1、Chapter 13.13-1.Absorption Spectrum 13-2.Quantitative calculations 13-3.Spectrometric instrumentation 13-4.Spectrometric error in measurementsIntroduction,particularly visible spectroscopy,is probably the most widely used analytical technique.It is based on the of the absorption of radiation(light)b
2、y molecules.Visible spectroscopy:380 nm 780 nm:200 nm 380 nmInfrared spectroscopy:780 nm 3105 nm1.Absorption SpectrumI.How does matter absorb radiation?When electromagnetic radiation(light)falls on a molecule,the molecule can be (from a ground state,)to a higher energy state(a excited state,),which
3、is called a.If the frequency of the radiation is (mu),it can raise the molecule to a state that differs in energy by an amount E,wherehE A molecule absorbs a photon(光子光子)by undergoing an energy transition exactly equal to the energy of the photon.The photon must have the right energy for this quanti
4、zed transition.1.Absorption SpectrumThere are three basic processes by which a molecule can absorb radiation:1.A molecule rotates about various axes,the energy of rotation being at definite energy levels,so the molecule may absorb radiation and be raised to a higher rotational energy level,in a.2.Th
5、e atoms or groups of atoms within a molecule vibrate relative to each other,and the energy of this vibration occurs at definite quantized levels.The molecule may then absorb a discrete amount of energy and be raised to a higher vibrational energy level,in a.1.Absorption SpectrumRotational transition
6、s can take place at very low energies(long wavelengths,that is,the microwave or far-infrared region),but vibrational transitions require higher energies in the near-infrared region,while.3.The electrons of a molecule may be raised to a higher electron energy,corresponding to an.KULeuvenAfdeling Foto
7、chemie en SpectroscopieProf.Dr.F.C.De SchryverSymmetricalstretchingAsymmetricalstretchingScissoring RockingWaggingTwisting Vibrations of methylene group(-CH2-)in a molecule for illustration：1.Absorption SpectrumAll the colors around us arise from the selective absorption and reflection of visible li
8、ght.When white light is passed through an object,the object will absorb certain of the wavelengths,leaving the unabsorbed wavelengths to be transmitted.These residual transmitted wavelengths will beseen as a color.This color is to the absorbed colors.In this color wheel,complementary colors are oppo
9、site each other.The numbers are approximate wavelength in nanometer.白光白光白光白光绿光绿光红光红光IntroductionI cannot pretend to be impartial about colors.I rejoice with the brilliant ones and am genuinely sorry for the poor browns.(30 November 1874-24 January 1965)KMnO4CuSO4KULeuvenAfdeling Fotochemie en Spectr
10、oscopieProf.Dr.F.C.De SchryverChemical structure of-carotene(-胡萝卜素).The eleven conjugated double bonds that form the chromophore(发色基团)of the molecule are highlighted in red.OHRetinol视黄醇视黄醇 -carotene-胡萝卜素胡萝卜素KULeuvenAfdeling Fotochemie en SpectroscopieProf.Dr.F.C.De SchryverGolden Rice 珀曲克斯 1.Absorpt
11、ion SpectrumII.Absorption spectrum:The extent that a matter or solution absorbs monochromatic light()。A plot of absorbance as a function of wavelength is called a an.The absorption spectra of chlorophyll a and chlorophyll b.1.Absorption Spectrum3003504004505005506006500.00.30.60.91.2 G2R1 G2R2 G2R3
12、G2R4AbsorbanceWavelength(nm)max:Qualitative analysisAbsorbance is proportional to the concentration:Quantitative analysis2.Quantitative CalculationsI.Transmittance and absorbanceItI。I0=Ia+It I0:Intensity of incident light()Ia:Intensity of absorbed light()It:Intensity of transmitted light()0tIIT 0tII
13、lglgTT1lgAThe fraction of radiant energy transmitted.2.Quantitative CalculationsII.Lambert-Beers LawThe amount of monochromatic light absorbed by a sample is described by the,commonly called.It says that the absorbance of a sample is proportional to the product of its concentration and the pathlengt
14、h(the thickness of the sample).bcAThis is not a beverage law!2.Quantitative CalculationsbcA:molar extinction coefficient(),unit:Lmol-1cm-1.It is dependent on the nature of the substance,the solvent,the wavelength of the incident light,and the temperature.c:concentration,unit:molL-1.b:pathlength,unit
15、:cm.Beers law is simple as abc.2.Quantitative CalculationsExample 13-1:A solution of Co(H2O)62+in a 1.00-cm cell was determined with a spectrometer to have an absorbance of 0.200 at 530nm.The extinction coefficient of Co(H2O)62+at this wavelength is known to be 1.00103 Lmol-1cm-1.Calculate the conce
16、ntration of Co(H2O)62+solution.bcAcm1.00cmmolL 101.000.200bAc-1-13Solution:-14Lmol102.002.Quantitative CalculationsExample 13-2:A solution containing 1.00 10-3 molL-1 Fe complex was observed to transmit 70.0%of the incident light using a 1.00-cm cell.(a)What is the absorbance of the solution at this
17、 wavelength?(b)What is the molar extinction coefficient of the Fe complex at this wavelength?(c)What fraction of light would be transmitted when a 2.00-cm cell is used?(d)What is the transmittance by a solution of Fe complex four times as concentrated using a 1.00-cm cell?Solution:(a)0.700T 0.1550.7
18、001lgT1lgA2.Quantitative Calculations(b)(c)The new cell is 2.00 cm,therefore0.3100.1552A0.310T1lg49.0%0.490T11213cmmolL101.55Lmol101.00cm1.000.155bcA(d)The new solution is four times as concentrated,therefore0.6200.1554A0.620T1lg0.240T 2.Quantitative CalculationsIII.Calculation of unknown from calib
19、ration curve1.Prepare a series of standard solutions of the analyte at known concentrations;2.Measure their absorbance using cells with the same pathlength at specific wavelength(normally max);3.Construct a calibration curve plotting absorbance versus concentrations.4.Determine the concentration of
20、the unknown from the absorbance(which is measured under the same conditions as standard solutions),using calibration curve.1 1系列标准溶液和待测溶液的配制系列标准溶液和待测溶液的配制 编号编号 空白空白 1 2 3 4 5 6 未知未知标准溶液标准溶液(ml)0.00 0.50 1.00 1.50 2.00 2.50 3.00 1.00(待测液待测液)10%磺基水杨酸磺基水杨酸(ml)5.00 5.00 5.00 5.00 5.00 5.00 5.00 5.0010%缓
21、冲液缓冲液(ml)10 10 10 10 10 10 10 10蒸馏水蒸馏水 稀释至刻度稀释至刻度0ACC样品样品A样品样品2 2标准曲线的制作标准曲线的制作2.Quantitative CalculationsExample 13-3:An iron complex that absorbs at 510 nm is determined spectrophotometrically.A series of standard solution give the following absorbance:0.100 ppm:0.081;0.200 ppm:0.171;0.500 ppm:0.4
22、32;1.00 ppm:0.857.The sample solution gives an absorbance of 0.463.Construct the calibration curve and calculate the concentration of the sample solution.Solution:0.00.40.80.00.40.8Ac2.Quantitative CalculationsIV.Mixture of absorbing speciesyyxxbcbcAThe absorbance of individual absorbing species are
23、 additive.yyxxyxbcbcAAA11111yyxxyxbcbcAAA22222We have two unknowns(cx and cy).We need to write two equations that can be solved simultaneously.2.Quantitative CalculationsExample 13-4:Metal complexes X and Y have overlapping absorption spectra.A mixture is analyzed by measuring the absorbance at wave
24、length 1 and 2 with the following results:A 1=0.533,A 2=0.590 in a 1-cm cell.The extinction coefficients of complexes X and Y at these wavelengths are shown in the right table.Calculate the concentration of X and Y in the sample solution.1 2 X 3.55103 5.64102 Y 2.96103 1.45104 2.Quantitative Calcula
25、tionsYXY,X,YXbc)(bc)(AAA11111YXY,X,YXbc)(bc)(AAA22222YXc)10(2.96c)10(3.550.53333YXc)10(1.45c)10(5.640.59042)L(mol103.60c15Y)L(mol101.20c14X Solution:3.Spectrometric InstrumentationLight sourceMonochromatorAbsorption cellDetectorReadout meteror recorderReference cell3.Spectrometric Instrumentation:Th
26、e source should have a readily detectable output of continuous radiation over the wavelength region for which the instrument is designed or operate.The most commonly employed source for the region is the,which have a useful wavelength range from about 320 nm to 3200 nm.3.Spectrometric Instrumentatio
27、n:A monochromator consists of to focus the radiation,to restrict unwanted radiation and help control the spectral purity of the radiation emitted from the monochromator,and (prism or diffraction grating)to“separate”the wavelength of the polychromatic radiation from the source.slitsmirrorprismprism3.
28、Spectrometric Instrumentation:The cell holding the sample(usually a solution)must be in the wavelength region being measured.The cells for use in visible spectrometers are usually cuvets 1 cm thick.:The detector is used to convert radiant energy into electrical energy.A )is commonly used in the visi
29、ble regions.4.Spectrometric Error in MeasurementsI.Deviation from Beers law.HA H+A-absorbstransparent1.Chemical deviations:CrO7 +H2O 2H +2CrO42-+2-2.Instrumental deviations:The basic assumption in applying Beers law is that is used.However,it is impossible to extract monochromatic radiation from a c
30、ontinuum source.Instead,a band of radiation is passed,the width of which depends on the dispersing element and the slit width.4.Spectrometric Error in MeasurementsII.Errors in measurementsBecause of the logarithmic relationship between transmittance and concentration,errors in measuring transmittanc
31、e can cause varying relative errors in the calculated concentration depending on the value of transmittances.lgTAbcATlgTT0.434cc0.95T 01.0T0.40T 0.03T 21%cc2.7%cc9.5%cc4.Spectrometric Error in MeasurementsTlgTT0.434ccThe transmittance should fall in the ()range in order to prevent large errors in sp
32、ectrometric readings.Relative concentration error as function of transmittance(T)error for 0.01.In practice:4.Spectrometric Error in MeasurementsT1lgA A10T2.0A 0.0110T2.03.0A 0.00110T3.00.005A 0.9910T0.0050.01A 0.9810T0.01Summary is based on the of the absorption of radiation(light)by substances.The
33、 quantity of light absorbed by a substance can be measured by a,which consists of a light source,a monochromator,a sample holder,and a detector.The amount of monochromic light absorbed is defined as the,where,the faction of light that passes through the sample.Absorbance of a sample can be described by:,which is the basis for using visible spectroscopy.
