119_4_Spectrophotometry_07F

119_4_Spectrophotometry_07F - IV SPECTROPHOTOMETRY Harris...

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IV. SPECTROPHOTOMETRY Harris, Quantitative Chemical Analysis, 6th Ed. Chap. 4: 65-72; 18 (378-386); 19(402-405); 20(426-434)
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2 SPECTROPHOTOMETRY P o , I o = radiant power, incident power or intensity = energy of radiation per second reaching the detector through the blank. P, I = transmitted power or intensity = energy of radiation per second reaching the detector through the sample. T = I/I o = P/P o = Transmittance A = –log(P/P o ) = –log(I/I o ) = –logT = log(1/T) = 2 - log(%T) = absorbance A = abc = ε bM (Beer-Lambert-Bouget Law)
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3 SPECTROPHOTOMETRY Range: Transmittance: 0 < T < 1 0 < %T < 100 Absorbance: > A > 0 A = abc a = A/bc a = absorptivity, absorption coefficient, extinction coefficient = 1/(distance*concentration) A = ε bM = ε b[X] ε = A/b[X] ε = molar absorptivity =(cm*mol/L) -1 = L/cm*mol ( cm 2 /mol)
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4 SPECTROPHOTOMETRY Concentration Effects: A = abc A 2 /A 1 = a 2 b 2 c 2 /a 1 b 1 c 1 = c 2 /c 1 because a 1 = a 2 b 2 = b 1 If c 2 /c 1 = 3, then A 2 /A 1 = 3 or A 2 = 3*A 1 A = -logT or log 1/T therefore log(1/T 2 ) = 3*log(1/T 1 ) or -log T 2 = -3*log T 1 3*logT 1 = log(T 1 ) 3 T 2 = (T 1 ) 3 T 2 < T 1 Path Length effects: same as concentration effects
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5 SPECTROPHOTOMETRY Why log T vs. C? Physical model: Harris, Box 18-1 Assume a beam of photons passing through thin layer of solution: Photons (Power, Intensity) absorbed initial number Photons (P, I) absorbed concentration of molecules, M Photons (P, I)absorbed thickness of layer I = - kI[M] x dI = - kI[M]dx k[M]x I I ln dx k[M] I dI k[M]dx I dI 0 I I x 0 0 - = - = - = kx[M] T 1 ln kx[M] lnT T I I 0 = - = = εbM abc A 2.303 kx[M] T 1 log = = =
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6 Calibration Curves Harris, 4.7 – 4.9 Calibration curve Fe, ppm A{cor} 0 0 0.75 0.324 0.75 0.323 0.75 0.325 1.50 0.557 1.50 0.554 1.50 0.552 2.25 0.855 2.25 0.857
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This note was uploaded on 04/13/2008 for the course CHEM 119H taught by Professor Munson during the Fall '07 term at University of Delaware.

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119_4_Spectrophotometry_07F - IV SPECTROPHOTOMETRY Harris...

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