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The SI unit of molar absorption coefficient is the square metre per mole (m 2 /mol), but in practice, quantities are usually expressed in terms of M −1 ⋅cm −1 or L⋅mol −1 ⋅cm −1 (the latter two units are both equal to 0.1 m 2 /mol).
The absorption coefficient can, in turn, be written as a product of either a molar absorptivity of the absorber, ε, and the concentration c of absorbing species in the material, or an absorption cross section, σ, and the (number) density N of absorbers. (see Beer Lambert Law link for full derivation)
absorption coefficient is essentially (but not quite always) synonymous with attenuation coefficient; see attenuation coefficient for details; molar absorption coefficient or molar extinction coefficient , also called molar absorptivity , is the attenuation coefficient divided by molarity (and usually multiplied by ln(10), i.e., decadic); see ...
ε is the molar attenuation coefficient of that material, and; c(z) is the molar concentration of that material at z. If c(z) is uniform along the path, the relation becomes =. The use of the term "molar absorptivity" for molar attenuation coefficient is discouraged. [1]
One can also use the molar attenuation coefficients = , where N A is the Avogadro constant, to describe the attenuation coefficient in a way independent of the amount concentrations = of the attenuating species of the material sample: = (= ()) = (= ()) = (= ()).
Before testing a sample, the Spectronic 20 is calibrated using a blank solution, which is the pure solvent that is used in the experimental sample. It is typically water or an organic solvent. In this calibration, the transmittance is set at 100% using the calibration knob of the instrument (the amplifier control knob in the figure at right).
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n i is the number density of the attenuating species i in the material sample; ε i is the molar attenuation coefficient of the attenuating species i in the material sample; c i is the amount concentration of the attenuating species i in the material sample, by definition of attenuation cross section and molar attenuation coefficient.