Chapter 14
Applications of Ultraviolet-Visible
Molecular Absorption Spectrometry
Key Topics
• Absorbing Species.
-Organics, Inorganics, Charge Transfers
• Qualitative Applications of UV-Vis
Spectroscopy.
– Solvents, Slit width, Detection
• Standard Addition Method.
Absorbing Species
• Absorption of UV-Vis radiation results in
excitation of bonding electrons.
– Aids in I.D of functional groups of molecules.
• Absorption by molecules occurs in electronic
absorption bands.
– Lines come from the transition of an electron from
ground state to a vibrational/rotational energy
state.
Sample States on UV-Vis
Organics
• All organic compounds can absorb Electro.Radiation thanks to valence electrons.
• Usually excitation promotes nonbonding
electrons (n) into σ*or π*.
• Chromophores are molecules that contain
unsaturated functional groups capable of
absorption. (n to π* or π to π*)
– This provides a rough I.D. of compounds (complex
spectra).
• Saturated functional groups can also be detected.
Energy Levels
Organic Spectra
Inorganics
• Inorganic anions also have absorption bands
from excited nonbonding electrons.
• Generally Ions and element complexes in the
first two transitions absorb bands of visible
light in an oxidation state and are usually
colored.
– d-orbitals typically, f-orbitals in lanthanide ions.
Charge-Transfer
• Based on a complex consisting of an electron
donor group bonded to an electron acceptor.
– Complex absorbs radiation and an electron from
the donor is transferred to an orbital that belongs
to the acceptor.
• In complexes involving a metal, the metal is
usually the proton acceptor.
Charge-Transfer Spectra
Qualitative Applications of UV-Vis
Spectroscopy
• Spectrophotometric measurements are great
at chromophoric group detection.
– Spectral comparison yields general conclusions
• UV-Vis spectra do not have enough detailed
structure to define identity of a compound
definitively.
– Usually paired with other techniques (mass spec,
IR, etc.)
Solvents
• Analyte is usually prepared in a diluted form.
• Gas-phase spectra are the most detailed.
– For volatile compounds.
• Transparency of a solvent is important:
– Can affect the absorbing system
– Polar solvents remove detailed graphical structure
Slit Width
• Slit widths should be at a minimum for
measurements.
– Peak heights and separation become distorted
with wider bandwidths.
Detection
• Absorption bands at specific wavelengths yield
clues as to the I.D. of a functional group.
• Examples include:
– Chromophores‘
– Aromatics
– Organic functional groups
• Some requiring slight solvent “tweaking” (pH, temp.,
concentration, etc.)
Standard Addition Method
• Used in the pursuit to find the relationship
between absorbance vs. concentration.
– Counters matrix affects .
• Involves adding or sampling one or more
increments of standard solution to sample
aliquots.
– Each sample is then diluted to a known volume.
• Discussed in excruciating detail in chapter 1D3.
•
Titration Curves
Titration curves are a function of absorbance vs. volume of titrant added.
a) Titration of non-absorbing analyte w/ absorbing titrant to form a non-absorbing
product.
b) Formation of absorbing product from non-absorbing reactants.
c) Absorbing analyte reacts w/ non-absorbing titrant to form non-absorbing products.
d) Absorbing analyte + titrant react to form non-absorbing product.
e) Absorbing analyte reacts with non-absorbing titrant to form absorbing product.
f) Absorbing titrant reacts with non-absorbing analyte to form absorbing product.
Instrumentation
• Ordinarily performed with a
spectrophotometer/photometer that has
been modified so that the titration sample is
not removed from the light path.
• The power of the radiation source as well as
the response of the transducer must remain
constant during a photometric titration
• The sample must not move so that the light
path remains constant.
Applications of Photometric Titrations
• Photometric titrations can provide more accurate
results than a direct photometric analysis of sample
– This is due to the data from several measurements
determines titration end point
• Advantages
– Experimental data for determining end point is collected
far from equivalence-point region where change in
absorbance value is slow.
• Therefore equilibrium constants do not need to be as large as that
required by titrations involving observations as to where end point
is reached.
• More dilute solutions can be used as well.
Applications of Photometric Titrations
• Photometric endpoints have been applied to many different
types of reactions
– Oxidizing agents
• Have characteristic absorption spectra that can be used to determine endpoints
– Acid/Base
• Although standard acids/bases do not absorb, introduction of various indicators
permit photometric neutralization titrations
• EDTA
– Precipitation (Turbidimetric titrations)
• Product precipitates as a solid, which causes a decrease in the amount of light
allowed to reach detector
• End point is determined when precipitate stops forming and amount of light
reaching detector remains constant.
• Also can be used along with indicator that reacts with precipitated solid and form
a colored complex at a specific wavelength.
The End
• Chemistry Cat!