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.
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.
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.
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 1D-3.
Titration CurvesTitration curves are a function of absorbance vs. volume of titrant added. • Titration of non-absorbing analyte w/ absorbing titrant to form a non-absorbing product. • Formation of absorbing product from non-absorbing reactants. • Absorbing analyte reacts w/ non-absorbing titrant to form non-absorbing products. • Absorbing analyte + titrant react to form non-absorbing product. • Absorbing analyte reacts with non-absorbing titrant to form absorbing product. • 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!