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Lecture

14spectrophotometry1.pdf

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Department
Chemistry
Course
ENCH 213
Professor
Diane Beauchemin
Semester
Fall

Description
Fundamentals of spectrophotometry • Spectro = spectrum • photo = photon • metry = measurement = display of radiation • intensity (emitted, absorbed or scattered) vs photon energy Properties of light • light = electromagnetic radiation = perpendicular oscillating electric and magnetic fields • wavenumber = # waves/cm = 1/λ= v • λ = wavelength • frequency, v ◦ # oscillations/s ◦ 1 oscillation/s = 1 Hz ◦ determined by the radiation source ◦ constant regardless of medium traversed Relation between frequency and wavelength • vi= velocity of propagation ◦ dependent on both frequency and the medium traversed • λi = v i • in vacuum (or air at 3 sig.fig.), v = i = speed of light = 3.00x10 cm/s 10 • if not in vacuum, v = c/n i ◦ where n=refractive index (>1) • as v= constant for a given light source • λ changes when light traverses medium vacuum Photons • Since light interacts with matter, it can be treated as packets of energy called photons • a photon carries energy, E = hv ◦ h= Planck’s constant = 6.63x10 -34J·s ◦ E = hc/λ = hcv ◦ longer λ involves less energy Light with a wavelength of 800 nm has an energy of a) 2.48 x 10 -19J. b) 2.48 x 10 -28J. -31 c) 5.30 x 10 J. Light with a frequency of 1.07×10 Hz has a wavenumber (in cm ) of -1 6 -1 a) 3.57 x 10 c-7. -1 b) 2.80 x 10 cm . c) 3.57 x 10 cm . -1 E = hcv 15 10 4 -1 v = v / c = 1.07x10 Hz / 3.00x10 m/s = 3.57 x 10 cm Molecular absorption • molecular analyte can absorb light of specific wavelengths • chromophore = part of the molecule responsible for absorption • if visible light is absorbed ◦ substance will be colored ◦ color = complementary color of light absorbed Absorption of visible light A liquid sample absorbs light of wavelength 510 nm. The solution will appear a) Green. b) Purple. c) colorless. Closer look at molecular absorption Which of the following types of transition would require the shortest wavelength radiation? a) Rotational transitions b) Vibrational transitions c) Electronic transitions (shortest wavelength = most energy) Result of molecular absorption • many possible transitions ◦ band spectrum ◦ envelope of multitude of possible transitions between vibrational-rotational levels in 2 electronic states Electromagnetic radiation in the range from 200 to 800 nm causes a) electronic, vibrational and rotational excitation. b) vibrational and rotational excitation. c) rotational excitation. Absorption process • P= power = energy of radiation beam that reaches a given area per second • Intensity = power per unit solid angle • Transmittance = T = P/P = 0raction of incident light transmitted by the solution ◦ 0 ≤ T ≤ 1 or 0% ≤ T ≤ 100% Beer’s law • absorbanceA= log (P /P10= 0logT = εbc ◦ b = path length ◦ ε = constant=molar absorptivity (L cm-1mol-1) ◦ c = analyte concentration (M) • strictly,A λ ε bcλ ◦ bothAand ε depend on λ ◦ if 10% T then –log(0.1) = 1 =A ◦ if 1% T then A= 2 If a sample transmits 75 % of the incident light, it has an absorbance of a) 0.125. b) 1.88. A= -log(0.75) = 0.125 c) -0.125. Asample in a 1.0 mm cell transmits 75.0% of the incident light at 510 nm. If the solution is 0.075 M, -1 -1 its molar absorptivity (M cm ) is a) 1.0 x l0-4 M cm . -1 E = -log0.75 / 0.10cmx0.075 = 17 b) 1.67 M cm . -1 -1 -1 c) 17 M cm . Experimental transmittances and absorbances • TrueAand T cannot be measured because the soluti
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