The Beer –Lambert Law

When a monochromatic light of initial intensity Io passes through a solution in a transparent vessel, some of the light is absorbed so that the intensity of the transmitted light I is less than Io .There is some loss of light intensity from scattering by particles in the solution and reflection at the interfaces, but mainly from absorption by the solution.

Therelationship between I and Io depends on the path length of theabsorbing medium, l, and the concentration of the absorbing solution,c. These factors are related in the laws of Lambert and Beer

Lambert’s law

When a ray of monochromatic light passes throughan absorbing medium its intensity decreases exponentially as the length of the absorbing medium increases.

Beer’s law :

When a monochromatic light passes through an absorbing medium its intensity decreases exponentially as the concentration of the absorbing medium increases.

Transmittance:

The ratio of intensities is known as the transmittance (T) and this is usually expressed as percentage

Extinction

If logarithms are taken of the equation instead of a ratio then

The expression log10 Io/I is known as the extinction (E) or absorbance(A). The extinction is some times referred as optical density.

Therefore

A (or) E = k cl

where k is molar extinction co-efficient for the absorbing material atwave length l, c = molar concentration of the absorbing solution,l = path length in the absorbing material in cm.

If the Beer- Lambert law is obeyed correctly and l is kept constant, then a plot of extinction against concentration gives a straight line passing through the origin

Extinction, E=log10 Io/I

Extinction

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PAPER CHROMATOGRAPHY and BEER’S LAW

THEORY

PAPER CHROMATOGRAPHY

Chromatography is a method for analyzing complex mixtures (such as ink) by separating them into the chemicals from which they are made. Chromatography is used to separate and identify all sorts of substances. Drugs from narcotics to aspirin can be identified in urine and blood samples, often with the aid of chromatography. The components to be separated are distributed between two phases: a stationary phase bed and a mobile phase which percolates through the stationary bed.

Paper chromatography is one method for testing the purity of compounds and identifying substances. Paper chromatography is a useful technique because it is relatively quick and requires small quantities of material. Separations in paper chromatography involve the same principles as those in thin layer chromatography. In paper chromatography, like thin layer chromatography, substances are distributed between a stationary phase and a mobile phase. The stationary phase is usually a piece of high quality filter paper. The mobile phase is a developing solution that travels up the stationary phase, carrying the samples with it. Components of the sample will separate on the stationary phase according to how strongly they adsorb to the stationary phase versus how much they dissolve in the mobile phase.The R f value for each spot should be calculated. R f stands for "ratio of fronts" and is characteristic for any given compound. Hence, known R f values can be compared to those of unknown substances to aid in their identifications.

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Spectrum of Electromagnetic Radiation

Photon Energy Units

Spectroscopic Experimental Concept

Spectral Bandwidth

The Spectrometer

Dispersive Spectrometers

Non-dispersive Spectrometers

Spectral Resolution

Measuring a Spectrum

Spectrometric Experiments

Scattering Experiments

Emission Experiments

Absorption Experiments

Absorbance or Transmittance

Beer’s Law

Limitation’s to Beer’s Law

Intermolecular Interactions

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One useful application of Beer’s Law is to determine the equilibrium constant of a reaction. Reactants (iron (III) and thiocyanate) are mixed in a ratio such that one reactant is presumed to be nearly 100% converted into a colored product. The product is assumed to follow Beer’s Law, and the relationship between relative absorbance and concentration is determined from the first experiment. After that, the reactant originally in excess is reduced in concentration. From the colorimetric determination of product concentration, the concentrations of reactant remaining at equilibrium is determined and the quantitative relationships among these concentrations are studied. One of these relationships, the mass action expression, is found to be nearly constant.

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