John B. Cooper

Microelectrode studies without supporting electrolyte: Model and experimental comparison for singly and multiply charged ions

Abstract The steady-state transport-limited voltammetry of ions has been modelled in the presence of excess supporting electrolyte and in its total absence. As expected, oxidation of a cation (or reduction of an anion) is predicted to result in a decrease in the limiting current when supporting electrolyte is removed. Alternatively, reduction of a cation (or oxidation of an anion) is predicted to result in an increase in the limiting current when supporting electrolyte is removed. When the product is an ion, its presence and that of the counter-ion diminish the importance of reactant migration, leading to the prediction that the limiting current will differ from that in the presence of excess supporting electrolyte by a known factor not very different from unity. Greater differences from unity are predicted when the number of electrons transferred in the electrode reaction becomes quite large or when the product is neutral. The model has been tested using a wide variety of reactions, comprising reductions of anions and both oxidations and reductions of cations. In the majority of cases the agreement with prediction is remarkably good. In many other cases, however, waves which are observed when excess supporting electrolyte is present almost disappear in its absence. At present we have no credible explanation for this phenomenon. The voltammetric waves generally satisfy the Tomes criterion of reversibility, confirming that the IR drop is small.

Comparison of Near-IR, Raman, and Mid-IR Spectroscopies for the Determination of BTEX in Petroleum Fuels

We report for the first time a direct comparison of the three most common vibrational analysis techniques for the determination of individual BTEX components (benzene, toluene, ethylbenzene, ortho-xylene, meta-xylene, and para-xylene) in blended commercial gasolines. Partial least-squares (PLS) regression models were constructed for each BTEX component by using each of the three spectroscopic techniques. A minimum of 120 types of blended gasolines were used in the training set for each BTEX component. Leave-one-out validation of the training sets yields lower standard errors for Raman and mid-IR spectroscopies when compared to near-IR for all six BTEX components. In general, mid-IR has slightly lower standard errors than Raman. These trends are upheld when the models are tested by using independent test sets with a minimum of 40 types of blended gasolines (all of which differ in composition from the training set).

Microelectrode studies in the absence of deliberately added supporting electrolyte: solvent dependence for a neutral and singly charged species

Abstract Steady state voltammetric studies utilizing a 5.6 μm radius platinum inlaid disk microelectrode have been carried out in a series of solvents containing ferrocene (FeCp 2 ) and/or cobaltocenium hexafluorophosphate ([CoCp 2 ] + [PF 6 ] − ) in both the presence and absence of intentionally added supporting electrolyte. The very low dependence of the ferrocene oxidation process (FeCp 2 ⇋ [FeCp 2 ] + + e − ) on the concentration of electrolyte enables this compound to be used as a convenient internal reference standard for charged compounds. For reduction of the charged [CoCp 2 ] + ion, ([CoCp 2 ] + + e − ⇋ CoCp 2 ), changes in the migration current as a function of solvent in the absence of electrolyte can be related to the dependence of ionic mobility on solvent viscosity. The ohmic IR drop effects observed in solvents with lower dielectrics are related to the degree of ionic association. Possible reasons for the lack of a simple relation for either migration or IR drop trends with solvent properties are discussed. A steady state analytical solution for mass transport for the charged system is compared to the experimental data. In practical terms, it has been shown that in the absence of deliberately added electrolyte, steady state voltammograms can be obtained for both neutral and singly charged species in solvents as non-polar as tetrahydrofuran with little distortion, whereas with toluene, which has only a slightly lower dielectric constant, the IR drop distortion is extremely large. As expected, the IR distortion for an ionic sample is less than that for a neutral sample. However, whether the IR drop is important or not in non-polar solvents is very dependent on the exact nature of the experimental conditions.

Redox and electroinsertion processes associated with the voltammetry of microcrystalline forms of Dawson molybdate anion salts mechanically attached to graphite electrodes and immersed in aqueous electrolyte media

Abstract The solid state redox chemistry of microcrystalline salts of the Dawson anion [S2Mo18O62]4− precipitated with large organic cations NR+4 (R Et, Bu, Hex) is reported. Voltammograms are obtained by mechanically attaching water insoluble heteropolymolybdate salts to a basal-plane pyrolytic graphite electrode which is then placed in aqueous electrolyte media. A considerable number of pH-dependent reduction processes are observed which fall into two categories. (i) Relatively weak responses that correspond to reversible two electron-two proton reduction steps. Despite differences in the mass transport mechanism these five processes are otherwise related to the solution phase voltammetric behaviour of [S2Mo18O62]4− in acidified organic solvent media and are summarized by the equation [ H n S 2 M o 18 O 62 ] x − + 2 e − + 2 H + ⇄ [ H n + 2 S 2 M o 18 O 62 ] x − The observed 60 mV pH−1 shift of voltammetric potentials is consistent with this mechanism. (ii) At negative potentials and in the presence of an appropriate electrolyte, processes with considerably larger peak currents are shown by electron microprobe analysis of the solid to be accompanied by cation (Cs+, Ba2+) uptake. Voltammetric data confirm that at the potentials where Cs+ and Ba2+ insertion occurs, that the solids are highly reduced. The pH dependence suggests that competition occurs between cation and proton uptake in the highly reduced solid. Therefore the major processes in the presence of a metal ion such as Cs+ are described by the equation ( N R 4 ) 4 [ S 2 M o 18 O 62 ] + x C s + + y H + + ( x + y ) e − ⇄ ( N R 4 ) 4 C s x H y [ S 2 M o 18 O 62 The initial voltammetric responses are interpreted in terms of electron transfer and charge neutralization processes that are confined to a region near to the surface of the solid heteropolymolybdate salt, whereas the major processes at more negative potentials that are facilitated by insertion of Cs+ or Ba2+ are associated with changes in the structure of the solid.

Determination of Weight Percent Oxygen in Commercial Gasoline: A Comparison between FT-Raman, FT-IR, and Dispersive Near-IR Spectroscopies

The weight percent oxygen in commercial gasoline samples has been determined by using partial least-squares (PLS) regression analysis combined with either FT-Raman, FT-IR, or dispersive near-IR spectroscopy. Calibration models were constructed with the use of 33 MTBE oxygenated commercial gasolines. The minimum standard errors of validation with the use of leave-one-out validation are 0.156, 0.188, and 0.119 wt % oxygen for FT-Raman, FT-IR, and near-IR, respectively. An independent test set of 36 MTBE oxygenated commercial gasolines was used to further validate the PLS models. The minimum standard errors of prediction for the test set are 0.155, 0.143, and 0.131 wt % oxygen for FT-Raman, FT-IR, and near-IR, respectively. The wt % oxygen in all samples ranges from 0.2 to 3.262%.

Self-Referencing Raman Probes for Quantitative Analysis

Diamond is evaluated as an internal reference for remote Raman spectroscopy using three different fiber-optic probe designs. The three probe designs include (1) a six-around-one fiber-optic probe with a flat diamond window; (2) a six-around-one fiber-optic probe with a quartz lens window with an embedded diamond particle, and (3) a filtered probe design with a flat diamond window. It is found that the second probe design provides a compact and inexpensive probe head which allows quantitative measurements of Raman-active analytes independent of changes in the incident laser power and independent of changes in the refractive index of the solution. In addition, the second probe design minimizes the glass Raman background from the fiber optics without the use of filters.

Remote Fiber-Optic Raman Analysis of Xylene Isomers in Mock Petroleum Fuels Using a Low-Cost Dispersive Instrument and Partial Least-Squares Regression Analysis

We report the use of a low-cost dispersive Raman instrument with charge-coupled-device (CCD) detection, near-infrared (near-IR) diode laser excitation, and remote fiber-optic sampling to analyze mock petroleum samples which contain high benzene, toluene, ethylbenzene, and xylene (BTEX) concentrations. Partial least-squares regression (PLSR) analysis is used to correlate the individual xylene isomer concentrations to the Raman spectral signal without the use of an internal standard. The resulting PLSR model is used to predict the concentration of individual xylene isomers, and it is found that, at a 95% confidence level, samples containing between ∼1.5 and 15% xylene isomer can be predicted to better than ±0.1% for meta- and para-xylene, and to ±0.15% for ortho-xylene. The use of PLS model leverage plots provides a facile statistical method by which to identify Raman spectra which involve diode laser mode hops or significant fiber backscatter.

Sequentially Shifted Excitation Raman Spectroscopy: Novel Algorithm and Instrumentation for Fluorescence-Free Raman Spectroscopy in Spectral Space

A novel Raman spectrometer is presented in a handheld format. The spectrometer utilizes a temperature-controlled, distributed Bragg reflector diode laser, which allows the instrument to operate in a sequentially shifted excitation mode to eliminate fluorescence backgrounds, fixed pattern noise, and room lights, while keeping the Raman data in true spectral space. The cost-efficient design of the instrument allows rapid acquisition of shifted excitation data with a shift time penalty of less than 2 s. The Raman data are extracted from the shifted excitation spectra using a novel algorithm that is typically three orders of magnitude faster than conventional shifted-excitation algorithms operating in spectral space. The superiority of the instrument and algorithm in terms of background removal and signal-to-noise ratio is demonstrated by comparison to FT-Raman, standard deviation spectra, shifted excitation Raman difference spectroscopy (SERDS), and conventional multiple-shift excitation methods.

Fabrication of Boron-Doped CVD Diamond Microelectrodes

Diamond microelectrodes are fabricated using microwave plasma CVD for the growth of electrically conducting single microcrystallite diamonds as well as diamond films on etched tungsten wires which are subsequently sealed in glass. The electroactive diamond is exposed by either mechanical polishing or by chemical etching of the glass. The resulting microelectrodes yield steady-state cyclic voltammograms at low scan rates.

Determination of octane numbers and Reid vapor pressure in commercial gasoline using dispersive fiber-optic Raman spectroscopy

Abstract Dispersive fiber-optic Raman spectroscopy utilizing CCD detection and near-IR DBR diode laser excitation is used to remotely analyze 205 petroleum fuels of varying composition for pump octane number, motor octane number (MON), research octane number (RON), and Reid vapor pressure (RVP). Partial least squares regression analysis in tandem with several preprocessing techniques was used to model pump octane, MON, RON, and RVP using 130 selected fuels for the training set. The resulting leave-one-out determined standard error of validation (SEV) values for the mean centered data are 0.542, 0.761 and 0.434 octane units, and 0.740 psi, for pump, RON, MON, and RVP, respectively. The remaining 75 samples were used as a test to further validate the models. For the test set, the standard errors (SE) for the mean centered data are 0.586, 0.773 and 0.492 octane units, and 0.869 psi, for pump, RON, MON, and RVP, respectively.