(3.1)--仪器分析第2章.ppt

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1、Chapter Absorption SpectrophotometrySection 1 OverviewSection 2 Basic PrinciplesSection 3 Ultraviolet-Visible SpectrophotometerSection 4 Visible SpectrophotometrySection 5 Visual colorimetrySection 6 Ultraviolet SpectrophotometryAbstractnThis chapter mainly introduces the basic principle of absorban

2、ce 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 selectiv

3、e absorption of light by molecules of the substance under testInstrumental AnalysisElectrochemical analysisChromatographic analysisOptical analysisSpectral analysis methodNon-spectral analysis methodEmission spectrometryRaman spectroscopyAbsorption spectroscopyEtc.Colorimetric methodVisible spectrop

4、hotometric 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(105:Ultra-high sensitivity;=(610)104:high sensitivity;60 nm.2.Coordin

5、ation color reactions When metal ions form coordination compounds with organic color reagents,charge transfer transition usually occurs,resulting in strong UV-Visible absorption spectrum.Section 3 Visible Spectrophotometry3.Oxidation-reduction color reaction Some elements in oxidized state like Mn(V

6、II)and Cr(VI)can be intensively absorbed in ultraviolet or visible light region,and ions can be determined by redox reaction.Take the determination of trace manganese in steel as an example.Mn2+can not be directly determined by photometric method.2 Mn2 5 S2O82-8 H2O=2 MnO4+10 SO42-16H+Mn2 should be

7、oxidized to MnO4,and then determined at 525 nm.Requirements for colour reactions*High sensitivity,generally 104*Good selectivity*Constant composition and stable properties*The color reagent has no obvious absorption at the measuring wavelength,the color difference is obvious,and the chromogenic valu

8、e is max60 nm.*The color condition is easy to control.4.Coloring agentInorganic chromogenic agent:thiocyanate,ammonium molybdate,hydrogen peroxide,etc.Organic color reagents:a wide varietyAzo color reagents:They themselves are color substances.After the formation of coordination compounds,the colors

9、 change obviously.They possess advantages of stable property,high color reaction sensitivity,good selectivity and high contrast.They are widely used.E.g.arsenazo III,PAR,etc.Triphenylmethane:chrome azurol S,xylenol orange,etc.ArsenazoTable 2-4:Several common inorganic color reagentsColoring agentThi

10、odeuterateAmmonium molybdateAmmonia waterHydrogen peroxideDetermination elementsIronmolybdenumtungstenniobiumrhenium,silicon phosphorus tungstensilicon phosphorus vanadium copper cobalt nickel titanium vanadium niobiumReaction mediumWeak acidityDiluted HNO3AcidityAmmonium hydroxide,concentratedAmmon

11、ium hydroxide,concentratedAmmonium hydroxide,concentratedComposition of colored compoundsSiliconmolybdenum bluePhosphomolybdenum bluePhosphotungstic blueSilicomolybdic acidPhosphomolybdic acidPhosphomolybdic acidRedOrangeYellowYellowYellowColorsBlueBlueBlueYellowYellowYellowBlueRedPurpleYellowReddis

12、h orangeYellowII.Selection of Color Conditions1.Dosage of chromogenic agent The relationship between absorbance A and the amount of color reagent C CR R is shown in the picture.Select the flat range of the curve2.Acidity of the reaction system Under the same experimental conditions,the absorbance of

13、 colored solutions with different pH values was measured.The pH range corresponding to the region with large absorbance and constant flatness in the curve is selected.3.Color development time and temperature Experimental determination4.Solvent Generally,water phase determination is the optimal choic

14、e.1.Selection of color reagent dosageToo few color reagents:the tested components cannot react completely,which leads to low determination results.Too many color reagents:increase reagent blank and interferenceThe appropriate amount of color reagents is determined by experiment.Take 9 volumetric fla

15、sks,add 5.00 ml 10.00 ggmLmL-1-1 iron standard solution into them respectively,and then add the same amount of other reagents1.5,2.0,2.5,3.0,3.5 Add 0,1.0,4.0,5.0 mL of color reagent respectivelyMeasure the absorbance at a given wavelengthDosage of chromogenic agent2.Selection of solution acidityIf

16、iron PH is too large,the iron ions will hydrolyze,leading to a low determination result.If iron PH is too small,o-diazofim will have very weak color-developing ability,resulting in low sensitivity.The appropriate acidity range is determined experimentally.Take 9 volumetric flasks,add 5.00 ml 10.00 g

17、gmLmL-1-1 iron standard solution into them respectively,and then add the same amount of other reagents.Add 0,1.0,4.0,5.0 mL of color reagent respectively.Measure the absorbance at a given wavelengthSolution aciditySolution acidity determination methodMake an A-pH curve and choose the appropriate pH

18、range from the curveThe relation curve between absorbance A and pH3.StabilityThrough the stability experiment,the curve of A and t is drawn.The optimum color development time is determined by selecting the flat range of absorbance.After adding o-diazo,how long will it take before the color reaction

19、is complete?This time period is called color development time.How long can the colored compounds remain and then decompose?This time period is called stability time.The determination should be conducted after color development time and before stability time.Most color reactions are carried out at th

20、e normal temperature,but some reactions must be done at a higher temperature.The solution temperature should stay the same when the working curve is drawn and the sample is tested.4.Color development temperature5.Solvent Organic solvents often reduce the dissociation of color substances and increase

21、 the dissolution of them.Thus,the determination sensitivity is improved.6.Elimination of co-existing ion interference6.Elimination of co-existing ion interference1)Add masking agent The principle of masking agent selection:masking agent does not react with the components to be tested;masking agent i

22、tself and the reaction products of masking agent and interfering components do not interfere with the determination of the components to be tested.For example,H H3 3POPO4 4 can be added as a masking agent to make FeFe3+3+(yellow)become Fe(POFe(PO)2 23-3-(colorless)when determining TiTi4 4 so as to e

23、liminate the interference of FeFe3+3+.Another example is that ascorbic acid can be added as a masking agent to reduce FeFe3+3+to FeFe2+2+when determining AlAl3+3+with chrome azurol S spectrophotometry so as to eliminate the interference of FeFe3+3+.2)Select appropriate reaction conditions3)Separate

24、interference ions1.Choose the appropriate incident light wavelength In general,the wavelength of incident light should be the lambda max.If there is interference of co-existing components at max,incident light wavelengths with less sensitivity but less interference should be considered.III.Selection

25、 of Measurement Conditions2.Control the appropriate absorbance range The relative error of concentration measurement value(c/c)is related not only T the transmittance error of the instrument,but also to the value of the transmittance reading T.The range of A should be controlled within 0.20.8.Contro

26、l methods:1)Control the concentration of the solution.A0.8,dilute the solution;A0.8,select the absorption cell with a short optical path;A0.2,select the absorption cell with a long optical path3.Select the appropriate reference solutionWhy is reference solution needed?The measured absorbance truly r

27、eflects the light absorption intensity of the solution to be tested.The selection of reference solution generally complies with the following principles:(1)If only components to be measured and color reagent reaction products have absorption at the determined wavelength while other reagents have no

28、absorption,pure solvent(water)is selected as the reference solution.(2)If the color reagent or other reagents have slight absorption at the measured wavelength,while the test solution itself does not have absorption,reagent blank(solution without sample)is selected as the reference solution.(3)If th

29、e test solution have absorption at the measured wavelength,while the color reagent does not have absorption,sample blank(without color reagent)is selected as the reference solution.(4)If the color reagent and other components in the test solution have absorption at the measured wavelength,appropriat

30、e masking agent can be added into the test solution to mask components to be tested,and then the color reagent is selected as the reference solution.Ways to improve sensitivity and selectivity of photometric determination1.Synthesize a new organic color reagent with high sensitivity2.Apply the combi

31、nation of separation and enrichment with determination3.Apply the colour system with ternary(multiplex)coordination compounds A coordination compound consisting of one central metal ion and two(or more)different ligands is called a ternary(multiplex)coordination compound.The color reaction of multip

32、lex coordination compounds has high sensitivity.On one hand,the cross section area of the multiplex coordination compound is larger than that of its corresponding binary coordination compound;on the other hand,the introduction of the second or third ligand may lead to the synergism among ligands or

33、between ligands and central metal ions,which increases the mobility and transition probability of the conjugated electron.The main types of ternary coordination compounds:ternary ion association compounds,ternary mixture coordination compounds and ternary micelle(solubility enhancement)coordination

34、compounds.1.Determination of a single componentDetermine quantitatively with A-C standard curve2.Simultaneous determination of multiple components(1)If absorption curves of each component do not overlap,it can be measured at the maximum absorption wavelength of each component.This is essentially not

35、 different from single-component determination.(2)If absorption curves of each component overlap each other,the content of each component can be obtained by solving the simultaneous equation group according to the addition of absorbance.A1=a1bca b1bcb A2=a2bca b2bcb 2.4.4 Quantitative methods2.4.4.1

36、 Analysis of single component samplesUnder the maximum absorption wavelength,appropriate reference solution is selected to measure the absorbance of the sample and results are obtained by using the law of absorption.A=KbcConcentrationWorking curve method(It is suitable for the determination of sampl

37、es in batches and the curve should be calibrated regularly.)*Prepare standard series of solutions(4 or more with different concentrations),measure the absorbance respectively,take the concentration of the standard solution as the horizontal coordinate and the absorbance as the vertical coordinate,an

38、d draw the curve on the coordinate paper,which is the working curve or the standard curve*The absorbance of the sample is measured under the same condition and the concentration is looked up in the working curve.C1.02.03.04.55.57.0A0.1000.2110.3040.4670.5580.719ConcentrationBook 90 pages 11 question

39、sm(ug)0.0016.0032.0048.0064.0080.00A0.0000.1020.2000.3040.4050.508Nickel is determined by dimethylglyoxime spectrophotometry.The standard nickel solution is made of pure nickel with concentration of 8.00 g ml-1.(1)Draw the working curve according to the following dataAdd Ni2+standard solution Volume

40、:V/mLAbsorbanceThe total color volume is 100 mL.(2)Weigh 0.6502g sample with nickel and put it into a 100mL volumetric flask after decomposition.Take 2.00mL test solution into the volumetric flask and determine it under same conditions of the standard solution.The absorbance result is 0.350.Q:What i

41、s the mass fraction of nickel in the sample?(Mni=58.70g mol-1)m(ug)0.0016.0032.0048.0064.0080.00A0.0000.1020.2000.3040.4050.508Working curveCheck the valueBook 90 pages 12 questionsm(ug)0.0010.0020.0030.0040.00A0.0000.0600.1200.1800.230The concentration of nickel standard solution is 10/g ml-1.Weigh

42、 0.00,1.00mL,2.00mL,3.00mL and 4.00mL of the solution accurately and put each of them into a 100mL volumetric flask respectively.The absorbance of each solution after dilution to the scale is 0,0.06,0.12,0.18,and 0.23.Take 0.3125g of a nickel-containing sample,put it into a 100 mL flask after dissol

43、ution and dilute to the scale.Under the same condition as the standard curve,the absorbance of the solution is 0.15.Now,lets solve the mass fraction of nickel in the sample(mg kg-1).Due to the influence of various factors,the points may not be exactly in a straight line,so the method of drawing a st

44、raight line is more random.If the least square method is used to determine the linear regression equation,it will be more accurate.The working curve can be expressed by a linear equation:y=a+bxx:concentration of the standard solution.y:the corresponding absorbance.a,b:regression coefficientsThe stra

45、ight line is called the regression line.Both a and b are obtained from the following formula respectively:The quality of the working curve can be expressed by the correlation coefficient of the regression line.The closer the correlation coefficient r is to 1,the better the linearity of the working c

46、urve is.It is usually required that r is larger than 0.999.Example 2-2 The following experimental data are obtained by the measurement of Fe+2 by phenanthroline method.Please determine the linear regression equation of the working curve and calculate the correlation coefficient.Standard solution con

47、centration c/mol L-1Absorbance AGiven linear regression equationThen,By CalculationGeneralThe linear regression equation:Correlation coefficient:It can be seen that the working curve of the experiment is in line with the requirements.Derived from the regression equationThereforeTherefore,as long as

48、under the same conditions,the sample concentration ccan be obtained by putting absorbance Acan into the above formula after measurement.Comparison method(applicable to the determination of individual samples)Ax=bcxAs=bcsConditions:Cx and Cs should be close.For example:The concentration of iron in th

49、e sample is between 10 20 ug/mL.What concentration is suitable to prepare a standard solution with the comparison method?(15.00 ug/mL)If As=0.455 and Ax=0.463,then Cx=?2.4.4.2 Quantitative Determination of Multicomponents The absorption spectrum curves do not overlap.Determinations are conducted res

50、pectively and there is no interference with each other.Do not overlapPartially overlap When the absorption spectrum curves overlap(see Fig.2-41),two wavelengths,1 and 2 are selected.Please determine the absorbance A1 and A2 at 1 and 2.According to the addition of absorbance,the following equation se