Publication List for Paul A. Flowers

1. Small-Scale Production of High-Density Dry Ice: A Variant Combination of Two Classic Demonstrations
   Flowers, Paul A.
   Journal of Chemical Education (2009), 86(4), 470-471
   Easily recoverable, thumb-sized pieces of high-density dry ice are conveniently produced by deposition of carbon dioxide within a test tube submerged in liquid nitrogen. A carbon dioxide-filled balloon sealed over the mouth of the test tube serves as a gas reservoir, and further permits a dramatic demonstration of both the gas-to-solid phase transition and Avogadro's law, with complete deflation of a 2–3 L balloon requiring only about 2–3 minutes. The dry ice "cube" that forms within the test tube is readily removed after slightly warming the tube's outer surface and is of sufficient density to sink in an aqueous indicator solution, permitting its subsequent use for the classic demonstration of carbon dioxide's sublimation and its acidic properties.
   
   
2. Fiber Optic Spectroelectrochemical Sensing for in situ Determination of Metal Ions
   Flowers, Paul A.; Arnett, Kristen A.
   Spectroscopy Letters (2007), 40(3), 501-511
   In situ chemical sensing techniques are increasingly used for a variety of applications, including industrial process control, on-site environmental assessment, and detection of explosives and chemical and biological weapons. A common category of sensors for such purposes entails the use of optical fibers for making spectral measurements of target compounds or species derived from these compounds via physical, chemical, enzymatic, or immunologic reactions. A less common but potentially advantageous approach involves the electrochemical conversion of the analyte subsequent to its spectroscopic detection. These spectroelectrochemical schemes represent versatile, essentially reagent-free analyses that could provide superior alternatives to existing methods. Reported here is a summary of progress made by the authors' group toward the development of fiber optic spectroelectrochemical sensors for in situ measurements. The aqueous copper (II)/copper (0) couple was chosen as a model system to investigate the merit of an analytical scheme involving (i) cathodic preconcentration of Cu²⁺ as Cu⁰ followed by (ii) anodic stripping of Cu⁰ to Cu²⁺, (iii) complexation of Cu²⁺ by an appropriate ligand, and finally (iv) absorbance determination of the copper-ligand complex or fluorescence determination of the unbound ligand. Results are encouraging and indicate the need for further refinement of the sensor's design and the experimental protocol in order to improve the method's sensitivity.
3. Membrane Extraction of Aqueous Transition Metal Ions Using Commercial Nafion Gas Dryers
   Bullock, Kimberly; Flowers, Paul A.
   Journal of Undergraduate Chemistry Research (2004), 3, 127-130
   Membrane extraction is a widely employed technique for improving the sensitivity and/or selectivity of a chemical analysis. This paper reports a study on the membrane extraction of aqueous transition metal ions using commercially available Nafion gas dryer assemblies. Though designed and marketed for use with gaseous mixtures, results this work indicate such assemblies can be effective for the selective extraction of aqueous mixtures after a simple pretreatment procedure not requiring physical modification of the unit. The analytical merit of this sampling approach was evaluated by examining various performance characteristics using aqueous Cu(II) as a model system.
4. Easily Constructed Spectroelectrochemical Cell for Batch and Flow Injection Analyses
   Flowers, Paul A.; Maynor, Margaret A.; Owens, Donald E.
   Analytical Chemistry (2002), 74(3), 720-723
   The design and performance of an easily constructed spectroelectrochem. cell suitable for batch and flow injection measurements are described.  The cell is fabricated from a com. available 5-mm quartz cuvette and employs 60 ppi reticulated vitreous carbon as the working electrode, resulting in a reasonable compromise between optical sensitivity and thin-layer electrochem. behavior.  The spectroelectrochem. traits of the cell in both batch and flow modes were evaluated using aq. Ferricyanide and compare favorably to those reported previously for similar cells.
   
   
5. Imaging Microelectrodes with High-Frequency Electrogenerated Chemiluminescence
   Wightman, R. Mark; Curtis, Corrine L.; Flowers, Paul A.; Maus, Russell G.; McDonald, Erin M.
   Journal of Physical Chemistry B (1998), 102(49), 9991-9996
   The chemiluminescence arising from reaction of electrogenerated intermediates of 9,10-diphenylanthracene (DPA) was used to generate images of microelectrodes with dimensions in the micrometer range.  The exptl. conditions were optimized to ensure high luminescent intensity with sharp focusing of the reaction zone to enable good optical resoln.  The soln. employed, benzonitrile (BN) contg. 0.1 M Bu4NPF6, promotes high intensity because it enables dissoln. of a high concn. (>25 mM) of DPA.  Radical anions of BN can serve as a reagent reservoir to ensure efficient reaction of DPA radical cations to the singlet excited state.  Under these conditions the measured intensity was 3.2 × 105 photons/s per mm2 of electrode area with a 1 kHz square-wave excitation.  Lateral resoln. is controlled using rapid potential pulses that maintain the reaction zone in close proximity to the electrode.  The images reveal that the electrode areas have quite different topog. than inferred from steady-state cyclic voltammograms.
   
   
6. Potentiometric measurement of transition ranges and titration errors for acid/base indicators
   Flowers, Paul A.
   Journal of Chemical Education (1997), 74(7), 846-847
   An expt. involving potentiometric measurement of transition ranges and titrn. errors for common acid/base indicators is described.  This procedure entails the potentiometric titrn. of a millimolar HCl soln. contg. ab indicator with comparably dil. NaOH.  The pH and visually assessed color of the reaction soln. are monitored as a function of added titrant vol., and the resultant data are plotted to permit detn. of the indicator's transition range and assocd. titrn. error.  By simultaneously monitoring soln. pH, the expt. reported here permits estn. of indicator transition ranges and, further, affords students addnl. experiences in potentiometric measurements and graphical data anal.
   
   
7. Variable Path Length Transmittance Cell for Ultraviolet, Visible, and Infrared Spectroscopy and Spectroelectrochemistry
   Flowers, Paul A.; Callender, Sherry-Ann
   Analytical Chemistry (1996), 68(1), 199-202
   The design and characteristics of a transmittance cell for UV, visible, and IR spectroscopy and spectroelectrochem. are described.  Through modification of a previously reported design, this cell employs threaded glass connectors as insertion ports for either quartz- or silicon-windowed tubes, thus permitting essentially continuous variation of the optical path length from ~0.050 to 200 mm.  Though the initial fabrication requires skillful glassblowing, once constructed, the cell's simple design allows for rapid and reproducible disassembly/reassembly between expts.  The utility of the cell for a diversity of fluid samples is demonstrated through applications to water, aq. ferricyanide, ferrocene in methylene chloride, and acetone vapor.
   
   
8. External reflectance cell for infrared spectroscopy of fluids at elevated pressure and temperature
   Flowers, Paul A.; Boaz, B. Gilley, III
   Analytical Chemistry (1992), 64(18), 2197-9
   A short pathlength cell for IR spectroscopy of fluids at high pressure and temp. has been constructed and characterized.  Though the upper pressure and temp. limits were not detd., the cell has been maintained for prolonged periods at 150 atm and 110° with no evidence of structural failure.  The cell design is relatively simple and allows for external reflectance sampling of a thin layer of fluid (~4 mm) at 30° incidence, with a resultant optical pathlength of approx. 10 mm.  Further, the design permits spectral observation of one of two fluid phases contained within the cell, thus facilitating studies of two-phase phenomena such as soly. and partition equil.  Applications to supercrit. fluids, molten salts, and two-phase fluid systems are demonstrated, and the potential utility of the cell for spectroelectrochem. studies is discussed.
   
   
9. Spectroscopic investigations of catalytic iridium carbonyl species in sodium chloroaluminate melts
   Tortorelli, Louis J.; Flowers, Paul A.; Harward, Brisco L.; Mamantov, Gleb; Klatt, Leon N.
   Journal of Organometallic Chemistry (1992), 429(1), 119-34
   In situ spectroscopic studies of the iridium chem. occurring during the catalytic hydrogenation of carbon monoxide employing IrCl(CO)3 and Ir4(CO)12 in an aluminum chloride-sodium chloride (63:37 mol%) melt were performed.  IR, UV-visible, Raman, and 1H NMR data indicate that similar monomeric species are generated during catalysis.  IR investigation of the introduction of IrCl(CO)3 into molten AlCl3:NaCl (63:37) under CO:H2 (1:3 mol ratio) and the reaction between IrCl(CO)3 and carbon monoxide in an acidic melt suggest the initial formation of an iridium-carbonyl species followed by the generation of a hydridocarbonyl complex in the initial stage of the catalysis.  IR data also indicate that the method of introduction for Ir4(CO)12 into the AlCl3:NaCl medium under 1 atm of CO:H2 (1:3 mol ratio) has a marked effect on the spectra obsd.  In an acidic melt IrCl(CO)3 reacts with hydrogen to form hydrogen chloride, methane and metallic iridium.
   
   
10. Ionically modified electrodes for use in nonpolar fluids
    Niehaus, David E.; Wightman, R. Mark; Flowers, Paul A.
    Analytical Chemistry (1991), 63(17), 1728-32
    The addn. of water to the supercrit. fluid of carbon dioxide or nitrous oxide allows for voltammetry to be performed on analytes dissolved in these solvents.  Exptl. data show that an amt. of water in excess of its soly. must be added to the electrochem. cell in order to minimize ohmic distortion.  This, along with the obsd. shapes of voltammograms taken in these systems, indicates that water film formation is taking place on the surface of the electrodes.  Electrodes coated with tetrahexylammonium nitrate ((THA)NO3) are shown to be useful in detecting species dissolved in supercrit. CO2 and N2O.  (THA)NO3 (m.p. 69°) is molten at ambient temps. under the supercrit. fluids due to dissoln. of the fluids in the molten salt.  Analytes dissolved in supercrit. CO2, N2O, and heptane partition into molten (THA)NO3 films on electrodes to give voltammograms free from ohmic distortion.  The voltammetric waves of ferrocene, anthracene, and 9,10-diphenylanthracene are shown to be chem. irreversible in the molten salt.  The chem. irreversibility of these analytes is not evident when (THA)NO3 is used as a conventional electrolyte in acetonitrile solns.  Post-SFC column detection of FeCp2 at a molten (THA)NO3 film electrode yields a detection limit of 0.1 ng.
    
    
11. Density of antifreeze-water mixtures:  a general chemistry experiment in compositional analysis
    Flowers, Paul A.
    Journal of Chemical Education (1990), 67(12), 1068-9
    A lab. expt. is described in the detn. of the d. of antifreeze-H2O binary mixts.  The expt. provides students with experience in some std. lab. procedures (pipetting and weighing) and exposes them to graphical anal.  Students should be warned that ethylene glycol, the major component of antifreeze, is a poison and they should avoid accidental ingestion by following lab. safety precautions.
    
    
12. Cylindrical thin-layer cell for spectroelectrochemistry
    Flowers, Paul A.; Nealy, Gary
    Analytical Chemistry (1990), 62(24), 2740-2
    The characteristics of a thin-layer spectroelectrochem. cell of cylindrical geometry are described.  The cell design permits double-pass transmittance sampling of the soln. layer, resulting in an enhanced optical sensitivity, allows accurate control of the optically transparent working electrode potential with minimal ohmic drop, and permits exhaustive electrolysis of the sample layer in less than one minute.  After its initial construction, the cell may be easily and reproducibly disassembled, cleaned, and reassembled between expts.  The unique cylindrical geometry of the cell permits its use with inexpensive fixed-wavelength spectrometers.
    
    
13. Infrared spectroscopic and spectroelectrochemical investigation of chloranil in molten sodium chloroaluminates
    Flowers, Paul A.; Mamantov, Gleb
    Journal of the Electrochemical Society (1989), 136(10), 2944-8
    The IR spectroscopic and spectroelectrochem. properties of tetrachloro-p-benzoquinone (chloranil) in melts [>50 mol percent (m/o) AlCl3] chloranil exists as a Lewis acid adduct with AlCl3 (or Al2Cl7-) coordinated at one of the carbonyl oxygens and at the C-C double bonds.  No evidence for adduct formation was obsd. in basic melts (<50 m/o AlCl3). Spectroelectrochem. data indicate that chloranil is reduced in an overall 2-electron process in basic melts, in agreement with previously reported electrochem. results.  In acidic melts, however, the data suggest stepwise one-electron redns. to produce the radical anion and dianion.  The presence of two closely spaced one-electron redn. steps is confirmed by differential pulse voltammetry.
    
    
14. Electrochemical and spectroscopic studies of tungsten hexachloride in an acidic sodium chloroaluminate melt
    Schoebrechts, Jean Paul; Flowers, Paul A.; Hance, Glen W.; Mamantov, Gleb
    Journal of the Electrochemical Society (1988), 135(12), 3057-62
    The chem. of WCl6 in an acidic Na chloroaluminate melt (63 mol % AlCl3) was investigated by electrochem. and spectroscopic techniques at 150 and 175°C.  WCl6 undergoes several redn. steps in the melt, the 1st of which is sensitive to the presence of oxide impurities.  UV-visible and IR absorption measurements indicate that WCl6 reacts with oxide to produce WOCl4.  Comparison of spectroscopic and electrochem. data indicates that the 1st voltammetric wave is due to the redn. of the hexachloride to WCl5 (E1/2 = +1.665 V vs. Al3+/Al in the same melt) and that WOCl4 is electroactive only below +1.5 V.  The equil. const. for the formation of WOCl4 and the initial amt. of oxide in the melt are derived from voltammetric data and compared to values obtained from IR measurements.
    
    
15. Thin-layer transmittance cell for infrared spectroelectrochemistry
    Flowers, Paul A.; Mamantov, Gleb
    Analytical Chemistry (1989), 61(2), 190-2
    A Si-windowed thin-layer transmittance cell for IR spectroelectrochem. is described and characterized using the system tetracyanoquinodimethane (TCNQ) in MeCN.  The cell was constructed by sealing the Si windows directly to a Pyrex glass body contg. threaded glass ports, resulting in an inert, vacuum-tight vessel suitable for use with a wide variety of solvents.  The cell geometry allows close placement of the ref. And counter electrodes, hence minimizing ohmic potential drop and permitting reasonably accurate control of the working electrode voltage.  Modifications to the described cell, designed for use in chloroaluminate melts, are proposed which would permit variation of the path length and operating temp.
    
    
16. Infrared spectroscopic determination of oxide in molten chloroaluminates
    Flowers, Paul A.; Mamantov, Gleb
    Analytical Chemistry (1987), 59(7), 1062-4
    Oxide impurities in molten chloroaluminates are difficult to avoid and may have pronounced effects on the behavior of other solute species of interest.  The detn. of oxide in these media thus represents an anal. problem of considerable importance.  IR emittance and external reflectance spectroscopy were examd. as means of detg. dissolved oxide.  IR emittance spectra of several AlCl3-NaClsat melt samples were obtained using a diamond-windowed cell; peak heights at 801 and 680 cm-1 were correlated to oxide concn. as detd. by voltammetric anal.  Std. addn. of H2O to a 65/35 mol% AlCl3-NaCl melt was performed to obtain a value for the initial oxide level of the sample.