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General Test Procedure (GTP) for Specific Optical Rotation (SOR)

PURPOSE
To lay down the procedure for Specific optical rotation

APPARATUS
Polarimeter
Sample cell/tube (50 mm or 100 mm)
Volumetric flasks
Thermostat

REAGENTS
Specified solvents: AR grade
Specified chemicals: AR Grade
Freshly distilled water

SAMPLE PREPARATION
  • Prepare sample solution of required concentration by transferring accurately weighed sample into a volumetric flask, dissolve and make up to volume with specified solvent or solution.
  • In the case of liquids, either use neat liquid or prepare a solution as specified.

PROCEDURE

For solutions (solid in liquid or liquid in liquid):
  • Adjust the temperature of the solvent or solution used for the preparation of the sample solution, to the specified temperature (±0.5°C).
  • Fill the sample cell with the solvent/solution, in which the sample solution is prepared.
  • Record the rotation and auto-zero the rotation value to nullify the interference of the solvent/solution.
  • Adjust the temperature of the sample solution to the specified temperature.
  • Rinse and then fill the sample cell with the sample solution.
  • Enter the exact concentration of the sample. Record five readings of specific optical rotation and report the mean value. The formula for calculating specific optical rotation is given below:
a = Observed rotation
λ = Wavelength used for measuring the rotation
L = Path length (i.e.= The length of the sample tube in decimeter)
t = Temperature at which the measurement is made
C = Concentration of substance in % w/v
  • Calculate the specific optical rotation on the anhydrous basis where water content is specified in the monograph or on a dried basis, where loss on drying is specified. The formula for calculating specific optical rotation on an anhydrous/dried basis is given below:
λ = Water content / Loss on drying content of sample (% w/w)
[α]λt = Specific optical rotation calculated as per above calculation.


For Neat Liquids:
  • Place the empty sample cell in the instrument and auto-zero the rotation value to nullify the interference of air.
  • Adjust the temperature of the sample to the specified temperature.
  • Record five readings of the specific optical rotation / optical rotation and report the mean value. 
  • The formula for calculating specific optical rotation is given below:
α = Observed rotation
λ = Wavelength used for measuring the rotation
L = Path length (i.e.= The length of the sample tube in decimeter)
t = Temperature at which the measurement is made
r = Density of the sample in g/ml at a temperature of measurement

REFERENCES
USP
BP
EP

Notes:
  1. Specified temperature as per USP is 25° and as per EP is 20°C.
  2. Optical rotation of the sample solution should be recorded within 30 min of the sample preparation unless otherwise specified. In the case of samples known to undergo racemization or mutarotation, care should be taken to standardize the time between the addition of the solute to the solvent and the introduction of sample solution into the sample cell.

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