Chem. 31 – 10/30 Lecture - PowerPoint Presentation

1. Chem. 31 – 10/30 Lecture

2. Announcements IAA Lab – Due todayQuiz 4On WednesdayOn Acids part of Ch. 6/Chapter 7 materials/Chapter 18 materials (extractions)Today’s LectureChapter 18 – SpectroscopyBeer’s LawSpectrometers

3. Announcements IIToday’s Lecture – cont.Chapter 23 – ChromatographyOverviewPartitioning (in extractions and chromatography)ChromatographsTransport (if time)

4. SpectroscopyBeer’s LawLight intensity in = PoLight intensity out = PTransmittance = T = P/PoAbsorbance = A = -logTLight sourceAbsorbance used because it is proportional to concentrationA = εbCWhere ε = molar absorptivity and b = path length (usually in cm) and C = concentration (M)bε = constant for given compound at specific λ valuesample in cuvette

5. SpectroscopyBeer’s Law QuestionHalf of the 284 nm light is absorbed when benzoic acid at a concentration of 0.0080 M is in a cuvette with a path length of 0.50 cm. What is the molar absorptivity of benzoic acid at this wavelength?

6. SpectroscopyMore on Beer’s LawUseful for determination of analyte concentrationsSome limitationsLaw not valid for high concentrationsDeviations to law appear to occur when multiple wavelengths of light used or when multiple species exist but absorb light differentlyUncertainties are lowest when 0.1 < A < 1Example of deviations to Beer’s Law: Unbuffered Indicator with ε(In-) = 300 M-1 cm-1, ε(HIn) = 20 M-1 cm-1; pKa = 4.0HIn ↔ H+ + In-

7. SpectroscopySpectrometerslight sourcesample in cuvettelight discriminator: monochromator (passes only a small range of wavelengths)light detector – measures light intensity by converting it to an electrical signalData processorComponents can look very different in different types of spectrometers, but spectrometers will have all of the major components (except other methods of wavelength discrimination may replace monochromators)

8. SpectroscopyExample Measurement: OzoneOzone (O3) is a pollutant (lower atmosphere) and in stratosphere provides UV protectionInstrument is used for measurement at station or in airplanecompares absorbance through sample cell vs.absorbance through reference cellCan also make measurements remotely (e.g. absorbance between two skyscrapers)light source (l = 254 nm)light detectorair inO3 scrubbersample cellreference cellchopper

9. Chromatography – Ch. 23IntroductionPurpose of ChromatographyTo separate and detect components of a mixtureAnalytical chemists are more interested in the detection partAdvantages of ChromatographyCan handle more complex samples than typical spectroscopic methodsAlso results in purification of mixtures (if desired)Disadvantage of ChromatographySeparation takes time (so generally not as fast as pure spectroscopic methods)Basis for Separation:differential partitioning between a stationary and a mobile phase

10. ChromatographyPartitioning – Ch. 23 Sect. 1Covering to understand partitioning in chromatographyPartitioning can occur between any two phases (as long as one phase is a fluid)Liquid-liquid is chosen as an examplePartitioning governed by equilibrium equationX(org)X(aq)K = partition coefficient (a constant)note: technically, upper conc. is for “raffinate” phase while lower is for “extractant” phase

11. ChromatographyPartitioningPartitioning coefficient depends on stability in solvents (related to solubility in solvents)Most common rule is likes dissolve likesExample of water – hexane partitioningOther Effects on PartitioningK gives distribution if compound does not react further in either phaseHowever, compounds may react further (e.g. acid HA → H+ + A- in aqueous phase)Ions (e.g. A-) will be found almost exclusively in aqueous phaseDistribution coefficient (D) gives ratio of total species concentration (only covering qualitatively)larger KAlthough both have K > 1, the OH group makes right molecule more polar (favors water more vs left)

12. ChromatographyPartitioningExample of Effect of Aqueous Reactions on Compound DistributionCompound A is nearly as polar as BHowever, acidity affects distribution between water and organic layerCompound B will undergo dissociation in water:HA ↔ H+ + A-Distribution of B given by:D = [HA]org/{[HA] + [A-]}aqCompound ACompound BK = 7.59K = 6.17pKa = 4.62Not very acidicIf aqueous phase is buffered at pH > pKa (e.g. pH = 6), most of B will be in anion form and very little of B will be in organic phaseWith a low pH buffer, D ~ K

13. ChromatographyPartitioning - QuestionsA compound with an octanol water partition coefficient of 52 is placed in a separatory funnel with water and octanol and shaken. The concentration of it in octanol is found to be 0.150 M. What is its concentration in water?It is desired to separate the following two compounds: CH3(CH2)3OH and CH3(CH2)3NH2. The two compounds have similar KOW values (around 11) but the second compound is basic. What can be done to separate the two?3. It is desired to transfer butanol (left compound in #2) from water to an organic phase. Would it be transferred most efficiently using 1-octanol, a less polar solvent (e.g. octane), or a more polar solvent (e.g. 1-hexanol) as the organic solvent?

14. ChromatographyPartitioning in ChromatographySeparation Occurs in ColumnPartitioning Requires Two Phases:Mobile phasefluid flowing through the columntype of fluid determines type of chromatographyfluid = gas means gas chromatography (GC)liquid chromatography (high performance liquid chromatography or HPLC)supercritical fluid (SFC) [supercritical fluid = fluid at high temperature and pressure with properties intermediate between liquid and gas]Stationary phase (solid or liquid within column)most commonly liquid-like substance on solid support

15. Chromatography More on Stationary PhasesOpen Tubular – in GC(end on, cross section view)Column WallMobile phaseStationary phase (wall coating)Packed column (side view) (e.g. Silica in normal phase HPLC)Packing Material (solid)Stationary phase is surface (larger area than shown because its porous)Bonded phase (liquid-like)Expanded ViewStationary PhaseChemically bonded to packing material Packing Material

16. ChromatographyOverview – The Good, the Bad, and the UglyThe Good: Differential partitioning of solute between a mobile and a stationary phasesThe Bad: Band broadeningThe Ugly: Non-ideal peak shapes (we will see this in the GC lab)More realistic pictureConcentration profile

17. Chromatography EquipmentChromatograph = instrumentChromatogram = detection vs. time (vol.) plotChromatograph ComponentsMobile Phase ReservoirFlow/Pressure ControlSample InInjectorChromatographic ColumnDetectorWaste or fraction collectionSignal to data recorderChromatogram

18. Chromatography Flow – Volume RelationRelationship between volume (used with gravity columns) and time (most common with more advanced instruments):V = t·uVV = volume passing through column part in time t at flow rate uVAlso, VR = tR·uV where R refers to retention time/volume (time it takes component to go through column or volume of solvent needed to elute compound)Can also use linear velocity (ux)ux = L/tR where L = column length

19. Chromatography More on VolumeHold-up volume = VM = volume occupied by mobile phase in columnStationary phase volume = VSCalculation of VM:VM = tM·uV, where tM = time needed for unretained compounds to elute from columnUnretained compound = compound 100% in mobile phase