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Ch 18 – Let There Be Light Ch 19 – Spectrophotometry: Instruments and Applications

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Properties of Light - Sec 18-1

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Relation between frequency and wavelength = c = wavelength (meters, cm, nm, etc) = frequency (cycles per second, Hertz, s -1 ) c = speed of light (2.997 x 10 8 m/s) Unit analysis:

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Example (p. 376) - Relating Wavelength and Frequency What is the wavelength of radiation in your microwave oven, whose frequency is 2.45 GHz?

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Light can also be thought of as a particle or PHOTON E = h E = energy (Joules) H = Planck’s constant (6.626 x 10 -34 J·s) = frequency (cycles per second, Hertz, s -1 ) And combining with = c -

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Electromagnetic Spectrum X-Rays: UV-Vis: Infrared: Microwave: Radio:

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Ground state: Excited state:

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Example (p. 377) – Photon Energies By how many joules is the energy of a molecule increased when it absorbs (a) visible light with a wavelength of 500 nm or (b) infrared radiation with a wavenumber of 1,251 cm -1 ?

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Absorption of Light – Sec 18-2 Spectrophotometer: Radiant power: Monochromator:

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Transmittance and Absorbance

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Example (p. 379) – Absorbance & Transmittance What absorbance corresponds to 99% transmittance? To 0.10% transmittance?

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Beer’s Law: absorbance is proportional to the concentration of light-absorbing molecules in the sample A = bc Beer’s Law for a mixture -

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Example (p. 381) – Using Beer’s Law The peak absorbance of 3.16 x 10 -3 M KMNO 4 at 555 nm in a 1.000 cm pathlength cell in Fig 18-5 is 6.54. (a) Find the molar absorptivity and percent transmittance of this solution. (b) What would the absorbance be if the pathlength was 0.100 cm? (c) What would the absorbance be in a 1.000 cm cell if the concentration was decreased by a factor of 4?

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Absorption Spectrum – CoCl 2 R O Y G B V

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Example (p. 382) – Finding Concentration from the Absorbance Gaseous ozone has a molar absorptivity of 2700 M -1 cm -1 at the absorption peak near 260 nm in the spectrum below. Find the concentration of ozone (mol/L) in air if a sample has an absorbance of 0.23 in a 10.0 cm cell. Air has negligible absorbance at 260 nm.

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Example (p. 383) – How Effective is Sunscreen? What fraction of ultraviolet radiation is transmitted through the sunscreen in the spectrum below at 300 nm?

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Using Beer’s Law – Sec 18-4 e.g. measuring NO 2 ¯ in aquarium water

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Analysis based on the absorbance of the colored product of this reaction-

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Absorbance Spectrum of the Colored Product Absorbance max = __________ nm

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Construction of a Calibration Curve (Standard Curve) Table 18-2 SampleAbsorbance at 543 nm in a 1.0 cm cuvette Corrected Absorbance (blank subtracted) Blank0.003 Standards 0.4575 ppm 0.9150 ppm 1.830 ppm 0.085 0.167 0.328 Unknown0.281 Unknown0.277

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Calibration Curve for Nitrite Analysis (blank subtracted)

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Example (p. 389) – Using the Standard Curve From the data from Table 18-2, find the molarity of nitrite in the aquarium.

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The Spectrophotometer – Sec 19-1

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Double-Beam Spectrophotometer

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Hitachi UV-Vis – U2000

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Sample Cuvettes

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Light Sources

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Monochromator

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Light Detectors

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A = total absorbance at wavelength 1 A = total absorbance at wavelength 2 X = molar absorptivity of molecule X at wavelength 1 X = molar absorptivity of molecule X at wavelength 2 Y = molar absorptivity of molecule Y at wavelength 1 Y = molar absorptivity of molecule Y at wavelength 2 Spectrophotometric Analysis of a Mixture – Sec 19-2

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A = A X + A Y because Beer’s Law is additive = X b [X] + Y b [Y] A″ = A″ X + A″ Y = ″ X b [X] + ″ Y b [Y]

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Spectrophotometry Ability of molecules to absorb and transmit light energy is the basis of one of the most widely used procedures for determining the concentration.

Spectrophotometry Ability of molecules to absorb and transmit light energy is the basis of one of the most widely used procedures for determining the concentration.

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