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Beer Lambert Law 4
Title : The Beer-Lambert Law and Its Limitation

Objective :
1. To determine the linear relationship between absorbance and concentration of an absorbing species.
2. To study the effects of molecular dissociation complex formation on the applicability of the Beer-Lambert Law.
3. To investigate the derivation and limitation of Beer-Lambert Law.

Introduction: In optics, the Beer–Lambert law, also known as Beer 's law, the Lambert–Beer law, or the Beer–Lambert–Bouguer law relates the absorption of light to the properties of the material through which the light is traveling. The general Beer-Lambert law is usually written as:
A = a() * b * c where A is the measured absorbance, a() is a wavelength-dependent absorptivity coefficient, b is the path length, and c is the analyte concentration. When working in concentration units of molarity, the Beer-Lambert law is written as:
A = * b * c where is the wavelength-dependent molar absorptivity coefficient with units of M-1 cm-1. The law states that there is a logarithmic dependence between the transmission (or transmissivity), T, of light through a substance and the product of the absorption coefficient of the substance, α, and the distance the light travels through the material (i.e., the path length), ℓ. The absorption coefficient can, in turn, be written as a product of either a molar absorptivity (extinction coefficient) of the absorber, ε, and the molar concentration c of absorbing species in the material, or an absorption cross section, σ, and the (number) density N ' of absorbers. Experimental measurements are usually made in terms of transmittance (T), which is defined as:
T = I / Io where I is the light intensity after it passes through the sample and Io is the initial light intensity. The relation between A and T is:
A = -log T = - log (I / Io).
Absorption of light by a sample

Modern absorption instruments can usually display the data as either transmittance, %-transmittance, or absorbance. An unknown



References: Anne,M. (2009). Beer Lambert. Available: http://www.chemistry.adelaide.edu.au/external/soc-rel/content/beerslaw.htm. Last accessed 2nd Feb 2014. Gwen,S. (2011). Beer Lambert. Available: http://www.medilexicon.com/medicaldictionary.php?t=48142. Last accessed 2nd Feb 2014. David N.Blauch. (2000). Beer Lambert. Available: https://www.princeton.edu/~achaney/tmve/100k/docs/Beer-Lambert_law.html. Last accessed 2nd Feb 2014.

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