Difference Between Absorptivity and Molar Absorptivity

There is no difference between absorptivity and molar absorptivity because the two terms express the same idea. Absorptivity, or molar absorptivity, is the absorbance of a solution per unit path length and concentration. The molar absorptivity can be determined when using the Beer Lambert Law.

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 travelled by the light beam. Refer the below equation,

A α lc

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

Figure 01: Beer Lambert Law in a Diagram

The absorbance is the ratio between the light intensity before (I₀) and after (I) it passes through the solution. Refer 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 unit-less). The molar absorptivity determines how strongly a solution can absorb a light beam. Furthermore, the molar absorptivity depends on the type of analyte in the solution.

Summary – Absorptivity vs Molar Absorptivity

The term absorptivity has applications in two fields, in chemistry as well as in physics. In chemistry, the absorptivity and molar absorptivity are the same. Therefore, there is no difference between absorptivity and molar absorptivity because they express the same idea; it is the absorbance of a solution per unit path length and concentration.

Reference:

1. Helmenstine, Anne Marie, Ph.D. “Absorptivity Definition.” ThoughtCo, Mar. 17, 2017. Available here
2. Libretexts. “The Beer-Lambert Law.” Chemistry LibreTexts, Libretexts, 16 Dec. 2017. Available here
3. “Molar Attenuation Coefficient.” Wikipedia, Wikimedia Foundation, 7 Apr. 2018. Available here 

Image Courtesy:

1.’Transmittance’By Marmot2019 – Own work, (CC BY-SA 4.0) via Commons Wikimedia 

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