Uv visible
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UV-Vis spectroscopy measures the absorption of ultraviolet and visible light by molecules. It can detect electronic transitions in molecules, which follow Beer's law - absorbance is proportional to concentration. UV-Vis is used to identify organic and inorganic compounds and determine concentrations. Instrumentation includes light sources, sample containers, and spectrometers to separate wavelengths and detect absorbances. Applications include titrations and measuring charge-transfer transitions between electron donors and acceptors.
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Uv visible
- 1. UV-Vis. Spectroscopy By- Saurav K. Rawat (Rawat DA Greatt) 1
- 2. Ultraviolet-Visible Spectroscopy 8-2 • Introduction to UV-Visible Absorption spectroscopy from 160 nm to 780 nm Measurement of transmittance Conversion to absorbance * A=-logT=ebc • Measurement of transmittance and absorbance • Beer’s law • Noise • Instrumentation
- 3. 8-3 Measurement • Scattering of light Refraction at interfaces Scatter in solution Large molecules Air bubbles • Normalized by comparison to reference cell Contains only solvent Measurement for transmittance is compared to results from reference cell
- 4. adn P n - ò = ò an - = 8-4 Beer’s Law • Based on absorption of light by a sample dPx/Px=dS/S dS/S=ratio of absorbance area to total area *Proportional to number of absorbing particles dS=adn * a is a constant, dn is number of particles n is total number of particles within a sample S an dP P x P P o P S P S P o x o 2.303 ln log 0 =
- 5. 8-5 Beer’s Law • Area S can be described by volume and length S=V/b (cm2) P Substitute for S o n/V = concentration Substitute concentration and collect constant into single term e anb • Beer’s law can be applied to mixtures Atot=SAx V P 2.303 log =
- 6. 8-6 Beer’s Law Limitations • Equilibrium shift pH indicators Need to consider speciation Weak acid equilibrium
- 7. 8-7 Beer’s Law Limitation • Polychromatic Light More than one wavelength
- 8. 8-8 Noise • Limited readout resolution • Dark current and electronic noise • Photon detector shot noise • Cell position uncertainty Changing samples • Flicker
- 9. 8-9 Instrumentation • Light source Deuterium and hydrogen lamps W filament lamp Xe arc lamps • Sample containers Cuvettes Plastic Glass Quartz
- 11. 8-11 Spectrometer Time separated double beam
- 12. 8-12 Spectrometer Multichannel photodiode array Dip probe
- 13. Application of UV-Visible Spectroscopy • Identification of inorganic and organic species • Widely used method 8-13 • Magnitude of molar absorptivities • Absorbing species • methods
- 14. 8-14 Molar Absorptivties • Range from 0 to 1E5 e=8.7E19PA P=transition probability A=target cross section (cm2) * Allowed transitions 0.1P1 e range 1E4 to 1E5 * Forbidden transition 0.01 • Absorbing species M+g-M* M* has a short lifetime (nanoseconds) Relaxation processes * Heat * Photo emission Fluorescence or phosphorescence
- 15. 8-15 Absorbing species • Electronic transitions p, s, and n electrons d and f electrons Charge transfer reactions p, s, and n (non-bonding) electrons
- 16. 8-16 Sigma and Pi orbitals
- 18. 8-18 Transitions s-s* UV photon required, high energy Methane at 125 nm Ethane at 135 nm • n- s* Saturated compounds with unshared e- Absorption between 150 nm to 250 nm e between 100 and 3000 L cm-1 mol-1 Shifts to shorter wavelengths with polar solvents * Minimum accessibility Halogens, N, O, S
- 19. 8-19 Transitions • n-p*, p-p* Organic compounds, wavelengths 200 to 700 nm Requires unsaturated groups n-p* low e (10 to 100) * Shorter wavelengths p-p* higher e (1000 to 10000)
- 21. 8-21 Transitions • d-d 3d and 4d 1st and 2nd transitions series Broad transitions Impacted by solution
- 22. 8-22 Transitions
- 23. 8-23 D transitions • Partially occupied d orbitals Transitions from lower to higher energy levels Splitting of levels due to spatial distribution similar Axial direction
- 24. 8-24 D transitions • Binding ligands on axis have greater effect on axial orbitals
- 25. 8-25 D transitions D value dependent upon ligand field strength Br-Cl-F-OH-C2O42-~H2OSCN- NH3enNO2-CN- D increases with increasing field strength • f-f 4f and 5f (lanthanides and actinides) Sharper transitions
- 26. 8-26 Actinide transitions 5 4 3 2 1 0 Pu6+(835 nm) Normal Heavy Light Pu4+ (489 nm) 400 500 600 700 800 Absorbance Wavelength (nm) Figure 2: UV-vis spectra of organic phases for 13M HNO3 system
- 27. 8-27 Charge-transfer Transitions • Electron donor and acceptor characteristics Absorption involves e- transitions from donor to acceptor SCN to Fe(III) *Fe(II) and neutral SCN Metal is acceptor Reduced metals can be exception
- 28. 8-28 Electronic Spectra • Cr(NH3)6 3+ d3 Weak low energy transition Spin forbidden 2 stronger transitions Spin allowed * t2g and eg transitions Lower energy to higher energy CT at higher energy Ligand to metal transition
- 29. 8-29 Charge transfer bands • High energy absorbance Energy greater than d-d transition Electron moves between orbitals * Metal to ligand * Ligand to metal Sensitive to solvent • LMCT High oxidation state metal ion Lone pair ligand donor • MLCT Low lying pi, aromatic Low oxidation state metal High d orbital energy
- 31. 8-31 Methods • Titration Change of absorbance with solution variation pH, ligand, metal • Photoacoustic effect Emission of sound
- 32. Rawat’s Creation-rwtdgreat@ gmail.com rwtdgreat@yahoo.co.uk RawatDAgreatt/LinkedIn www.slideshare.net/ RawatDAgreatt Google+/blogger/Facebook / Twitter-@RawatDAgreatt +919808050301 +919958249693