Optical Rotation and Specific Rotation (Polarimetry):Basics, Operation and Calibration

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Polarimetry is the most widely used technique in the pharmaceutical industry for the identification test or to calculate the % purity of as sample (to calculate assay) by measuring its optical rotation and specific optical rotation at a specified temperature.

 

Instrument used to measure this phenomenon known as polarimeter.

  • Optical rotation: The optical rotation of a substance is the angle of rotation when a plane polarized light is passed through it. This rotation can be exhibit by only those substance which contains chiral carbon (Carbon congaing four different functional groups) or optically active compound. The rotation of the plane of polarization may be either clockwise, to the right (dextro rotatry d), or left levo rotarty l)
  • Specific rotation : It is the degree of rotation or angle of rotation when plane polarized light passes through a solution of sample place in a 1dm tube, concentration of sample is 1g/ml, conducted at 25 oC using the D-line of the sodium lamp (λ=589.3 nm). given as below:

 

αTλ  =       α / C x  1                                                                                                                                                    

α = observed optical rotation

c =  Concentration of solution

1 = length of tube 1dm(10cm)

T = Temperature

λ= wavelength of D line of sodium lamp

 

General operation of Polarimeter :

  • Switch ON the main instrument and wait at least for 15 minute for proper initialization of instrument
  • Open the software in computer connected to instrument.
  • Prepare the sample solution of known concentration in proper solvent so as to form a clear solution (or as directed in STP)
  • Fill the solvent alone in tube cell as a blank and press ‘Zero Clear the screen will show measurement as 0.000.
  • Fill the sample solution in the tube and place in the sample compartment enter the sample identification and sample concertation, dip the temperature probe in sample solution select start.

 

Precautions:  

  • Sample solution should be clear.
  • Make sure there is no air bubbles entrapped in the tube before measurement.
  • Handle the sample tube very carefully as it is expensive.
  • Always use the proper clean and dried sample tube for measurement.
  • Never open the sample chamber during operation.

 

Calibration Procedure:

 

Method I – Calibration with Sucrose:

  • Prepare 10%, 20 % , 30%, 40%, 50 %, w/v solution of sucrose (previously dried at 60ºc for 1 hours) in purified water.
  • Measure the optical rotation of above prepared solution in a 1 dm tube at 25ºC using water as a blank.

 

Acceptance Criteria :

Concentration Limit
10 % +6.60º to +6.73º
20 % +13.17º to +13.44º
30% +19.73º to +20.13º
40% +26.26º to +26.80º
50 % +32.78º to +33.45º

Method II – Calibration with Fructose

  • Preparation of 5 M ammonia solution: 75 ml of strong ammonia solution to 1000 ml of water.
  • Dissolve 10.0 g of Fructose in 80 ml of water, add 0.2 ml of 5 M Ammonia Solution, allow to stand for 30 minutes and dilute to 100.0 ml with water.
  • Rinse cell with 5 M ammonia solution make necessary blank correction.
  • Place the sample cell, containing the Fructose solution in the sample compartment. When the temperature reaches 20 °C ± 0.5 °C, Click OK, and record the Optical rotation at 20°C ± 0.5 °C.
  • Frequency : Monthly

Acceptance Criteria : The optical rotation of Fructose is between – 9.05 ° to – 9.25°.

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