BEER-LAMBERT'S LAW (2).pptx

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1. BEER- LAMBERT'S LAW

2. The Beer –Lambert Law When a monochromatic light of initial intensity I o 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. The relationship between I and Io depends on the path length of the absorbing medium, l, and the concentration of the absorbing solution, c . These factors are related in the laws of Lambert and Beer

3. Lambert’s law When a ray of monochromatic light passes through an absorbing medium its intensity decreases exponentially as the length of the absorbing medium increases

4. Beer’s law : When a monochromatic light passes through an absorbing medium its intensity decreases exponentially as the concentration of the absorbing medium increases.

5. The Beer-Lambert Law, also known simply as Beer's Law, is a fundamental principle in analytical chemistry and spectroscopy that describes the relationship between the concentration of a substance in a solution and the amount of light it absorbs. This law is particularly important in the field of UV-visible spectroscopy, where it is used to quantitatively analyze the concentration of a solute in a solution based on its absorbance of light.

6. Mathematically, the Beer-Lambert Law is expressed as: A = ε * c * l Where: A is the absorbance of the solution, a dimensionless quantity that indicates how much light is absorbed by the solution. ε (epsilon) is the molar absorptivity (also called molar extinction coefficient), which is a constant specific to the substance being analyzed. It represents how strongly the substance absorbs light at a particular wavelength. c is the concentration of the substance in the solution, typically measured in molarity (moles per liter). l is the path length that the light travels through the solution, usually measured in centimeters.

7. ABSORBANCE VS CONCENTRATION

8. LIMITATIONS The limitations of Beer-lambert law are given as: Beer-Lambert law is only valid on monochromatic light This law is applicable under a low concentration range where interactions between molecules are not considered. This law is also invalid when radiations of very high intensities are used.

9. DEVIATION OF BEER-LAMBERT’S LAW When a plot of absorbance as a function of concentration at a particular path and wavelength of monochromatic is drawn, a straight line passing through the origin is obtained. But when concentration is very high, a plot of absorbance and concentration deviates from linear behavior.

10. The main causes of such deviation from Beer Lambert’s law are given as: The deviation may occur when the light of a single wavelength is not used Polymerization of solutes during the measurements Association, dissociation, or ionization of solutes causes deviation Presence of some other substance that absorbs at the same wavelength as the solute may cause deviation. The presence of some impurities in the colored compounds may cause deviation

11. FRANCK- CONDON PRINCIPLE

12. The Franck-Condon principle is a fundamental concept in molecular spectroscopy that describes the transitions of a molecule between its different electronic energy levels during a spectroscopic process, such as absorption or emission of light. This principle is named after the scientists James Franck and Edward Condon, who developed it in the early 20th century.

13. In 1925, before the development of the Schrödinger equation , Franck put forward qualitative arguments to explain the various types of intensity distributions found in vibronic transitions. His conclusions were based on the fact that an electronic transition in a molecule takes place much more rapidly than a vibrational motion of the nuclei that the instantaneous internuclear distance and the velocity of the nuclei can be considered remain unchanged during the electronic transition (later used as Born-Oppenheimer Approximation).

14. This means in the diagrams showing the potential energy curve of the two electronic states of the molecule, the transition must be represented by the vertical lines, i,e. the most probable or most intense transition will be those represented by the vertical lines.

15. Figure demonstrating the Franck principle: for r e ’ > r e ’’ (left) and r e ’ = r e ’’ (right). The vibronic transition A → B is the most probable in both cases.

16. When a molecule absorbs or emits light, it undergoes a change in its electronic energy levels. The electrons in the molecule move from one energy level to another. Since the mass of the atomic nuclei is much larger than that of the electrons, the nuclei do not move appreciably during these rapid electronic transitions. This is because the timescale for nuclear motion is typically much slower compared to the timescale of electronic transitions. IN SIMPLE TERMS

17. The Franck-Condon principle tells us that the probability of observing a particular vibrational transition depends on the overlap between the vibrational wavefunctions of the initial and final electronic states. In other words, the more similar the vibrational states are in the initial and final electronic energy levels, the more likely the transition is to occur. Overall, the Franck-Condon principle provides a useful framework for understanding and predicting the outcomes of electronic and vibrational transitions in molecules during spectroscopic processes. It's an important concept in fields such as spectroscopy, photochemistry, and molecular physics.

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