Molar Absorptivity
Definition and meaning of Molar Absorptivity in chemistry.
Molar absorptivity is a constant that quantifies how strongly a chemical substance absorbs electromagnetic radiation at a specific wavelength. It is the absorbance per unit of molar concentration and path length, derived from the Beer-Lambert Law.
In more detail
The molar absorptivity (symbol ε, epsilon) is a quantitative constant defined by the Beer-Lambert Law: A = ε × b × c, where A is absorbance, b is the path length in centimeters, and c is the molar concentration in moles per liter. This property is wavelength-dependent and characteristic of each chemical compound, reflecting the energy of electronic transitions in its molecules. The units are typically liters per mole per centimeter (L mol⁻¹ cm⁻¹). Molar absorptivity remains constant only under fixed conditions and is temperature-dependent, making it invaluable for identifying compounds and quantifying their concentrations in spectrophotometric analysis.
Key facts
| Symbol | ε (epsilon) |
|---|---|
| Units | L mol⁻¹ cm⁻¹ |
| Field | Analytical Chemistry |
| Beer-Lambert Law | A = ε·b·c |
Potassium permanganate (KMnO4), a vivid purple solution, has high molar absorptivity in the visible spectrum and is commonly used as a standard in quantitative analysis because its strong light absorption enables precise concentration measurements through UV-visible spectrophotometry.
Frequently asked questions
How does molar absorptivity differ from absorbance?
Absorbance (A) is the amount of light absorbed by a specific solution under particular conditions, while molar absorptivity (ε) is an intrinsic property of the substance itself at a given wavelength. Absorbance depends on concentration and path length; molar absorptivity does not.
Can molar absorptivity be used to identify unknown substances?
Yes, molar absorptivity is wavelength-dependent and unique to each compound. By measuring the absorbance of a solution at specific wavelengths, scientists can identify unknown substances and determine their purity by comparing their absorption spectra to known standards.