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UV SPECTROSCOPY
Explain Single Beam Spectrophotometer with a neat labelled
diagram
This spectrophotometer is used to measure the relative intensity of light before and after inserting a test sample. It functions by either blanking the instrument or standardising it with respect to the reference. There are four basic single beam spectrophotometer components: โ Monochromator โ Detector โ Light source (should be stable over time, low cost, wide wavelength range brightness, and long service life) โ Sample (object) The light enters the instrument through an entrance slit, is collimated and focused on to the dispersing element, typically a diffraction grating. The light of desired wavelength is selected simply by rotating the monochromator and impinged on the sample. The intensity of the radiation transmitted through the sample is measured and converted to absorbance or transmittance Working: The light source consisting of a hollow cathode lamp emits a sharp atomic line of the element whose determination is required. The light is modulated (switched on and off) rapidly by means of a rotating chopper located between the light source and the flame.
Modulation can also be achieved by pulsing the power (switched on and off rapidly) to the light source. Modulation serves to differentiate the light coming from the source lamp from the emission from the flame. The modulated light is led to the flame where ground state atoms of the element of interest are present and after absorption is led to the monochromator which isolates the wavelength of interest which is then led to the detector.
General working of Spectrophotometer
WORKING OF THE SPECTROPHOTOMETER โ When using a Spectrophotometer, it requires being calibrated first which is done by using the standard solutions of the known concentration of the solute that has to be determined in the test solution. For this, the standard solutions are filled in the Cuvettes and placed in the Cuvette holder in the spectrophotometer that is similar to the colorimeter. โ There is a ray of light with a certain wavelength that is specific for the assay is directed towards the solution. Before reaching the solution the ray of light passes through a series of the diffraction grating, prism, and mirrors. These mirrors are used for navigation of the light in the spectrophotometer and the prism splits the beam of
absorbance of Standard and the Test solution can be measured at the same time and any no. of test solutions can be analyzed against one standard. It gives more accurate and precise results, eliminates the errors which occur due to the fluctuations in the light output and the sensitivity of the detector.
What are the different types of transitions?
Electronic transitions When molecule is getting excited by the absorption of electromagnetic radiation in UV-visible region then its electrons are promoted from ground state to excited state or from bonding orbital to anti-bonding orbital. Types of electronic transitions a) ๐ โ ๐ * Transition: transition of an electron from bonding sigma orbital (๐) to anti-bonding sigma orbital (๐ * ), is represented by ๐ โ ๐ * transition. For example, alkanes because in alkane all the atoms are held together with sigma bond.
b) ๐ โ ๐ * & ๐ โ ๐ * Transition: Transitions from non-bonding molecular (๐) orbital to anti-bonding sigma orbital or anti-bonding pi orbital (๐ * ), are represented by ๐ โ ๐ * or ๐ โ ๐ * transition respectively. These transition required less energy than ๐ โ ๐ * transition. For example, alkyl halide, aldehydes, ketones etc. c) ๐ โ ๐ * Transition: This type of transition generally show in unsaturated molecules like alkenes, alkynes, aromatics, carbonyl compounds etc. This transition required less energy as compare to ๐ โ ๐ * transition. Examples
APPLICATIONS OF UV SPECTROSCOPY
Chemical kinetics: Study of chemical reactions Ultraviolet/visible
spectroscopy can also be used to study reaction rates.
Spectrometric rate determination involves the measurement of
absorption of either the reactants or products at a fixed
wavelength, when a reaction occurs. If a reactant or reagent or
product of the reaction absorbs radiation at a particular
wavelength, the spectrophotometer can be set to measure the
absorption at that wavelength as a function of time and hence the
rate of a reaction can be measured.
For eg: Nitroethane is taken in a cuvette containing a basic
solution, and the rate of the reaction is determined by monitoring
the increase in absorbance at 240 nm
