Visible Spectroscopy, Lecture notes of Chemistry

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UV-visible Spectrocopy

• Theories on molecular excitation,

electronic transition.

• Application of uv techniques in

chemical analysis and quantitative

analyses.

SPECTROSCOPY

• Interaction of Radiation with a sample. Eg

NMR spectroscopy, IR, AAS, AES.

• The study of molecular or atomic structure

of a substance by observation of its

interaction with electromagnetic radiation

• QUANTITATIVELY - For determining the amount

of material in a sample

• QUALITATIVELY – For identifying the chemical

structure of a sample

The Electromagnetic

Spectrum

Radio waves Microwave Infra-red Visible Ultraviolet X-rays Gamma rays Wavelength Energy Frequency Sho rt Hig h Hig h Lon g Low Low

• (^) LOW ENERGY RADIATION has a LONG WAVELENGTH
• (^) Eg. Radiowaves (long wavelength, low energy)
• (^) HIGH ENERGY RADIATION has a SHORT WAVELENGTH
• (^) Eg. Gamma ray (short wavelength, high energy)

RADIATION IS TRANSMITTED IN

A WAVEFORM

FREQUENCY (Hz) WAVELENGTH (m) ENERGY ( kJ/mol) ELECTROMAGNETIC SPECTRUM EFFECT OF ENERGY ON A MOLECULE Cosmic rays γ ray s x rays Ultra violet visibl e Infrared^ Microwave^ Radio waves e- Electronic excitation 10 -12^10 -11^10 -9^10 -6^10 -3^10 - 1016 1014 1012 108 10 1018 20

x10^5

x10^7 (^12000 ) 0 15 0

2

2

S 0 Ground state Energy S 1 Vibrational levels rotational levels absorption Vibrational levels rotational levels UV-Vis IR^ mW

Effects of the energy levels

depending on the nature of

the energy received

Energy Levels

• (^) With UV-Vis spectroscopy, the UV-Vis light is passed through the sample and transmittance of light by a sample is measured.
• The amount of light transmitted with respect to the incident light is called TRANSMITTANCE (T) ie.,
• (^) transmittance often quoted as a percentage eg., SAMPLE INCIDENT LIGHT Intensity (I o ) TRANSMITTED LIGHT Intensity (I (^) t ) T = I (^) o I (^) t T = I (^) o I (^) t % X 100 UV / VISIBLE SPECTROSCOPY - THEORY

UV / VISIBLE SPECTROSCOPY - THEORY 0 2 220 380 Wavelength(nm) A B ABSORBANCE A = - log 10 T A = - log 10 A = log 10 I (^) t I o I (^) o I t plotting Absorbance vs wavelength an ABSORBANCE SPECTRUM is nerated. The absorbance spectra for the compounds A and B are own. th the advantage that absorbance measurements are usually linear with ncentration, this absorbance spectra are now used For of %T = 0 and 100 the corresponding absorbance values will be 0 and 2 respectively

UV / VISIBLE SPECTROSCOPY - THEORY

UNITS OF THE MOLAR EXTINCTION COEFFICIENT

• (^) CONCENTRATION (c) - Moles litre-
• (^) PATHLENGTH (l) - cm

A = Ecl Hence E = A

c l

E = 1 ˛

mole litre-1 x cm

E = mole-1 litre x cm -

But 1 litre = 1000cm

E = 1000 mole -1 cm3 x cm -

Hence Units of E = 1000 cm2 mole -

UV / VISIBLE SPECTROSCOPY - THEORY IMPORTANCE OF THE BEER LAMBERT LAW A = Ecl but if E and l are constant BSORBANCE  CONCENTRATION and should be linear relationship pare standards of the analyte to be quantified at known concentrations measure absorbance at a specified wavelength. pare calibration curve. m measuring absorbance of sample centration of analyte in sample be obtained from the calibration curve n be obtained from the slope of the bration curve for a given wavelength () x x x x x ABSORBANCE AT 300nm CONCENTRATION (moles litre-1^ )

16

Given the following set of data for a compound C.

What is the concentration of C if an Absorbance obtained

is 0.3321? Abs Blank sample = 0. 13800

Conc (M) Abs 0.1 0. 0.2 0. 0.3 0. 0.4 0. 0.5 0.

Quantitative application of

uv/visible spectroscopy (

Quantitative application

of uv/visible

spectroscopy

17

What is the concentration of C

when we obtain an Absorbance

of 0.3321?

The concentration is: Abs= 1.0137 *

Conc + 0.

Abs= 0.3321 – Abs blank= 0.3321-

0.13800 = 0.1941Conc= Abs – 0.1378 = 0.1941 –

0.1378 = 0.055 M 1.0137 1.