Optical Rotation vs. Specific Rotation

Attribute	Optical Rotation	Specific Rotation
Definition	Measure of the angle of rotation of plane-polarized light as it passes through a sample	Measure of the specific rotation of a compound, calculated using the formula [α] = α / (c * l)
Units	Degrees (°)	Degrees per millimeter per gram (°/mm/g)
Dependence on Concentration	Depends on the concentration of the sample	Specific rotation is independent of concentration
Dependence on Wavelength	Depends on the wavelength of light used	Specific rotation is independent of wavelength
Measurement	Measured using a polarimeter	Measured using a polarimeter and a sample with known concentration and path length

Definition

Optical rotation and specific rotation are terms used in the field of chemistry to describe the ability of a substance to rotate the plane of polarized light. Optical rotation is a general term that refers to the rotation of plane-polarized light by a substance, while specific rotation is a specific measurement of this rotation for a particular substance at a given concentration, temperature, and wavelength of light.

Measurement

Optical rotation is measured using a polarimeter, which consists of a light source, polarizer, sample tube, and analyzer. The amount of rotation is typically expressed in degrees, either clockwise (dextrorotatory) or counterclockwise (levorotatory). Specific rotation, on the other hand, is a standardized measurement that takes into account the concentration of the substance and the path length of the sample tube. It is expressed in degrees per decimeter per gram (°/dm/g).

Units

While optical rotation is measured in degrees, specific rotation is expressed in degrees per decimeter per gram. This difference in units reflects the fact that specific rotation is a more precise measurement that takes into account the concentration and path length of the sample, whereas optical rotation is a more general measurement of the ability of a substance to rotate polarized light.

Temperature Dependence

Both optical rotation and specific rotation are temperature-dependent. As the temperature of a sample changes, its ability to rotate polarized light may also change. This is an important consideration when conducting experiments or analyzing data involving optical rotation or specific rotation, as temperature fluctuations can affect the accuracy of the measurements.

Wavelength Dependence

Another factor that can affect both optical rotation and specific rotation is the wavelength of the light used in the measurement. Different substances may exhibit different levels of rotation at different wavelengths, so it is important to specify the wavelength when reporting optical rotation or specific rotation values. This ensures that the measurements are accurate and comparable across different experiments.

Applications

Optical rotation and specific rotation are important concepts in the field of chemistry and are used in a variety of applications. For example, they are commonly used to determine the purity and concentration of chiral compounds, as well as to identify unknown substances based on their optical properties. Specific rotation values are often included in chemical databases and used as reference values for comparison in research and analysis.

Conclusion

In conclusion, optical rotation and specific rotation are both important measurements of a substance's ability to rotate polarized light. While optical rotation is a general term that describes this phenomenon, specific rotation is a more precise measurement that takes into account the concentration and path length of the sample. Both measurements are temperature and wavelength-dependent, and are used in a variety of applications in the field of chemistry.