Absorbance and the Beer-Lambert Law

Absorption bands in molecular electronic spectra are often broad, and may be described in terms of wavelength, A ax (nm), and the molar extinction (or absorption) coefficient, niax (dm moF cm )  

It can be seen from eq. 4.9 that absorbance is dimensionless. The absorbance is related to the concentration of the solution by the Beer-Lambert law (eq. 4.10) where e is the molar extinction (or absorption) coefficient of the dissolved sample. The extinction coefficient is a property of the compound and is independent of concentration for dilute solutions. Solution concentrations of the order of 10 moldm are typical, giving absorbance values 3. 

Since the concentration, c, is measured in mol dm and the cell path length, , in cm, the units of e are dm mol cm . V alues of e iax range from close to zero (a very weak absorption) to 10000dm moP cm (an intense absorption). 

Electronic absorption spectra may be presented as plots of absorbance against wavenumber or wavelength, or as plots of max against wavenumber or wavelength (Fig. 4.17). 

The data in Fig. 4.17 are redrawn as a plot of e ,ax against wavenumber. Sketch the appearance of this absorption spectrum, and include the scales. 

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However, from the definition of absorbance and the Beer-Lambert law ... 

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