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(UV Vis) Spectros

UV-Vis spectroscopy is used for qualitative and quantitative analysis of organic and inorganic compounds by identifying functional groups and their effects on absorption spectra. It works by exciting electrons in molecules to higher energy orbitals when they absorb light in the UV or visible range. A solvent must be carefully chosen that is transparent in the region being analyzed. The intensity of light passing through the sample is measured and absorbance is calculated based on the Beer-Lambert law, where absorbance is directly proportional to concentration and path length.

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246 views4 pages

(UV Vis) Spectros

UV-Vis spectroscopy is used for qualitative and quantitative analysis of organic and inorganic compounds by identifying functional groups and their effects on absorption spectra. It works by exciting electrons in molecules to higher energy orbitals when they absorb light in the UV or visible range. A solvent must be carefully chosen that is transparent in the region being analyzed. The intensity of light passing through the sample is measured and absorbance is calculated based on the Beer-Lambert law, where absorbance is directly proportional to concentration and path length.

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Garion Charles
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(UV-Vis) Spectroscopy

Friday, September 8, 2017 9:30 PM

1. What is UV-Vis Spectroscopy use for?

• Widely used (qualitative/quantitative analysis);


organic/inorganic compounds. Identify/study functional groups/substituents effects.
Comparison of samples. R&D.

• Color evaluation (colorimetric analysis): Plastics, pigments ,paints, textiles ,papers, windows, filters.
• Whiteness of samples: e.g.: paper industry (white/near white paper samples)
• petrochemicals: plastics (PVC production )
• construction: marble, limestone.
• Quantitative analysis (using Beer-Lambert law).

Electrons absorb energy (excited to a higher energy orbital).


Absorption of light: promotes electronic excitation.
Detection device: photoelectric device.
Spectra of absorbance vs. wavelength are obtained.
Ranges: Vis 40-72 kcal/mole; UV 75 — 150 Kcal/mole; (only and )

2. Choosing a Solvent

Solvent must be carefully chosen (transparent - does not absorb in region).


Usually: pure alcohols (ethanol), ethers, water, hexane, cyclohexane.
Operates with:
— Liquid samples (commonly).
— Solids (directly; sometimes dissolved ).
— Gases (some cases)
Limitations:
— Solvent/solute interactions
— Low concentrations preferable.

 Examples:

Notes Page 1
3. Two types of groups: chromophores & auxochromes.

ABSORBTION VS WAVELENGTH

Notes Page 2
4. Beer Lambert Law

The intensity of the light passing through the sample cell is also measured for that wavelength - given
the symbol, I. If I is less than Io, then the sample has absorbed some of the light (neglecting reflection of
light off the cuvetter surface). A simple bit of math is then done in the computer to convert this into
something called the absorbance of the sample - given the symbol, A. The absorbance of a transition
depends on two external assumptions.
• The absorbance is directly proportional to the concentration (c) of the solution of the the sample

Notes Page 3
• The absorbance is directly proportional to the concentration (c) of the solution of the the sample
used in the experiment.
• The absorbance is directly proportional to the length of the light path (l), which is equal to the
width of the cuvette.

Notes Page 4

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