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Ch 18 – Let There Be Light

Ch 19 – Spectrophotometry: Instruments and Applications

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 108 m/s)

Unit analysis:

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 -

Electromagnetic Spectrum

X-Rays:

UV-Vis:

Infrared:

Microwave:

Radio:

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Ground state:

Excited state:

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:

Transmittance and Absorbance

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Example (p. 379) – Absorbance & Transmittance

What absorbance corresponds to 99% transmittance? To 0.10% transmittance?

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 KMNO4 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 – CoCl2

R

O

Y

G

B

V

Example (p. 382) – Finding Concentration from the Absorbance

Gaseous ozone has a molar absorptivity of 2700 M-1cm-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?

Using Beer’s Law – Sec 18-4

e.g. measuring NO2¯ in aquarium water

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

Absorbance Spectrum of the Colored Product

Absorbance max = __________ nm

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Construction of a Calibration Curve (Standard Curve)

Table 18-2

Sample Absorbance at 543 nm in a

1.0 cm cuvette

Corrected Absorbance

(blank subtracted)

Blank 0.003

Standards

0.4575 ppm0.9150 ppm1.830 ppm

0.0850.1670.328

Unknown 0.281

Unknown 0.277

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.

The Spectrophotometer – Sec 19-1

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Double-Beam Spectrophotometer Hitachi UV-Vis – U2000

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Sample Cuvettes Light Sources

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Monochromator 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 A = AX + AY 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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