What is the Difference Between Molar Absorptivity and Specific Absorbance

The key difference between molar absorptivity and specific absorbance is that molar absorptivity is the absorbance of a solution per unit path length and concentration, whereas specific absorbance is the maximum absorbance of a 1% solution over a 1 cm path length measured via spectroscopy.

Molar absorptivity and specific absorbance are two important terms in chemistry that are related to the optical properties of substances.

1. Overview and Key Difference
2. What is Molar Absorptivity
3. What is Specific Absorbance
4. Molar Absorptivity vs Specific Absorbance in Tabular Form
5. Summary – Molar Absorptivity vs Specific Absorbance

What is Molar Absorptivity?

Absorptivity or molar absorptivity is the absorbance of a solution per unit path length and concentration. It originates from the Beer-Lambert Law. Beer-Lambert Law states that the absorbance of electromagnetic waves by a solution is directly proportional to the concentration of the solution and the distance traveled by the light beam.

A α lc

Here, A is the absorbance, l is the path length (distance traveled by the light beam), and c is the concentration of the solution. A proportionality constant is used to obtain the equation for absorbance.

The absorbance is the ratio between the light intensity before (I0), and after (I) it passes through the solution. Refer to the below equation,

A = εbc

Here, ε is the molar absorptivity. It is also known as the molar absorption coefficient. The unit of the molar absorptivity can be obtained from the above equation, while the unit of concentration is mol/L (moles per liter), and the unit of path length is cm (centimeter). The unit of molar absorptivity is L mol-1 cm-1 (since the absorbance is unitless). The molar absorptivity determines how strongly a solution can absorb a light beam. Furthermore, molar absorptivity depends on the type of analyte in the solution.

What is Specific Absorbance?

Specific absorbance is the maximum absorbance of a 1% solution over a 1 cm path length measured via spectroscopy. It is denoted by A¦. Determining the specific absorbance of a particular drug provides an important tool in quantitative verification of the concentration of analytical standards. The formula for the specific absorbance is as follows:

A(1 percent, 1 cm) = A/cl

Where c is the concentration of the absorbing substance that is given as a percentage w/v, and I is the thickness of the absorbing layer (taken in cm). We can say that A(1 percent, 1 cm) or specific absorbance in a given solution is a property of the absorbing substance.

In spectroscopy, specific absorbance is the measurement of light absorbance in mixed media at one specific wavelength. Moreover, the light absorbance that is measured at a particular wavelength can attribute to a specific component or to a group of components.

What is the Difference Between Molar Absorptivity and Specific Absorbance?

Molar absorptivity and specific absorbance are related to each other. The key difference between molar absorptivity and specific absorbance is that molar absorptivity is the absorbance of a solution per unit path length and concentration, whereas specific absorbance is the maximum absorbance of a 1% solution over a 1 cm path length measured via spectroscopy. Higher the molar absorptivity, the higher the absorbance.

The following table summarizes the difference between molar absorptivity and specific absorbance.

Summary – Molar Absorptivity vs Specific Absorbance

The key difference between molar absorptivity and specific absorbance is that molar absorptivity is the absorbance of a solution per unit path length and concentration, whereas specific absorbance is the maximum absorbance of a 1% solution over a 1 cm path length measured via spectroscopy. The higher the molar absorptivity, the higher the absorbance.

Reference:

1. C.H. Seman. “What Is Molar Absorptivity?” All the Science, 1 Oct. 2022.

Image Courtesy:

1. “Tyrosine abs flu” By Mark Somoza – Own work (CC BY 2.5) via Commons Wikimedia
2. “Spectroscopy overview” By Jon Chui – Own work (CC BY-SA 3.0) via Commons Wikimedia