(3.4)--仪器分析第2章.ppt
Chapter Absorption SpectrophotometrySection 1 OverviewSection 2 Basic PrinciplesSection 3 Ultraviolet-Visible SpectrophotometerSection 4 Visible SpectrophotometrySection 5 Visual colorimetrySection 6 Ultraviolet SpectrophotometryAbstractn This chapter mainly introduces the basic principle of absorbance photometric analysis,the condition of color reaction,the error of analysis,the choice of measuring condition,the structure,performance,usage and application of common spectrophotometer.Section I OverviewAnalytical chemistryAbsorption Spectrophotometry is an analytical method based on the selective absorption of light by molecules of the substance under testInstrumental AnalysisElectrochemical analysisChromatographic analysisOptical analysisSpectral analysis methodNon-spectral analysis methodEmission spectrometryRaman spectroscopyAbsorption spectroscopyEtc.Colorimetric methodVisible spectrophotometric methodUltraviolet spectrophotometric methodEtc.Chemical AnalysisChemical Analysis:Constant components(1%),Er 0.1%0.2%Based on chemical reactions,using glass instrumentsHigh accuracyInstrumental Analysis:Trace components(1%),Er 1%5%Special instruments are required based on physical or physicochemical propertiesHigh sensitivityExample:Samples containing about 0.05%Fe,weigh 0.2 g,then m(Fe)0.1 mgWeight method m(Fe2O3)0.14mg,not accurateTitration method V(K2Cr2O7)0.02mL,inaccurateThe result of spectrophotometry 0.048%0.052%,which meets the requirement.In spectroscopic analysis,the analytical method based on the selective absorption of light by matter is called absorbance photometry,which is divided into different wavelengths of the light used:Infrared absorption spectrum:molecular vibrational spectrum,absorption wavelength range 2.5 1000m,mainly used for the structure identification of organic compounds.UV absorption spectrum:electron transition spectrum,absorption wavelength range of 200 400 nm(near ultraviolet region),can be used for structural identification and quantitative analysis.Visible absorption spectrum:electron transition spectrum,absorption wavelength range of 400 750 nm,mainly used for the quantitative analysis of colored matter.This chapter focuses on UV-Visible absorption photometry.Main spectral analysis:High sensitivity:the lower limit of determination can be up to 105106mol/L-The accuracy can meet the requirements of determination of trace components:Relative error 25%-The operation is simple and quick,the selectivity is good,and the instrument equipment is simple.-It is widely used to determine most inorganic substances and organic compounds with conjugated double bonds.Characteristics of spectrophotometric methodFeatures of the method:1.High sensitivity and low detection limit of 10-6mol/L.2.High accuracy,relative error 2%5%3.Easy to operate,cheap and widely used.Ultraviolet and Visible SpectrophotometryA method based on the absorption of light radiation in the 200780 nm region by matter molecules.200 400 nm ultraviolet spectrophotometry400 780nm visible spectrophotometryVisual colorimetryVisual colorimetryMake use of natural lightComplementary color light with comparative absorptionLow accuracy(semi-quantitative)Indistinguishable multicomponentThe method is simple and sensitive.Standard seriesUnknown sampleCharacteristic Compared with titration analysis and gravimetric analysis,colorimetric analysis and visible light photometry are mainly used for the determination of trace components in samples.1.High sensitivity-Lower limit:10-5 10-6 mol/L2.High accuracy-Relative error:colorimetric method 56%,photometric method 25%3.Easy operation and fast determination4.Widespread application The analytical method based on the photochemical properties of substances is called photochemical analysis.It is divided into spectral analysis method and non-spectral analysis method.Spectral analysis is the method which measures the wavelength and intensity of the emitted,absorbed,or scattered light produced by substances under the effect of light(or other energy).1.Overview of UV-Visible SpectrophotometryHeight LowFrequencyEnergyIntra-atomic electron transitionIntra-molecular electron transition Vibrational transitionRotational transitionAutorotation of atomic nucleusRotation of electronsX-Rays Ultraviolet visibleInfraredNear Medium FarMicrowaveRadio frequency regionElectron spectrumUltravioletvisibleVibrational infraredNuclear magnetic resonance imagingShort Long Wavelength Light spectral region and energy transition correlation diagramAirwave2.Basic characteristics of light Wave particle duality Polarization Electromagnetism Interference and diffractionBasic properties of light Light is an electromagnetic wave with wave-particle duality.Table 2-1:Electromagnetic wave spectrumSpectral region nameWave length rangeWavenumberFrequencyPhoton energyTransition energy level typeNuclear energy levelInner electron levelEnergy level of atoms,valence electrons of molecules and bonding electronsMolecular vibrational levelMolecular rotational levelNuclear autorotation energy level-Raysx-RaysFar ultraviolet lightNear ultraviolet lightVisible lightNear infrared lightMedium infrared lightFar infrared lightMicrowaveRadio frequencyWave-particle duality of lightV olatilityParticle property E ERefraction of lightDiffraction of lightPolarization of LightInterference of lightPhotoelectric effectE:photon energy(J,Joule):Photon frequency(Hz,Hz):Photon wavelength(cm)c:speed of light(2.99791010 cm.s-1)h:Plank constant(6.625610-34 J.s Joule.second)Optical spectral region(vacuum ultraviolet)Far infraredMid-infraredNear infraredIt can be seenNear ultravioletFar ultraviolet10nm200nmMid-level Electronics200nm 400nmValence electrons400nm 750nmValence electrons750 nm 2.5 mMolecular Vibration2.5 m 50 mMolecular Vibration50 m 300 mMolecular rotationIt is mainly used for structural identification of organic compoundsIt can be used for structural identification and quantitative analysisIt is mainly used for quantitative analysis of color substancesMonochromatic lightComposite lightLight complementationSingle wavelength lightLight composed of light at different wavelengthsIf two kinds of monochromatic light in different colors are mixed to obtain white light at a certain intensity ratio,then the two kinds of monochromatic light are a pair of complementary color light.This phenomenon is called light complementation.BlueYellowPurplish redGreenPurpleYellow GreenGreen blueOrangeRedBlue green3.Substance colors and selective absorption of lightRedOrangeYellowYellow GreenGreenDark greenCyanIndigoBluePurplePinkish redPurplish redWhite light(sunlight):composite light consisting of a variety of monochromatic lightMonochromatic light:light at a single wavelength(consisting of photons of same energy)Visible light region:400-760 nmUltraviolet region:near ultraviolet region 200-400 nmFar ultraviolet region:10-200 nm(vacuum ultraviolet region)Wavelength/nm400430 430480 480500 500560 560590 590620 620760ColorsPurple Blue Cyan Green Yellow Orange RedThe relation between colors of The relation between colors of substances and light substances and lightComplete absorptionFull penetrationAbsorption of yellow lightSpectral indication Spectral indication Apparent phenomenon indicationComposite Composite light lightWhen white light passes through a colored solution,the solution will selectively absorb light of certain wavelengths and transmits the unabsorbed light,which is the color of the transmitted light and the color of the light complementary to its absorbed light.For example,the KMnO4 solution selectively absorbs green light(500560nm)from white light,and the purple light complementary to green light passes through the solution because it is not absorbed,so the KMnO4 solution appears purple.The color of a substance depends on the composition of the incident light and the structure of the substance itself.Why is the copper sulfate solution blue?The substance absorbs yellow light and lets blue light pass through:CuSO4 absorbs the yellow light,and thus appears blue.The substance absorbs blue light and lets yellow light passes through.Complementary light:blue light and yellow light The complexity of molecular structure makes the absorption degree of various wavelengths of light different;the absorbance varies as monochromatic light at different wavelengths irradiates-absorption curve and maximum absorption of wavelength maxComplementarity of light and colors of substancesColors of substances:Colors of substances:Substances Substances selectively absorb light at different selectively absorb light at different wavelengths.wavelengths.The absorbed light is in the The absorbed light is in the visible visible light region light region.The color of absorbed light.The color of absorbed light and that of transmitted light present a and that of transmitted light present a complementary plementary relationship.The The substance appears the color of substance appears the color of transmitted light.transmitted light.CuSO4 solution:absorb the yellow light solution:absorb the yellow light in white light and appear blue;in white light and appear blue;KMnO4 solution:absorb the green light solution:absorb the green light in white light and appear purple in white light and appear purpleIf the absorbed light is in the If the absorbed light is in the ultraviolet ultraviolet band band,the substance does,the substance does not appear any not appear any color color.What is the range of light absorbed by potassium permanganate solution?What is the range of light absorbed by copper sulfate solution?Absorption curveDefinition:A curve that indicates the absorptive capacity of a solution absorbant to light at different wavelengths is called an absorption curve,or absorption spectrum.AbsorbanceWavelength/nmFig.2-3 Light Absorption Curve of KMnO4 Solution Absorption spectrum curve or light absorption curveTake the wavelength as the horizontal coordinate and the absorbance as the vertical coordinate Maximum absorption wavelength:a wavelength at the highest level of light absorption,expressed in max In visible light,the KMnO4 solution absorbs most green lights at a wavelength of 525 nm,but its absorptive sensibility to purple and red light is weak.max=525 nm When the concentration varies,the shape of light absorption curve remains unchanged.Meanwhile,max keeps same,but absorbance becomes different.Fig.2-3 Light Absorption Curve of KMnO4 SolutionAbsorbanceWavelength/nmThe absorption curve of substances can be obtained by experiments.Here are the specific steps:（nm）500 510 515 520 525 530 535 540 550 560A 0.213 0.325 0.378 0.431 0.476 0.423 0.377 0.312 0.201 0.152（nm）.521 522 523 525 526 527 A.0.221 0.235 0.247 0.251 0.259 0.262 Absorption curveAbsorbanceWavelength/nmFig.2-3 Light Absorption Curve of KMnO4 SolutionThe absorption curve of lightDiscussion of the absorption curve:(2)Absorption curves of the same substance with different concentrations look similar and max is invariant.For different substances,the shapes of absorption curves and max are different.(Animation)(1)The absorbance of the same substance to light at different wavelengths is different.The corresponding wavelength at the maximum absorbance is called the maximum absorbance wavelength max.(4)The same substance of different concentrations has different absorbance A at certain wavelength,and the difference of absorbance A reaches the highest point at max.This property can be used as a basis for quantitative analysis of substances.(5)As the concentration changes,the largest variation range of absorbance appears at max,when the determination is most sensitive.The absorption curve is an important basis to select the wavelength of incident light in quantitative analysis.Discussion of the absorption curve:(3)The absorption curve can provide the structure information of substances and serve as a basis of qualitative analysis of substances.4.Law of Light Absorption1.Lambert-Beers law In 1729 and 1760,Bouguer and Lambert explained the relationship between the absorption degree of light and the thickness of the absorption layer.Ab In 1852 Beer proposed that there was a similar relationship between the absorption of light and the concentration of absorbents.Ab The combination of the two is called Lambert-Beers law.A parallel monochromatic beam passes through a colored solution.The absorbance of the solution is proportional to the concentration and thickness of the solution.The mathematical expression is:A lg（I0/It)=b c The light intensity of reflected light can be eliminated by the reference solution.n Transmittance(transmittance)Tn Permeability T:Describe the degree of transmittance of incident light through the solution:n T=I t/I0（T：01）n In order to show the absorption degree of the substance more directly,the negative logarithm of the transmittance T is defined as the absorbance,expressed in A.Then,absorbance A is equal to A=lg(I0/It).The greater the value A is,the greater the absorption is.What is the relationship between light absorption degree and the length of the optical path?Lets look at an animation-Lamberts Law.Lamberts law1-Light source 2-Monochromator 3-Absorption cell 4-Receiver 5-Measuring system When a beam of parallel monochromatic light irradiates vertically into a homogeneous transparent solution of a certain concentration,the relationship between the absorption degree of the incident light and the thickness of the solution is:kbb is solution liquid layer thickness,or optical path length;k is a proportional constant,which is related to incident light wavelength,solution property,concentration and temperature.This is Lamberts law.Beer lawWhat is the relationship between light absorption degree and concentration?Lets look at an animation-Beer Law.1-Light source 2-Monochromator 3-Absorption cell 4-Receiver 5-Measuring systemWhen a beam of parallel monochromatic light irradiates vertically to homogeneous transparent solutions of a substance with different concentrations but same thickness of the liquid layer,the relationship between the absorption degree of the incident light and the concentration of the solution is:kc k is another proportional constant,which is related to incident light wavelength,solution property,thickness and temperature.This is Beer law.Beer law is only suitable for a certain concentration range.(Too high or too low)1.Lambert-Beers law In 1729 and 1760,Bouguer and Lambert explained the relationship between the absorption degree of light and the thickness of the absorption layer.Ab In 1852 Beer proposed that there was a similar relationship between the absorption of light and the concentration of absorbents.Ac The combination of the two is called Lambert-Beers law.A parallel monochromatic beam passes through a colored solution.The absorbance of the solution is proportional to the concentration and thickness of the solution.The mathematical expression is:A lg（I0/It)=b c Incident light I0Transmitted lightLambert-Beers lawWh
