Polarimetry is an analytical technique used to measure the optical activity of organic and inorganic compounds in solution. A substance is considered optically active if it can rotate the plane of linearly polarized light. This rotation depends on the structure of its chiral molecules and their concentration. Every optically active substance has a characteristic angle of rotation, which allows identification and quantification of compounds without altering or damaging the sample. Because of its simplicity, non-destructive nature, and reliability, polarimetry is widely used in pharmaceutical, food, chemical, and cosmetic industries.
Several factors influence polarimetric measurement, including the composition of the sample, its concentration, the length of the observation tube, temperature, and the wavelength of light used. Understanding these variables is essential for obtaining accurate and reproducible results.
Polarised Light
Natural light consists of electromagnetic waves vibrating in all possible planes perpendicular to the direction of propagation. These vibrations can be represented as a combination of two perpendicular components. In contrast, polarized light vibrates only in a single plane. This selective vibration occurs when natural light passes through a polarizing material, such as a Nicol prism or Polaroid sheet, which absorbs one component of vibration and transmits the other.
Types of Polarised Light
a) Linearly or Plane Polarised Light
When natural light passes through a polariser, only one vibration plane is allowed to pass. The resulting beam is called plane-polarised light. Upon passing through an optically active substance, this beam rotates by an angle dependent on molecular symmetry, wavelength, temperature, and sample concentration.
b) Circularly Polarised Light
Circularly polarized light consists of two perpendicular electric field components of equal amplitude but differing in phase. The combined effect produces a rotating electric field that traces a circular path along the direction of light propagation. Depending on the direction of rotation, it may be right-handed or left-handed.
c) Elliptically Polarised Light
Elliptically polarized light is produced when two perpendicular light components differ in amplitude and/or phase. The resulting electric field describes an ellipse and represents a general form of polarization widely encountered in optical research.
Optical Rotatory Dispersion (ORD) and Circular Dichroism (CD)
Optical Rotatory Dispersion (ORD) refers to the change in optical rotation as a function of wavelength. When white light passes through an optically active sample, shorter wavelengths rotate more strongly than longer ones. This variation is called optical rotatory dispersion.
The phenomenon is closely linked with circular birefringence, where right and left circularly polarized light travel through the sample at different velocities. Absorption also influences ORD. When optically active molecules absorb certain wavelengths, the two circular components experience unequal absorption, a phenomenon known as Circular Dichroism (CD).
CD converts linearly polarized light into elliptically polarized light. ORD and CD spectra are mathematically related, and knowing one allows prediction of the other.
Polarimeter
A polarimeter is an instrument used to measure the angle of rotation produced by an optically active substance. A typical polarimeter consists of:
- Light source (usually sodium or mercury vapor lamp)
- Polarizer (commonly Nicol prism)
- Sample tube with parallel glass plates
- Analyzer
- Graduated scale or digital detector
The length of the sample tube is often calibrated using a standard solution at a fixed temperature. Nicol prisms are widely used for both polarizer and analyzer because they produce highly pure polarized light.
Functioning of a Polarimeter
Light from the source passes through a collimating lens, ensuring that rays become parallel before entering the polarizer. The plane-polarized beam then passes through the optically active sample inside the tube. Depending on the concentration and nature of the substance, the plane of polarization rotates either to the left (levorotatory) or to the right (dextrorotatory).
The analyser is rotated to find the position where the two halves of the split field appear equally illuminated. The rotation required to achieve this balance corresponds to the observed optical rotation. Modern photoelectric polarimeters automate this process for greater accuracy.
Measurement Procedure
To measure optical rotation:
- Fill the polarimeter tube with the sample solution.
- Ensure there are no bubbles and that end plates remain clean and parallel.
- Insert the tube into the polarimeter.
- Rotate the analyser until uniform brightness is observed.
- Read the angle of rotation directly from the scale.
This angle, along with sample concentration and cell length, is used to calculate specific rotation.
Applications of Polarimetry
1) Food Industries
Polarimetry is crucial in food analysis for determining concentrations, evaluating purity, and performing quality control. It is especially important in:
- Sugar industry: sucrose, glucose, sugar syrups
- Dairy industry: lactose, lactic acid
- Juice and wine production: tartaric and malic acids
- Essential oil analysis
2) Pharmaceutical Industries
The technique supports purity analysis and concentration measurements as required by regulatory pharmacopoeias. It is applied to:
- Alkaloids: codeine, morphine, nicotine
- Amino acids and organic acids
- Vitamins, steroids, antibiotics
3) Medicine
- Analysis of glucose and albumin in urine
- Hormone studies
- Enzyme and toxicology research
4) Cosmetic Industries
Used for identifying optically active essential oils such as lavender, spearmint, and citrus oils.
5) Chemical Industries
- Purity control of polymers and biopolymers
- Monitoring chemical reactions
- Characterization of organic compounds
6) Research Applications
- Determination of optical configuration
- Tracking changes in macromolecular structures
- Monitoring reaction kinetics via optical rotation
- Differentiating optical isomers
Additional Concepts
Optical Activity
Optical activity arises from molecules that lack mirror-plane symmetry. Chiral compounds rotate the plane of polarized light due to their asymmetric 3-dimensional structure.
Specific Rotation
Specific rotation is the observed rotation standardized for concentration, wavelength, and path length. It allows comparison of rotation values across different experimental conditions.
Factors Affecting Optical Activity
Optical rotation depends on temperature, wavelength, concentration, solvent polarity, and molecular structure.
ORD and CD Instruments
Spectropolarimeters measure ORD curves, while CD spectrometers measure differential absorption of left and right circularly polarized light. Both techniques provide complementary stereochemical information.
Detailed Notes:
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