Determination of Specific optical rotation

Determination of Specific optical rotation

Determination of Specific optical rotation

Determination of Specific optical rotation

Optical rotation, specifically the specific optical rotation, is a fascinating phenomenon that has intrigued scientists, chemists, and researchers for centuries. In this article, we will delve into the world of specific optical rotation, its historical significance, measurement techniques, and its various applications in different industries.


Specific optical rotation is a property of chiral substances, which means that these compounds exist in both right-handed and left-handed forms, similar to how our hands mirror each other. This property is a fundamental concept in chemistry, providing valuable insights into the structural and chemical properties of compounds.

What Is Specific Optical Rotation?

Specific optical rotation, often referred to as optical rotation, is a measure of the degree to which a substance rotates plane-polarized light. It is expressed in degrees and is a unique identifier for each chiral compound. This rotation occurs due to the interaction between the compound’s molecules and the incident polarized light.

Measurement Techniques for Specific Optical Rotation


Polarimetry is the most common method used to measure specific optical rotation. It involves passing plane-polarized light through a sample and measuring the angle by which the plane of polarization is rotated. This technique is highly sensitive and is widely used in the pharmaceutical and chemical industries.

Circular Dichroism (CD)

Circular Dichroism is another technique employed to determine specific optical rotation. It measures the difference in absorption of left and right circularly polarized light. CD spectroscopy is useful in studying the secondary structure of biomolecules and chiral compounds.

Equipment: Polarimeter


Optical Rotation:  ( ά  ) is the property shown by certain substances of rotating the plain of polarization of polarized light. Such substances are said to be optically active in the sense that they cause incident polarized light to emerge in a plane forming a measurable angle with a plane of the incident light.

The optical rotation of a substance is the angle through which a plane of polarization is rotated when polarized light passes through the substance.

Substances are described as Dextro-rotatory or Leavo-rotatory according to the plane of polarization, clockwise or anticlockwise.

The optical rotation,

unless otherwise specified, is measured as the wave length of D line of sodium (λ=589.3nm) measured at 250C., on a layer of 1-dm thick. It is expressed in degrees.


Weigh accurately a suitable quantity of the substance being examined to obtain the solution of the strength specified in the individual monograph and transfer to a volumetric flask by means of water or other solvent, if specified. If a solvent is used, reserve a portion of it for the blank determination.

Unless otherwise specified, adjust the contents of the flask to 250 by suspending the flask in a constant temperature bath. Make up to the volume with the solvent at 250 and mix well.

Transfer the solution to the polarimeter tube with in thirty minutes from the time the substance was dissolved and during this time interval, maintain the temp. at 250C.

Determine the zero point of the polarimeter and then make five readings of the observed rotation of the test solution at 250.

Take equal number of readings in the same tube with the solvent in place of the test solution.

The zero correction is the average of the blank readings, and is subtracted from the average observed rotation if the two figures are of the same sign or added if they are opposite in sign to obtain the corrected observed rotation.

Factors Affecting Specific Optical Rotation

Factors affecting specific optical rotation include the concentration of the substance, the wavelength of light, and the temperature. These variables can influence the observed rotation of plane-polarized light passing through a chiral compound, and it’s essential to consider them for accurate measurements.

Application in the Pharmaceutical Industry

The pharmaceutical industry extensively uses specific optical rotation to analyze the purity and enantiomeric composition of drugs. Chiral purity is vital in drug development as the biological activity of enantiomers can vary significantly.


1. How is specific optical rotation different from optical rotation?

Specific optical rotation is a property inherent to a substance, while optical rotation can vary with factors like concentration and path length.

2. Why is specific optical rotation important in the pharmaceutical industry?

It is crucial for assessing the purity and effectiveness of chiral drugs, ensuring that patients receive the correct enantiomer.

3. Can specific optical rotation be used to identify unknown substances?

Yes, it can be used to identify and characterize chiral compounds, aiding in their identification.

4. What are some modern instruments used for measuring specific optical rotation?

Automated polarimeters and CD spectrometers are commonly used in modern laboratories.

5. How does specific optical rotation impact the food industry?

It is used to assess sugar content, flavor compounds, and the authenticity of food products, especially those with chiral components.

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