UV / visible UV / visible Spectroscopy Spectroscopy Introduction Introduction Identification of organic species Identification of organic species Quantitation of inorganic species Quantitation of inorganic species Colorimetric analysis Colorimetric analysis
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UV / visible Spectroscopy l Introduction l Identification of organic species l Quantitation of inorganic species Colorimetric analysis.
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The origin of the analytical signalThe origin of the analytical signal
Excitation of an atom or molecule by Excitation of an atom or molecule by ultraviolet or visible radiation.ultraviolet or visible radiation.
190 - 900nm190 - 900nm
UV / visible SpectroscopyUV / visible Spectroscopy The radiation which is absorbed has an energy The radiation which is absorbed has an energy
which exactly matches the energy difference which exactly matches the energy difference between the ground state and the excited state.between the ground state and the excited state.
These absorptions correspond to electronic These absorptions correspond to electronic transitions.transitions.
Electronic transitions involve the promotion Electronic transitions involve the promotion of electrons from an occupied orbital to an of electrons from an occupied orbital to an
unoccupied orbital.unoccupied orbital.
Energy differences of 125 - 650 kJ/mole.Energy differences of 125 - 650 kJ/mole.
A = Absorbance (optical density)A = Absorbance (optical density) IIOO = Intensity of light on the sample cell = Intensity of light on the sample cell
I = Intensity of light leaving the sample cellI = Intensity of light leaving the sample cell c = molar concentration of solutec = molar concentration of solute l = length of sample cell (cm)l = length of sample cell (cm) = molar absorptivity (molar extinction coefficient)= molar absorptivity (molar extinction coefficient)
The Beer-Lambert Law is rigorously obeyed The Beer-Lambert Law is rigorously obeyed when a single species is present at relatively when a single species is present at relatively
The size of the absorbing system and the The size of the absorbing system and the probability that the transition will take place probability that the transition will take place
control the absorptivity (control the absorptivity ().).
Values above 10Values above 1044 are termed high intensity are termed high intensity absorptions.absorptions.
Values below 1000 indicate low intensity Values below 1000 indicate low intensity absorptions which are forbidden transitions.absorptions which are forbidden transitions.
The characteristic energy of a transition and the The characteristic energy of a transition and the wavelength of radiation absorbed are properties wavelength of radiation absorbed are properties
of a group of atoms rather than of electrons of a group of atoms rather than of electrons themselves.themselves.
The group of atoms producing such an The group of atoms producing such an absorption is called aabsorption is called a
It is often difficult to extract a great deal It is often difficult to extract a great deal of information from a UV spectrum by of information from a UV spectrum by
itself.itself.
Generally you can only pick out Generally you can only pick out conjugated systems.conjugated systems.
As structural changes occur in a As structural changes occur in a chromophore it is difficult to predict chromophore it is difficult to predict exact energy and intensity changes.exact energy and intensity changes.
Use empirical rules.Use empirical rules.
Woodward-Fieser Rules for dienesWoodward-Fieser Rules for dienes
Woodward’s Rules for enonesWoodward’s Rules for enones