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!