专业英语13单元.doc

专业英语13单元.doc

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1、UltravioletandvisiblemolecularspectroscopyProbablythefirstphysicalmethodusedinanalyticalchemistrywasbasedonthequalityofthecolorincoloredsolution.Thefirstthingsweobserveregardingcoloredsolutionsaretheirhue,orcolor,andthecolor’sdepth,orintensity.Theseobservat

2、ionsledtothetechniquehistoricallycalledcolorimetry;thecolorofthesolutioncouldidentifyspecies(qualitativeanalysis)whiletheintensityofthecolorcouldidentifytheconcentrationofthespeciespresent(quantitativeanalysis).Thistechniquewasthefirstuseofwhatwenowundersta

3、ndtobeabsorptionspectroscopyforchemicalanalysis'.Whenwhitelightpassesthroughasolutionandemergesasredlight,wesaythatthesolutionisred.Whathasactuallyhappenedisthatthesolutionhasallowedtheredcomponentofwhitelighttopassthrough,whereasithasabsorbedthecomplementa

4、rycolor,yellowandblue.Themoreconcentratedthesamplesolution,themoreyellowandbluelightisabsorbedandthemoreintenselyredthesolutionappearstotheeye.Foralongtime,experimentalworkmadeuseofthehumaneyeasthedetectortomeasurethehueandintensityofcolorsinsolutions.Howev

5、er,eventhebestanalystcanhavedifficultycomparingtheintensityoftwocolorswithslightlydifferenthues,andthereareofcoursepeoplewhoarecolor-blindandcannotseecertaincolors.Instrumentshavebeendevelopedtoperformthemeasurementsmoreaccuratelyandreliablythanthehumaneye.

6、Whilethehumaneyecanonlydetectvisiblelight,thischapterwillfocusonboththeultraviolet(UV)andthevisible(Vis)portionsofthespectrum.ThewavelengthrangeofUVradiationstartsattheblueendofvisiblelight(about400nm)andendsatapproximately200nmforspectrometersoperatedinair

7、.Theradiationhassufficientenergytoexcitevalenceelectronsinmanyatomsandmolecules;consequently,UVradiationisinvolvedwithelectronicexcitation.Visiblelight,consideredtobelightwithwavelengthsfrom800to400nm,actsinthesamewayasUVlight.Itisalsoconsideredpartoftheele

8、ctronicexcitationregion.Forthisreasonwefindcommercialspectroscopicinstrumentationoftenoperateswithwavelengthsbetween800and200nm.SpectrometersofthistypearecalledUV/Visible(orUV/Vis)spectrometers.Thevacu

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