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

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

Properties of Light - Sec 18-1

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:

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

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:

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 ?

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

Transmittance and Absorbance

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 -

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?

Absorption Spectrum – CoCl 2 R O Y G B V

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.

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 NO 2 ¯ in aquarium water

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

Absorbance Spectrum of the Colored Product Absorbance max = __________ nm

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

Calibration Curve for Nitrite Analysis (blank subtracted)

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

Double-Beam Spectrophotometer

Hitachi UV-Vis – U2000

Sample Cuvettes

Light Sources

Monochromator

Light Detectors

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 = 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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