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Polarimetry

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Polarimetry

INTRODUCTION
Polarimetry is a sensitive non-destructive method for the measurement of the optical activity of the compounds which are organic or inorganic in nature. This measures the rotation of the polarized light as it passes through an optically active compound. This method i s mainly used in the determination of the unknown solution concentrations. A substance is said to be optically active if it rotates the plane of the polarized light.
Optical activity phenomenon
Optical activity phenomenon
When the polarized light passes through the optically active substance and rotates the plane of polarized light to the left side, or clockwise, then the compound is known as the dextrorotatory substance. If the rotation is observed in the right side, or anti-clockwise direction, then the compound is known as the levorotatory substance.
THEORY
The rotation of the plane polarized light is mainly based upon the asymmetric molecules and the steric configuration. The rotation is directly proportional to the concentration and the path length. The angle of rotation is calculated by Biot's formula:
α = [α]D25 × C × d
where C = concentration of the sample solution; d = path length of the sample cell; [α]D25 = specific rotation of the sample at D line of the sodium at 25°C.
Factors affecting the optical activity:
  • Nature of the sample
  • Concentration
  • Length of the sample tube
  • Wavelength of the incident light
  • Temperature of the sample solution
CONCEPT OF THE CIRCULAR DICHROISM (CD) AND OPTICAL ROTATORY DISPERSION (ORD)
The measurement of the optical rotation as the function of the wavelength is known as the ORD, and the measurement of the unequal absorption of the right and left circularly polarized light is known as the CD.
This means that by changing the wavelength, the rotation of the optically active compound is changed in ORD.
ORD and CD phenomenon
ORD and CD phenomenon
The change in the specific rotation with the wavelength change is known as the cotton effect. Above the zero is known as the positive cotton effect, and below the zero is known as the negative cotton effect. The concept of the ORD was first given by Crabbe and Djerassi.
In CD, the two components of the polarized light have two different absorptivities, that is, circularly polarized light is converted to the elliptically polarized light. This mainly measures the difference of the optical rotation between the dextrorotatory and the levorotatory substances. Hence the CD is the plot between the molar ellipticity and the wavelength.
Molar ellipticity (θ) = [ψ]M/100
                        = ∊1−∊r
where Ψ = specific ellipticity; M = molecular weight; εl and εr are the molar absorptivities of the left and right circularly polarized light.
CD is an absorptive quantity and ORD is a dispersive quantity. CD has a higher resolution than ORD.
INSTRUMENTATION
The optical activity is determined by the polarimeter which consists of the following components:
  • Source
  • Filter
  • Sample cell
  • Analyzer
  • Detector
Image
Polarimeter

Source

Generally, sodium vapour lamp is employed. This produces wavelengths above 450 nm.

Filter

Filter is mainly used to polychromatic light into monochromatic light by absorbing the undesired radiation.

Sample Cell

Sample cells are long tubes which are made up of glass.

Analyzer

A nicol prism is used for this purpose. This is mainly used to analyze the samples whether they rotate the plane of polarized light on the right or left side.

Detector

The photomultiplier tube is commonly employed for the detection of the wavelength.
The procedure followed is sample tube filled with the sample solution which is placed between the polarizer and the analyzer. Then, allow the source of light to pass through the radiation. This ordinary light is polarized by the polarizer and the polarized light is passed through the sample solution.
The optically active substance present in the sample solution rotates the plane polarized light into clockwise or in an anti-clockwise direction. Then, the analyzer measures the angle of rotation and is detected by the detector.
APPLICATIONS
  • Used in the determination of the unknown solutions concentration
  • Used in the identification of several compounds
  • Used in quantitative determinations in the sugar industry
  • Used in proteomics
  • Used in the determination of the optical purity of substances
  • Used in the determination of chiral compounds
ADVANTAGES
  • Simple
  • Random analysis
  • Simple sample preparation
DISADVANTAGES
  • Only optically active substances can be measured
  • Less accurate
  • Less reproducibility
  • High interferences
REVIEW QUESTIONS
  1. What is optical activity? Give one example.
  2. What is plane polarized light?
  3. Explain Biot's formula.
  4. What are the factors affecting optical activity?
  5. Explain circular dichroism and optical rotator dispersion.
  6. Explain the different components of a polarimeter instrument.
  7. What are the advantages and disadvantages of polarimetry?


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