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INSTRUMENTAL ANALYSIS

LAB.

Mr Haydar A.M.S

BSc, MSc

Faculty of Science

Chemistry Department

Email: Haydar.kovly@gmail.com

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Infrared Spectroscopy: Analyse the functional groups

of benzoic acid

Lab 7

Aim of the experiment

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To analyze the functional groups of benzoic acid using infrared spectroscopy

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Introduction

IR spectroscopy deals with the interaction of infrared radiation with matter.

It is a light with a longer wavelength and lower frequency than visible light.

Typical IR wavelengths range from 8x10-5 cm to 1x10-2 cm, and this corresponds to energies of around 1-10 kcal.

This energy is sufficient to make atoms vibrate, but not enough to cause electronic transitions.

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IR Spectroscopy

Symmetrical stretching Antisymmetrical stretching Scissoring

Rocking Wagging Twisting

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IR Spectroscopy

Heavier atoms vibrate more slowly than lighter ones,

so a C-D bond will vibrate at a lower frequency than a C-H bond.

Thus, stronger bonds generally vibrate faster than weaker bonds.

So O-H bonds which are stronger than C-H bonds vibrate at higher frequencies.

IR Spectrum

• A plot of % transmittance (%T) vsvibrational frequency in wavenumbers (cm-1)

c

c

1

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λ = wavelength

υ = frequency

c = speed of light in a vacuum

1 wavenumber

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40004000--30003000

cmcm--11

30003000--20002000

cmcm--11

20002000--15001500

cmcm--11

15001500--10001000

cmcm--11

OO--H H

NN--HH

CC--HH

CCCC

CCNN

C=CC=C

C=OC=O

CC--OO

CC--FF

CC--ClCl

deformationsdeformations

40004000--30003000

cmcm--11

30003000--20002000

cmcm--11

20002000--15001500

cmcm--11

15001500--10001000

cmcm--11

OO--H H

NN--HH

CC--HH

CCCC

CCNN

C=CC=C

C=OC=O

CC--OO

CC--FF

CC--ClCl

deformationsdeformations

Increasing energy

Increasing frequency

IR Spectrum

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IR Spectrum

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IR Spectrum

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Instrumentation

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Sample Analysis of Solid Compounds

KBr is used mostly in IR Spectroscopy techniques

In IR spectroscopy, samples are analyzed by grinding with powdered potassium bromide and pressing into a disc.

Potassium bromide is transparent from the near UV to long-wave IR wavelength (0.25-25 µm) and has no significant optical absorption lines in its high transmission region.

It is used widely as infrared optical windows and components for general spectroscopy because of its wide spectral range.

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Sample Analysis of Benzoic acid

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Procedure

• Transfer some KBr out of the desiccator into a mortar and grind it into a very fine powder using a clean pestle.

• Add about 1 to 2 % of your sample (Benzoic acid), mix and grind it well again.

• The mixture is now ready for placement in a stainless steel die that should have already been assembled as per the following figure.

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Procedure

KBr die – Side

View

KBr die – Cross Section

ram

upper pellet

lower pellet

KBr+sample

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Procedure

• Apply 8 to 10 tons of pressure for 3 to 5 minutes. Leave the vacuum on during this step also.

• Slowly bleed pressure from the die by slowly opening the bleed screw and remove the disk from the die

• Using tweezers transfer the pressed KBr disk to the appropriate disk holder for IR analysis. Avoid contact with your fingers as the disk may pick up moisture.