Satish Pradhan Dnyanasadhana College, Thane Department of Chemistry T.Y.B.Sc. Analytical Chemistry Paper-IV Sem-V UV-Visible spectroscopy

Contents 4.2 Molecular Spectroscopy – III (04 L) 4.2.1 Recapitulation of basic concepts 4.2.2 Instrumentation, Principle and working of single and double beam spectrophotometers. 4.2.3 Applications of UV- Visible Spectrophotometery

4.2.1 Recapitulation of basic concepts What is Electromagnetic Radiation Electromagnetic radiation, or light, is a form of energy whose behavior is described by the properties of both waves and particles. The optical properties of electromagnetic radiation, such as diffraction, are explained best by describing light as a wave.

Properties of electromagnetic radiation Frequency: The number of oscillations of an electromagnetic wave per second (n). Wavelength: The distance between any two consecutive maxima or minima of an electromagnetic wave (l). Wave number: The reciprocal of wavelength (–n).

The electromagnetic spectrum The electromagnetic spectrum consists of radiation that ranges in wavelength from 10-12 m (high energy) to 104 m (low energy). The physical principles and mathematical description of radiation across the whole of the electromagnetic spectrum is the same, however, it is convenient to divide it into a number of different regions depending on the origin of the waves, i.e., cosmic rays, gamma rays, x-rays, ultraviolet, visible, infrared, microwaves, and radio waves.

Energy Increases Wavelength Decreases Electromagnetic Spectrum

In this energy is supplied by photons. Sources of Energy In this energy is supplied by photons. Absorption Spectroscopy Scattering Techniques

Thermal or chemical energy source . Sources of Energy Thermal or chemical energy source . Emission Spectroscopy luminescence spectroscopy

Quantum theory of Radiation Black Body White Body

Energy for Atom and Molecule Electronic Energy Electronic Energy Atom Atom Rotational Energy Vibrational Energy

Electronic Energy of Molecule * Antibonding * Antibonding ∏ n *  * n *  * n Nonbonding  Bonding Energy  Bonding

ELECTRONIC TRANSITIONS , , and n electrons d and f electrons charge transfer electrons Organic Molecule Inorganic molecule Organic + Inorganic

Electromagnetic spectrum Radiation Source H2 and D2 lamp Wavelength region continuum source from 160–380 nm Useful for UV molecular absorption UV Spectroscopy

Electromagnetic spectrum Radiation Source Tungsten lamp Wavelength region 400-800 nm Useful for Vis molecular absorption Visible Spectroscopy

Light get reflected back Scattering of Light Transmission of light 0.003 Absorbance Light get reflected back

Regions of Absorption U.V. 200-400 nm Visible 400-800 nm

INSTRUMENT Colorimeter OR Photometer 400-800 nm Spectrophotometer 200-800 nm.

4.2.2 Instrumentation, Principle and working of single and double beam spectrophotometers.

Light Dispersing devices Instrumentation, Radiation Source Light Dispersing devices Sample Cells Detectors Readout Device

Electromagnetic spectrum Radiation Source H2 and D2 lamp Wavelength region continuum source from 160–380 nm Useful for UV molecular absorption UV Spectroscopy

Electromagnetic spectrum Radiation Source Tungsten lamp Wavelength region 400-800 nm Useful for Vis molecular absorption Visible Spectroscopy

LIGHT DISPERSING DEVICES MONOCHROMATORS SPECTROPHOTOMETERS FILTERS (COLORIMETERS ) MONOCHROMATORS SPECTROPHOTOMETERS

Filter: A filter transmits the monochromatic beam of light and absorbs other light Polychromatic light Monochromatic light

FOCUSING LENS COLLIMATING LENS PRISM ENTRANCE SLIT EXIT SLIT LAMBDA-1 LAMBDA-2 PRISM ENTRANCE SLIT EXIT SLIT PRISM MONOCHROMATOR

MAGINIFIED VIEW ENLARGED VIEW GRATING MONOCHROMATOR

Sample Cells Rectangular Cylindrical

PHOTO CELL DETECTOR Detectors:- PHOTO MULTIPLIER TUBE PHOTOTUBES

Semiconductor Selenium B PHOTOCELL DETECTOR (--) D D G C E- G B A + Iron plate A Semiconductor Selenium B Thin layer of silver C

Construction: It consists of Iron plate A on which a thin layer of a semiconductor like Selenium B is deposited. The layer is covered by very thin layer of silver C that acts as collector electrode. A ring D can hold the silver plate in its place. Working:-This cell operates without battery. When the transmitted beam of light passes through thin film of silver metal to selenium layer, electrons released from semiconductor surface .These electrons pass through a hypothetical barrier layer in between silver and selenium layer and are collected by silver electrode. Thus under the action of light a cell is formed with iron plate as positive electrode Metal ring as negative electrode. The current flow is detected in galvanometer. This current is directly proportional to absorbance.

PHOTO TUBE DETECTOR - AMPLIFIER RECORDER Collector Anode (+) Photo Cathode (-) Collector Anode (+) - AMPLIFIER RECORDER

PHOTOMULTIPLIER TUBE DETECTOR

Signal Processors The electrical signal generated by the transducer is sent to a signal processor where it is displayed in a more convenient form for the analyst. Examples of signal processors include analog or digital meters, recorders, and computers equipped with digital acquisition boards. The signal processor also may be used to calibrate the detector’s response, to amplify the signal from the detector, to remove noise by filtering, or to mathematically transform the signal.

Instruments

SINGLE BEAM SPECTROPHOTOMETER Radiation Source Collimating Lense O.2 Read out Meter Amplifier PMT Detector Sample Cuvette Grating SINGLE BEAM SPECTROPHOTOMETER

DOUBLE BEAM SPECTROPHOTOMETER PMT Detector Blank Cuvette Mirror Grating Tungsten Lamp Read Out Meter Mirror Sample Cuvette Mirror Deuterium Lamp PMT Detector DOUBLE BEAM SPECTROPHOTOMETER

Distinguish Between Photometer and Spectrophotometer Radiation source is Tungsten filament lamp. Absorbance of light is measured in the wavelength 400-800 nm.( Visible region) Filters are used to select monochromatic light. Sample cells made from glass. Detectors used are photocell or photoemmisive tube. Absorbance of coloured solution is measured SPECTROPHOTOMETER. Radiation source is Hydrogen or deuterium lamp. Absorbance of light is measured in the wavelength 200--800 nm. (U.V.& Visible region) Prism or Gratings are used to select monochromatic light. Sample cells made from quartz. Detectors used are photomultiplier tube. Absorbance of coloured as well as colorless solution can be measured

Applications of U.V. & Visible Spectroscopy Qualitative Analysis Identification of Structural Groups in Molecules. Spectroscopic analysis of a substance is carried out using radiation of a particular wavelength this wave length is called as Lambda Max. The Constituent groups in a molecule absorbed to their characteristic wavelengths. It is possible to determine a particular group in a molecule by determining its Lambda Max. Lambda Max values of important groups are given in following table.

A B S O N C E Wavelength --- 200 300 400 500 600 700 800 The Wavelength at which absorbance of highly concentrated solution is maximum is called as Lambda max. A B S O N C E Lambda Max 200 300 400 500 600 700 800 Wavelength ---

Orbital's containing electrons are called as Bonding Orbital's Types of Orbital's Bonding Orbital's : Orbital's containing electrons are called as Bonding Orbital's Antibonding Orbital's vacant or unounoccupied Orbital's ( Not containing electrons) are called as anti Bonding Orbital's .

Closed shell electrons Covalent single bonded electrons Types of Electrons in a molecule Closed shell electrons Not absorbing / Not in bonding Covalent single bonded electrons σ Sigma Electrons in π orbital's Paired nonbonding outershell electrons n electrons C---C Sigma bond σ Sigma electrons C==C C C H—O---H

The three include transitions involving ELECTRONIC TRANSITIONS There are three types of electronic transitions. The three include transitions involving 1) , , and n electrons (Organic Molecule) 2) d and f electrons (Inorganic molecule) 3) Charge transfer electrons.(Org+Inorgainc)

ELECTRONIC TRANSITIONS , , and n electrons d and f electrons charge transfer electrons Organic Molecule Inorganic molecule Organic + Inorganic

Energy levels for Electronic transitions in a Molecule * Antibonding * Antibonding ∏ n *  * n *  * n Nonbonding  Bonding Energy  Bonding

Example Functional Groups Lambda Max ( nm) Solvents Acetic Acid --COOH 208 Ethyl Alcohol Acetyl Chloride --COCl 220 Hexane Acetamide --CONH2 178 Nitromethane --NO2 201 Methyl Alcohol Azomethane --N=N-- 338 Acetaldehyde >C=O 290 Acetone 189

B) Chromopores and Auxochromes: Chromopores are unsaturated groups responsible for the absorption of visible light.e.g. >C=O (carbonyl group). The molecule which contain Chromopores are called as chromogen and the colour of the compound is due to unsaturated groups present in Chromopore. The intensity of the colour of the chromogen increases with the number of chromophores present. For ex.ethylene (CH2= CH2) is colorless but CH3-( CH= CH)6--CH3 is yellow. As the number of groups increase in a molecule absorption shifts to longer wavelength and the colour deepens.

Example Functional Groups Lambda Max ( nm) Solvents Ethylene >C=C< 171 Vapour Acetaldehyde >C=C 290 Hexane Acetic Acid --COOH 208 Ethyl Alcohol Nitromethane --NO2 201 Napthalene Unsaturated comp. 275 Methyl Vinyl Ketone 215 Cinnamic Acid

Applications of U.V. & Visible Spectroscopy Quantitative Analysis By Calibration Curve Method

Other Applications UV and visible spectroscopy can be used up to concentrations up to 10-6 and some times up to 10-7 M . Qualitative analysis can be performed because each absorbing species absorb at different wave length. It used in water and air pollution studies. It is also used to study the complexes and in the determination of complex. It is used in industry to monitor various process controls. It is also used to study to distinguish between cis-and trans isomerism. C- is isomer absorbs at shorter wave length and trans isomer absorb at longer wave length.

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