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Spectroscopic Methods of Analysis

UV-Vis spectroscopyElectronic spectroscopy

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Principle

(Absorption or emission)

Electromagnetic radiation

SampleQuanlitative

Quantitative

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Electromagnetic radiation

Wave Properties Particle Properties

Phng truyen

wavelength (cm, m, nm, A) frequency (s 1) the velocity of light cc = . = 3 x 10 10 cm/s wavenumber (cm 1) = 1/ = /c

E = h = hc/ = h.c.

The energy of a unit of radition (photon)

E (eV, kcal/mol)

h: Planck constant = 6,626.10 34J.s = 6,626.10 27 erg.s = 6,59 eV.s

Direction of propagation

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Electromagnetic spectrum

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Electromagnetic spectrum

(Gamma rays) X rays Ultraviolet Visible Infrared Radio waves

UV-VIS IR

RedOrangeYellowViolet GreenBlue

760 nm380 nm

E = h = hc/ = hc

NMR

NMR: Nuclear Magnetic Resonance

Colorimetry

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Visible lights

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Colours of Visible Light

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Principle

(Absorption or emission)

Electromagnetic radiation

SampleQuanlitative

Quantitative

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Absorption and emission

(The excited state)

(The ground state)

h

E1

E0

Ee = E1 E0

E = Ee + Ev + Er

Ee : (electron energy)Ev: (vibration energy)Er: (rotation energy)

UV-Vis spectroscopy Electronic spectroscopy

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Instrument

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Instrument

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Instrument

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Intrument

UV/VIS instrument

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Instrument

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Instrument

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Instrument

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Grating monochromator

Typical grating monochromator

polychromatic radiation

monochromatic radiation

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Source

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Detectors

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VIS spectrum

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IR spectrum

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UV/Vis Spectra for Molecules and Ions

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UV/Vis Spectra for Molecules and Ions

E

**

n

Energy

- * > n - * > - * > n - *

- * (200 300 nm)

- * (150 nm)n - * (150 200 nm)

n - * (> 300 nm)

Occupied level

E

Atommic Orbital

* Unoccupied level

- * - *n - *

n - *

Molecular orbitals

Molecular orbitals

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Chromophores

Chromophores are groups of atoms within a molecule, which absorb electromagnetic radiation.

The most important chromophores are: Conjugated double bonds, such as:

Aromatic systems, such as:

O

N

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Chromophore Notation of transition max(nm)-bonded electronsC-C and C-H * ~150

lone pair electrons-O- n * ~185-N< n * ~195-S- n * ~195>C=O n * ~300>C=O n * ~190

-bonded electrons>C=C< (isolated) * ~190>C=O * ~190

of these, mainly the >C=O absorption can be seen in a normal UV spectrum

Absorption of Simple Unconjugated Chromophores

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HOMO - highest occupied molecular orbital

LUMO - lowest unoccupied molecular orbital

Conjugated Systems Absorb at Longer Wavelength

*

*

*

2

1

A B

isolateddouble bond two conjugated

double bonds

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Conjugated Systems Absorb at Longer Wavelength

Increasing the conjugation shifts the absorption maximum (lmax) towards longer wavelengths, called a red or bathochromic shift. This has the advantage that a standard UV detector is now able to observe this absorption.

Decreasing conjugation has the opposite effect.. .a blue or hypsochromic shift.

Also the intensity of the absorption (max) increases with increasing conjugation.

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Conjugated Systems Absorb at Longer Wavelength

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Absorption

Chromophore Auxochrome Bathochromic shift, red shift hypsochromic effect, blue

shift Hyperchromic effect hypochromic effect

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Solvent Choice The choice of solvent in UV-Vis detection is

dependent on a few factors: The solvent should not absorb light in the same

wavelength region as the substance that is being analysed.

The solvent should be transparent at the wavelengths that are being used in the analysis.

The solvent should not form a complex with the analyte, subsequently disturbing the absorption spectrum.

The solvent can be used to shift the absorption wavelengths to either longer or shorter transition wavelengths.

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Solvent Lower wavelength limit (nm)

Water 205Ethanol 210Hexane 210Cyclohexane 210Methanol 210Diethyl ether 210Acetonitrile 210Tetrahydrofuran 220Dichloromethane 235Chloroform 245Carbon tetrachloride 265Benzene 280

solvents of choice - no significant interference

Solvent Choice

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Effect of Solvent

The solvent can influence the position (max) and the molar absorptivity (max) of the absorbance spectra, through changes in: pH Polarity Electrolyte concentration

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In Conclusion

Alkanes, alcohols and ethers cannot be observed in UV-Vis, as the transitions involved are * and n*

Ketones generally show weak n* transitions and are visible in the UV region

Dienes and enons show strong * absorptions and are also visible in the UV region.

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Absorbance and Concentration: Beers LawI R

I o I A I T

IO = IR + IA + IT = IA + IT

T (transmittance)

T = IT/Io or

T% = 100 x IT/Io

A (Absorbance)

A = log I0/IT = log 1/T = log 100/ T% = 2 log T%

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I R

I o I A I T

The morlar absorptivity, (L x mol -1 x cm -1)

b: pathlength

C: concentration, mol/L

A = b C

Depends on analyte, wavelength, temperature, matrix

a The analytes absorptivity, (L x g -1 x cm -1)

C: concentration, g/L or mg/L

A = a b C

Absorbance and Concentration: Beers Law

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Example

A 5.00 104 M solution of an analyte is placed in a sample cell that has a pathlength of 1.00 cm. When measured at a wavelength of 490 nm, the absorbance of the solution is found to be 0.338. What is the analytes molar absorptivity at this wavelength?

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Limitations to Beers Law

Concentration

pH or dilution

Solvent

Temperature

Time

Ligand

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Applications

Qualitative Quantitative

One componentMultiple component

Determination of Equilibrium Constants the acid-baz equilibrium constant

Stoichiometry of a Metal - Ligand Complex

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VIS spectrum

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(1) A = bc. (2) Ac = bCc, Am = bCm ; Am/ Ac = Cm/Cc Cm = Cc x Am/Ac (3) Am = bCm, Am = b(Cm + Cc); Am- Am = bCc ;

Am/ Am - Am = Cm/C Cm = Cc x Am/ Am - Am

(4) C0 C1 C2 C3 C4 C5 M0 M1A0 A1 A2 A3 A4 A5 A(M0) A(M1)A

C, mol/L

A1

C1

A2

C2

A3

C3

A5

C5

Cm

A(M1) A(M0)

A4

C4

Quantitative Analysis for a Single Analyte

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The determination of Fe in an industrial waste stream was carried out by the o-phenanthroline. Using the data shown in the following table, determine the concentration of Fe in the waste stream.

ppm Fe Absorbance0.00 0.0001.00 0.1832.00 0.3643.00 0.5464.00 0.727unknown 0.269

Example: Determination of Iron in Water and Wastewater

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Quantitative Analysis of Mixtures (Two components)

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IIIIII

IIIIII

IIIIII

IIIII

IIIIIIIII

IIIIIIIII

AAC

AAC

bCbCAAA

bCbCAAA

2112

2112

2112

2112

22222

11111

Quantitative Analysis of Mixtures (Two components)

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Quantitative Analysis of Mixtures (Two components) - Example

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Quantitative Analysis of Mixtures (Two components) Example