UV - Visible SpectroscopyTypes of Transitions UV Spectroscopy σ σ∗ alkanes 150 nm σ π∗ carbonyls 170 nm π π∗ unsaturated cmpds. 180 nm n σ∗ O, N, S, halogens 190 nm n π∗ carbonyls 300 nm √ - if conjugated! √ Electronic Transitions . Chromophores: functional groups C=C. Hypsochromic shift: shift to shorter λ. also called blue shift. C=O etc.. SH . Hyperchromism: increase in ε of a band. that give electronic transitions. 4.. NH. Auxochromes: substituent's with unshared pair 's like OH. when attached to π chromophore they generally move the absorption max. 3... 6. also called red shift.Terms describing UV absorptions 1. 5. Bathochromic shift: shift to longer λ. Hypochromism: decrease in ε of a band. to longer λ. 2. . . and absorption of 490560 nm light makes it red. diametrically opposite each other.Colorimetry When white light passes through a colored substance. a characteristic portion of the mixed wavelengths is absorbed. absorption of 400430 nm light renders a substance yellow. The remaining light will then assume the complementary color to the wavelength(s) absorbed Complementary colors are . Thus. . . Colorimetry is the technique which uses perception of colour as a tool to find the quantity of a substance Copper Sulphate .Principle of colorimetric determination The colour of a solution depends on the amount of the substance dissolved. .Requirements for substances to be measured colorimetricaly 1. 2-If the substance to be analysed is colourless. the substance must be converted to a certain derivative which has a suitable chromogen( Ferrous to be converted to ferric then used as above). . organic dyes etc. 3-If there is no suitable chromogen.Substance must be coloured e.g CuSO4. it must react with certain reagent (known as chromogen) to produce equivalent coloured product (Ferric with thiocyanate). Spectrophotometer . Nessler ’s Tube Photoelectric Colorimeter Ex.Principle of colorimetric Determination In colourimetry the intensity of colour of an unknown substance is compared with the intensity of colour of same substance in standard condition (ie known) Visual Colorimeter Ex. 4mg/dm3 .8mg/dm3 0.Colorimetery (Nessler’s Tube) Unknown 4mg/dm3 2mg/dm3 0. Spectrophotometer The basic components of spectrophotometer are Source Monochromator Cell Detector Signal Processing Unit (recorder) . Block Diagram of Spectrophotometer Source Lamp Detector Signal Processor and Readout (Recorder) sample Wavelength Selector (Monochromator) . Source Thermal : W filament lamp – visible region Electric Discharge : Low pressure Hydrogen tubes – UV (alternatively deuterium lamps are used for 165 – 375nm) Usually a heat absorbing material (filter) is placed between the source and the sample ( to trap unwanted IR radiation) . Filters Absorption Filters – Absorbs unwanted radiation (Ex.e. Coloured glass) Interference Filters – Semi reflecting material ( CaF2 / MgF2 coated with Ag) . Pigmented Glass i. Monochromators 2 Essentially consists of Entrance slit (1) 1 Grating / Prism Exit slit (2) . Cell / Cuvette Should be transparent to radiations of interest Example Quartz Fused Silica . Ag and silver oxide Detector converts the radiation into current. alkali metal oxide .Detectors Either Photo tube or Photo multiplier is used commonly The cathode of the detector is coated with mixture of alkali metal . . .Recorder The signal from the detector is received by the recording system. Prism Entrance Slit Exit Slit recorder Source Monochromator Sample Detector . Determining the dissociation constants of acids and bases Determining the trace metal ion present in water (Fe) .Applications Qualitative analysis Finds application in Characterizing and identifying aromatic compounds and conjugated olefins. Elucidating the structure of several vitamins Understanding the kinetics of chemical reactions. Find out the poisoning of fruit juice (Tin in canned fruit juice) Find the adulteration in oil (Vanadium in lubricating oil) To determine the amount of isomers (keto – enol ) in a recemic mixture .Quantitative analysis Used to Determine the concentration of compounds based on Beer – Lamberts law equation using the formula A = εCl Estimate hardness in water Find the trace elements in contaminated soil. Detection of impurities The bands due to impurities are very intense. Additional peaks can be observed due to impurities in the sample and it can be compared with that of standard compound. . Study of tautomeric equilibrium The percentage of various keto and enol forms present in a tautomeric equilibrium can be determined by measuring the different wavelength. Estimation of Iron (Fe 3+) The two main steps are Step 1 : Determination of λmax Step 2 : Estimating the Amount in water . Determination of λmax A known concentration of ferric is taken To this required amount of thiocyanate is added to form a red colored [Fe (CNS) 6 ]3 – complex The solution is taken in the cuvette and absorbance of the solution at different wavelengths is noted. A graph is drawn by taking λ in x axis and Absorbance in y axis The peak gives the λmax value (480nm) . A calibration graph is drawn with the standard solutions absorbance Vs their concentration. ▬ [Fe (CNS) ] 3▬ 6 . knowing the absorbance of unknown solution . From the calibration graph .Determination of Fe A series of standard ferric solutions are treated with excess of thiocyanate and their absorptions are determined at 480 nm The unknown sample is also treated in the same way and its absorbance is also noted at 480 nm. the concentration can be found out. The reaction may be represented as H+ Fe 3+ + 6 (CNS) – ▬► [Fe (CNS) 6 ]3 – . When thiocyanate is used in excess. the only complex that exist will be [Fe (CNS) 6 ]3 – This complex is stable . red in colour and has a λmax of 480 . Iron forms complex with thiocyanate in acidic medium A series of complexes are possible with the general formula [Fe (CNS) n ]3 – n where n = 1 – 6 . . Calibration Graph .