What Is Molar Absorptivity?

In chemistry, molar absorptivity is defined as a measure of a chemical's ability to absorb light at a specified wavelength. The molar absorptivity coefficient, ε, depends on the chemical species; actual absorption depends on chemical concentration and the path length. These variables are used in the Beer-Lambert Law. Molar absorptivity also is known as the molar extinction coefficient and the molar absorption coefficient.

The Beer-Lambert Law is an equation relating absorption to chemical concentration, path length and molar absorptivity. Mathematically, the Beer-Lambert Law can be expressed as A = εcl. The most common units for the molar absorptivity coefficient are M^-1cm^-1, although the units can be different depending on the units used for chemical concentration and path length. The International System of Units (SI) for this measurement are m²/mol.

Different chemical species usually have different molar absorptivity coefficients. These specific values for different chemicals at specified wavelengths of light can be found in chemical reference manuals. In case the absorptivity values are not listed or cannot be found, they can be determined experimentally by measuring the absorbance of several solutions of the chemical at known concentrations.

Determining the molar absorptivity of a chemical species can be accomplished by measuring the absorption of varying solution concentrations with a spectrometer. The spectrometer measures the total absorbance of the solution, which increases as the chemical concentration increases. Many spectrometers measure transmittance, which is the inverse of absorbance. Absorbance must be used for Beer-Lambert's Law; if transmittance is displayed, the inverse must be found first.

In a mixture of chemical species, each component contributes to the mixture's overall absorbance. The Beer-Lambert Law can be expanded for solutions with multiple components and can be expressed as A = (e₁c₁ + ... + e_nc_n)l, with the subscript n denoting the number of species present. This expanded equation applies to the absorbing species in the solution.

The molar absorption coefficient is related to the absorption cross section, σ, via Avogadro's constant, N_A. If the units of the molar absorption coefficient are taken to be L mol^-1cm^-1 and the units of the absorption cross section are in cm2, then σ = 1000ln(10) x ε/N_A, or 3.82 x 10-21 x ε. The absorption cross section is related to the probability of an absorption process in a solution.

Molar absorptivity is particularly useful in spectrometry for measuring the concentration of chemical solutions. Measuring absorbance is a very fast method of determining chemical concentrations, although the specific chemical species in the solution must be known. Other methods of measuring concentration, such as titration, can take more time and may require additional chemicals.