24 Introduction to Spectrochemical Methods

Transcription

1 24 Introduction to Spectrochemical Methods Spectroscopic method: based on measurement of the electromagnetic radiation produced or absorbed by analytes. electromagnetic radiation: include γ-ray, X-ray, ultraviolet(uv), visible, infrared(ir), microwave and radio-frequency(rf). 24A roperties of Electromagnetic Radiation photons or quanta 24A-1 Wave properties Fig wave nature of a beam of single-frequency electromagnetic radiation. In (a) a plane-polarized wave is shown propagating along the x-axis. In (b), only the electric field oscillations are shown. Wave Characteristics V i = νλ i V i : velocity; ν: frequency(1/period p), Hz(= S -1 ); λ i : wavelength; Wavelength units for various spectral regions (Table 24-1) Region Unit Definition X-ray Angstron unit, Å 1-1 m Ultraviolet/visible Nanometer, nm 1-9 m Infrared Micrometer, μm 1-6 m The speed of light c = νλ = cm/s = m/s ν: frequency, wavenumber, ν, (cm -1 ) no. of wave/cm = 1/λ Fig Change in wavelength as radiation passes from air into a dense glass and back to air. 148

2 Ex Calculate the wavenumber of a beam of infrared radiation with a wavelength of 5. μm. ν = 1 5. μm 1-1 = 2cm -4 cm/μm Radiant ower and Intensity power, : the energy of a beam that reaches a given area per second intensity: power-per-unit solid angle 24A-2 The article Nature of Light hotons hc E = hν = = hcν λ h: lanck s constant ( J s) Joules (J): the work done by a force of one newton (N) acting over a distance of one meter. Ex 24-2 Calculate the energy in joules of one photon of the radiation described in ex E = hcν = J s 3. 1 cm 2 cm s = B Interaction of Radiation and Matter 24B-1 The Electromagnetic Spectrum Regions of the UV/Visible and IR Spectrum (Table 24-2) Region Wavelength Range UV nm Visible nm Near-IR μm Mid-IR μm J Fig 24-3 the regions of the electromagnetic spectrum. 149

3 24B-2 Spectroscopic Measurements Emission or chemiluminescence spectroscopy Absorption spectroscopy hotoluminescence spectroscopy (fluorescence and phosphorescence spectroscopy) Fig Emission or chemiluminescence processes. In (a), the sample is excited by the application of thermal, electrical, or chemical energy. These processes do not involve radiant energy and are therefore called nonradiative processes. In the energylevel diagram (b), the dashed lines with upward-pointing arrows symbolize these nonradiative excitation process, whereas the solid lines with downward-pointing arrows indicate that the analyte loses its energy by emission of a photon. In (c), the resulting spectrum is shown as a measurement of the radiant power emitted E as a function of wavelength, λ. Fig Absorption methods. Radiation of incident radiant power, can be absorbed by the analyte, resulting in a transmitted beam of lower radiant power. For absorption to occur, the energy of the incident beam must correspond to one of the energy differences shown in (b). The resulting absorption spectrum is shown in (c). Fig hotoluminescence methods (fluorescence and phosphorescence). Fluorescence and phosphorescence result from absorption of electromagnetic radiation and then dissipation of the energy by emission of radiation (a). In (b), the absorption can cause excitation of the analyte to state 1or state 2. Once excited, the excess energy can be lost by emission of a photon (luminescence shown as solid line) or by nonradiative processes (dashed lines). The emission occurs over all angles, and the wavelengths emitted (c) correspond to energy differences between levels. The major distinction between fluorescence and phosphorescence is the time scale of emission, with fluorescence being prompt emission and phosphorescence being delayed emission. 15

4 24C Radiation Absorption 24C-1 The Absorption rocess T = A = -log T = log Absorbing solution of Concentration c Measuring Transmittance and Absorbances A = log log solvent solution BeerS Law A = log = abc A = εbc Fig.24-7 Fig Table 24-3 Important Terms and Symbols used in Absorption Measurements Term and Symbol Definition Alternative Name and Symbol Incident radiant power, o Transmitted radiant power, Radiant power in watts incident on sample Radiant power transmitted by sample 151 Incident intensity, I Transmitted intensity, I Absorbance, A log( /) Optical density, D; extinction, E Transmittance, T / Transmission T athlength of sample, Length over which l, d b attenuation occurs Absorptivity a A/bc, c(g/l) Extinction coefficient, k Molar absorptivity, ε A/bc b(cm), Molar extinction coefficient (L/cm, mol) c(mol/l) Using Beer s Law Ex A M solution of KMnO 4 has a transmittance of 44.1% when measured in a 2.1-cm cell at a wavelength of 525 nm. Calculate (a) the absorbance of this solution; (b) the molar absorptivity of KMnO 4. (a) A = -log T = -log.441 = (.3554) =.355 (b) = bc A ε.3554 = 2.1 cm = L mol cm -5-1 mol L

5 Applying Beers Law to Mixtures A total = A 1 + A A n = ε 1 bc 1 + ε 2 bc 2 + +ε n bc n 24C-2 Absorption Spectra Atomic Absorption Fig.24-1 Methods for plotting absorption spectra. The numbers for the curves indicate ppm of KMnO 4 ; b = 2. cm. Fig artial energy-level diagram for sodium, showing the transition resulting from absorption at 59, 33 and 285 nm.. Ex The energy difference between the 3p and the 3s orbitals in Figure 24-11b is 2.17 ev. Calculate the wavelength of radiation that would be absorbed in exciting the 3s electron to the 3p state (1 ev = J). λ = hc E = J s 3. 1 cm/s ev J/eV 7 nm/cm = 59 nm Molecular Absorption ground state: the lowest energy state of an atom or molecule excited state: excitation M + hν M* a chemical species absorbs thermal, electrical, or radiant energy and is promoted to a higher energy state called an excited state. Relaxation: an excited species gives up its excess energy and returns to a lower energy state. M* M + heat photochemical decomposition: by fluorescent or phosphorescent reemission of radiation. 152

6 electronic transition: transition of an electron between two orbitals vibrational transitions rotational transitions Fig Energy-level diagram shoeing some of the energy changes that occur during absorption of infrared (IR) radiation, visible (VIS), and ultraviolet (UV) radiation. E = Eelectronic + Evibrational + Erotational ΔEelectronic 1 Evibrational 1 Erotational (a) Stretching vibrations Fig Types of molecular vibrations. The plus sign indicates motion from the page to-ward the reader, the minus sign indicates motion in the opposite direction. (b) Bending vibrations Absorption of Ultraviolet and Visible Radiation Fig Typical ultraviolet absorption spectra. The compound is 1,2,4,5-tetrazine. In (a), the spectrum is shown in the gas phase where many lines due to electronic, vibrational, and rotational transitions are seen. In a nonpolar solvent (b), the electronic transitions can be observed, but the vibrational and rotational structure have been lost. In a polar solvent (c), the strong intermolecular forces have caused the electronic peaks to blend together to give only a single, smooth absorption peak. 153

7 24C-3 Limits to Beers Law 1. Real Limitations to Beers Law in relatively concentrated solution of the analyte or in concentrated electrolyte solutions 2. Chemical Deviations The absorbing species participates in a concentration-dependent equilibrium such as a dissociation or association reaction. Ex Solutions containing various concentrations of the acidic indicator HIn (Ka = ) were prepared in.1 M HCl and.1 M NaOH. In both media, a linear relationship between absorbance and concentration was observed at 43 and 57 nm. From the magnitude of the acid dissociation constant, it is apparent that, for all practical purposes, the indicator is entirely in the undissociated form (HIn) in the HCl solution and completely dissociated as In - in NaOH. The molar absorptivities at the two wavelengths were found to be ε 43 ε 57 HIn (HCl solution) In - (NaOH solution) Derive absorbance data (1.-cm cell) at the two wavelengths for unbuffered solutions with indicator conc. ranging from to M. lot the data. in a unbuffered M solution of indicator [H + ] = [In - ], [HIn] = M - [In - ] [ H3O ][In ] [In ] 5 K HIn = = = [HIn] [In ] [In - ] = [HIn] = = A 43 = ( ) + ( ) =.236 A 57 = ( ) + ( ) =.73 Table 24-4 C HIn, M [HIn] [In - ] A 43 A (Fig )

8 Fig Chemical deviations from Beer s law for unbuffered solutions of the indicator HIn. 3. Instrumental Deviations: olychromatic Radiation ε bc A = log = ε bc or = 1 bc For λ = 1, bc For λ = 1 A m + = log + = log( when ε = ε A m = log( = log( = log 1 + ) log( 1 = ε bc = ε bc + ) log[( + ) log( bc bc Fig Deviations from Beer s law with polychromatic radiation. The absorber has the indicated molar absorptivities at the two wavelengths λ and λ )(1 bc 155 bc + ) log(1 bc )] ) bc ) Fig The effect of polychromatic radiation on Beer s low. 4. Instrumental Deviations: Stray Light + A = log s s : power of the + s stray radiation Fig Deviation from Beer s low caused by various levels of stray light. 5. Mismatched Cells A = εbc + k

9 24D Emission of Electromagnetic Radiation 24D-1 Emission Spectra Line spectra: 1-2 ~ 1-3 nm, Bands for Na, K, Sr, Ca and Mg in Fig Fig Emission spectrum of a brine sample obtained with an oxyhydrogen flame. Fig Energy-level diagram for Na. Band spectra: made up of many closely spaced lines that are difficult to resolve Bands for OH, MgOH and MgO in Fig Continuum spectra: no line character and generally produced by heating solids to a high temperature. Blackbody radiation The energy peak shift to shorter wavelength with increasing temperature. Fug Blackbody radiation curves for various light sources. 24D-2 Emission by Fluorescence and hosphorescence Atomic Fluorescene: Molecular Fluorescene: Fluorescene: an analytically important emission process in which atoms or molecules are excited by the absorption of electromagnetic radiation. The excited species then relax to the ground state, giving up their excess energy as photons. 156

10 Relaxation rocesses (Fig ) a. Nonradiative relaxation 1. Vibrational deactiviation, or relaxation: collisions between excited molecules and molecules of the solvent temp. of the medium average lifetime : 1-1 ~ 1-11 s 2. Internal conversion: between the lowest vibrational level of an excited electronic state and the upper vibrational level of another electronic state average lifetime : 1-12 s b. Fluorescece emission electronically excited molecules relax to any of the several vibrational states of the ground electronic state. average lifetime : 1-5 ~ 1-1 s Fig Energy-level diagram shows some of the processes that occur during absorption, nonradiative relaxation and fluorescence by a molecular species. Resonance Lines and the Stokes Shift Resonance fluorescence has an identical wavelength to the radiation that caused the fluorescence. Stokes-shifted fluorescence is longer in wavelength than the radiation that caused the fluorescence. 157