Molar absorption is a measure of a chemical’s ability to absorb light at a specified wavelength. The Beer-Lambert law relates absorption to chemical concentration, path length, and molar absorption. Different chemical species have different molar absorption coefficients, which can be determined experimentally or found in reference manuals. Molar absorption is useful in spectrometry for measuring chemical concentrations.
In chemistry, molar absorption is defined as a measure of a chemical’s ability to absorb light at a specified wavelength. The molar absorption coefficient, , depends on the chemical species; actual absorption depends on chemical concentration and path length. These variables are used in the Beer-Lambert law. Molar absorption is also known as molar extinction coefficient and molar absorption coefficient.
Beer-Lambert’s law is an equation relating absorption to chemical concentration, path length, and molar absorption. Mathematically, the Beer-Lambert law can be expressed as A = εcl. The most common units for the molar absorption coefficient are M-1 cm-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 is m2/mol.
Different chemical species usually have different molar absorption coefficients. These specific values for different chemicals at specified wavelengths of light can be found in chemical reference manuals. In the event that absorbance values are not listed or cannot be found, they can be determined experimentally by measuring the absorbance of multiple solutions of the chemical at known concentrations.
The determination of the molar absorption of a chemical species can be obtained by measuring the absorption of different concentrations of solution 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. The absorbance must be used for the Beer-Lambert law; if transmittance is displayed, the inverse must first be found.
In a mixture of chemical species, each component contributes to the overall absorbance of the mixture. The Beer-Lambert law can be expanded for solutions with multiple components and can be expressed as A = (e1c1 + … + encn)l, with the subscript n denoting the number of species present. This expanded equation applies to the absorbent species in the solution.
The molar absorption coefficient is related to the absorption cross section, , via Avogadro’s constant, NA. If the units of the molar absorption coefficient are L mol-1cm-1 and the units of the absorption cross section are cm2, then σ = 1000ln(10) x ε/NA, 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 absorption is particularly useful in spectrometry for measuring the concentration of chemical solutions. Absorbance measurement is a very rapid method of determining chemical concentrations, although the specific chemical species in the solution must be known. Other concentration measurement methods, such as titration, can take longer and may require additional chemicals.
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