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 1D- 3.

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!