Ch18-TCF

Ch18-TCF - Quantitative Chemical Analysis Quantitative...

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Quantitative Chemical Analysis Quantitative Chemical Analysis Chapter 18: Fundamentals of Spectroscopy Fundamentals of Spectroscopy
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2 Electromagnetic radiation interacts with matter. • Frequency ( ν , nu) = waves (cycles) per second Hz (hertz) λ (lambda) is wavelength – units depend on the part of the spectrum of interest C = speed of light = 2.998 × 10 8 meters/sec 3.00 × 10 8 m/s • Ultraviolet-visible ( UV-Vis ) is usually in nanometers ( nm ) sometimes in angstroms ( Å , 1 nm = 10 Å) λ × = C in a vacuum; In any medium, λ × = C/ n where n is the refractive index of the medium n > 1, so light slows down; λ is shorter but is constant in all media
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3 The energy of a photon, E = h ν where is Planck’s constant . So, E = C λ = C ~ Where ≡1 ~ λ When λ is in cm, is in units of cm 1 , called wavenumbers ~
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4 In this section of the course we discuss … electronic / vibrational behavior of molecules • electronic spectroscopy • Fourier Transform spectroscopy – infrared spectroscopy – nuclear magnetic resonance spectroscopy • atomic spectroscopy ( exam ? ) ( exam ? )
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5 Electronic spectroscopy • The basic parts of a spectrometer: • At a specific wavelength, can quantitate the ability of a substance to absorb light • The absorbing molecule is often called the chromophore if we are detecting the analyte in the presence of other molecules • More specifically, the chromophore is the functional group in the molecule that absorbs the light
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6 P o is irradiance (formerly intensity ) • units of Joules/second·unit area • one J/s is a watt , a unit of power • so, irradiance is power/area or watts/area The ratio P = P o P = T, transmittance P o P o 100 × T = %T, percent transmittance Transmittance is not linearly proportional to either path length or concentration
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Consider, dP = −β PCdx dx is the incremental path length P is the irradiance incident on dx dP is the amount of light absorbed across dx β is a proportionality constant characteristic of the specific compound at a specific wave length of light C is the concentration of the chromophore (usually mol/L) negative sign because P decreases with increasing x So, dP P P P o = b 0 β Cdx ln P P o = β Cb 7
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8 log P P o ( ) = log T 1 ( ) = { absorbance, A P P o β Cb ln ε Cb = Beer-Lambert Law: A = ε Cb ( aka Beer’s law) ε = molar absorptivity (includes β and ln log conversion factor) previously, extinction coefficient By convention, C in mol/L, and b in centimeters Can also use the Beer’s law in the form A = a Cb where a is specific absorptivity, in grams /liter Note that absorbance is linear in path length and in concentration
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9 To measure the concentration of a substance in solution … Make a standard solution of known concentration Measure the absorbance of the standard and calculate ε from A = ε Cb Measure the absorbance of a spectrum of the solution of the unknown Use the known ε and the measured absorbance to calculate the unknown concentration from A = ε C b
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9a More about the spectrometer itself Dispersive Instruments Source:
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This note was uploaded on 03/28/2011 for the course CHEM 300L at USC.

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Ch18-TCF - Quantitative Chemical Analysis Quantitative...

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