optical rotation of a solid substance is the angle of rotation of the plane of polarisation at the wavelength of the 'D' line of Sodium, measured at 25°C unless otherwise specified, calculated with reference to a 1 dm thick layer of a solution containing one gm of the substance per ml. The specific optical rotation of a solid is always expressed with reference to a given solvent and concentration.
Apparatus
The apparatus for measurement of rotation of a compound is known as the, Polarimeter.
It Consist of following parts
a) Light source -The source of light is usually Sodium vapour lamp, which emits monochromatic light.
b) Polariser - The polariser consists of a fixed Nicol prism at one end to convert ordinary light into plane polarised light.
c) Sample compartment -In this compartment the sample is placed in 1 dm or 2 dm tube. In the case of solids, a suitable solution is made, through which the plane polarised light is passed.
d) Analyser - The analyser consists of a movable Nicol prism, with a scale marked off in degrees. This is placed at the other end of the polarimeter.
e) Eye piece -The eye piece is placed at the analyser end.
Calibration -The polarimeter is calibrated using a solution of previously dried Sucrose and measuring the optical rotation in a 2 dm tube at 25°C for concentrations ranging from 10% w/v to 50% w/v.
The angle of rotation is as given in following table :
Concentration. Angle of rotation
g /100ml. at 25°C10.0 - 13.33°
20.0 - 26.61°
30.0. - 39.86°
40.0 - 53.06°
50.0. - 66.23°
Alternatively a quartz control plate with known optical activity can be used for calibration.
Precautions
The accuracy and precision of optical rotation measurements can be increased, if following precautions are taken
a) The instrument must be in a good condition. The optical elements must be very clean and in exact alignment.
b) Specific attention should be paid to the temperature control of the solution and the polarimeter.
c) Five consecutive readings are taken and the mean of these five readings is used for calculations to improve precision.
d) Polarimeter tube must be filled in such a way as to avoid air bubbles.
e) For tube with removable end plates fitted with gaskets and caps, the end plates must be tightened to ensure a leak proof seal between the end plate and the body of the tube.
f) For substances with low rotatory power, the end plates should be loosened and tightened again after each reading.
g) Spillage of the sample must be avoided.
h) The tube having the sample must be thoroughly cleaned. Liquid and solution of solids must be clear.
