POLARIMETRY

  Polarimetry is a type of qualitative and quantitative technique, used for optically active compounds

• the tendency of the molecules to rotate the plane of plane polarized light (clockwise or anticlockwise) and the extent of rotation is measured

• these properties are unique for a molecule, thus polarimetry can be used to identify and estimate the compounds

 

Definations:

Polarimetry is one of the important instrumental methods employed in analysis. This measures the rotation of the polarized light as it passes through an optically active compound. This technique involves the measurement of change in the direction of vibration of polarized light when interact with an optically active compound. A substance is said to be optically active if it rotates the plane of the polarized light.

 

 

Some other terms:

●Plane polarized light: Light that has been passed through a Nicol prism or other polarizing medium and vibrations take place in only one plane.

●Non-Plane polarized light: Light that has not been passed through a Nicol prism or other polarizing medium and vibrations take place in different plane.

 

 

OPTICAL ACTIVITY

l  It is the property of a chemical substance to rotate the plane of polarization of plane-polarized light.

ü  Dextro= right designated by ‘d’, (+),

ü  clockwise Levo= left designated by ‘l’, (-), counterclockwise

 

SPECIFIC ROTATION

The specific rotation may be defined as angle of rotation of plane of polarized light produced by a liquid, which in a volume of 1 ml, contain 1 g of the active substance when the length of column through which light passes is one decimeter (100 mm). it is calculated by the following general formula:

 

For, liquid substance, 

Principle:

Instruments measures the rotation of of polarized light as it passes through an optically active substance and the tendency of the molecules to rotate the plane polarized light towards clock-wise or anti-clock wise direction whose extent of the rotation can be measured. These compounds are called as optically active compounds or substances.

Optically active is unique character for a molecule.

 

 

INSTRUMENTATION: POLARIMETER

The instrument used to measure the angle of rotation is called polarimeter. There are following basic components:

1. Light source

2. Polarizer

3. Analyzer attached to a disc graduated in degrees

4. Sample tube

5. Detector

 

Light Source:

In Polarimetry instrumentation, we used monochromatic radiation from a sodium lamp and mercury vapor lamps with a wavelength of 589 nm for sources of light. 

Polarizer

It is a Nicol prism made of calcite. In polarimetry, Nicol prism or fixed polarizer converts unpolarized light to a linearly polarized light. It can convert a linearly polarized signal to a circularly-polarized signal in a clockwise or counterclockwise direction.

Sample tube

It contains optically active molecules for study. These optically active molecules rotate the plane of polarized light due to their optical activity. There are two types of optical rotation of light found in the sample cells.

1.      When the molecule present on the sample cell rotates the plane-polarized light in the clockwise direction, we called it dextrorotatory rotation.

2.      However, the molecules rotate the light counterclockwise, we call it levorotatory rotation.

Analyzer

Analyzing rotator Nicol prism also made by calcite. The analyzing rotator prism is fixed in such a way that it can be rotated easily about the axis of the incident light. It is used to measure the rotatory power or angle of rotation of a particular optically active compound.

Detector

The detector is a device or instrument used to detect the presence of a particular object or substance. In a polarimeter instrument, we used an eye or photoelectric cells as a detector that detects the optical rotation of light.

Working of Polarimeter:

Monochromatic radiation from a source is allowed to fall on the lens that renders the rays of the light parallel. The light then passes through the [Polarizer] Nicol prism. The light emerging from the Nicol prism is displaced from the original beam and enters into another auxiliary half shade of Nicol prism. It intercepts one - half of the beam. The radiation then passes through the polarimeter tube and the analyzer. The analyzer is rotate about the axis of the tube. There exists a particular position of the analyzer at which the two halves of the field are just equal in power which is considered as a blank point. The movement of the analyzer is recorded by means of Vernier fixed to the analyzer.

Application of polarimetry:

-          Polarimetry is a sensitive, non-destructive technique for measuring the optical activity exhibited by inorganic and organic compounds. It has following uses:

1. it is useful for the qualitative analysis of various drugs. It is used for evaluating and characterizing optically active compounds by measuring their specific rotation and comparing this value with the theoretical values found in literature. This specific rotation of some sugars are given below,

 

Sugar                           Specific rotation

Sucrose                        +66.54

Glucose                       +52.54

Fructose                      - 93.78

Maltose                       +137.5

Lactose                        +55.3

Dextrose                      +194.9 

 

2.  It is useful for determination of purity of various drugs.

3. Optical activity is helpful in selecting biologically active isomer.

4. It is also helpful in distinguishing dextro and levo isomers.

5. Polarimetry is frequently used for determining the quality of sugar products.

 

Experiment: Quantitative analysis of dextrose solution by polarimetric method

1. Dissolve accurately weighed amount (about 1, 2, 3, 4, 5 and 6g) of dextrose in water into six 100 ml volumetric flask.

2. Add 0.2 ml of 10% ammonia solution, dilute to 100 ml with water, and allow to stand at room temperature for 30 minutes.

3. Measure the optical rotation of each solution with the help of polarimeter.

4. Make a graph of optical rotation against dextrose concentration.

5. Measure the optical rotation of unknown solution in a similar way.

6. Read the concentration of dextrose from standard curve.