Do Concentration Cells Store Charge in Water? Comment on Can Water Store Charge?

In a recent article, Ovchinnikova and Pollack (O&P)(1) reported that the persistent pH gradients (>100 min after electrolysis) generated upon charging a simple electrolytic cell (Pt electrodes in dilute aqueous NaCl solutions) imply that “water can store charge”, in apparent violation of the principle of electroneutrality in bulk macroscopic fluid phases

Pyrolysis of phenyl acetate: a concerted reaction

A kinetic study reveals that the unimolecular decomposition of phenyl acetate into phenol and ketone occurs concertedly, probably via a four-cyclic transition state

Dispersion de Radiacion y Transferencia de Calor en Espumas Plasticas: Conductividades Termicas a Partir de Espectros Infrarrojos = Radiation Scattering and Heat Transfer in Cellular Plastics: Thermal Conductivities from Infrared Spectra

The mechanism of radiative heat flow in cellular materials is analyzed in terms of their spectral properties in the infrared region. Specifically, it was found that commercial polystyrene foams having average cell diameters of 100-150 μm behave as optically dense scattering media up to about 8 μm. At longer wavelengths, i.e. in the region where black bodies display their maximum emissive power at ambient temperatures, the scattering coefficient a markedly decreases and the material becomes almost transparent above 50 μm. The behavior of σ in this critical region does not follow a simple λ^(-n) law, revealing that the process should be classified as Mie scattering. It is shown that from this information, encoded as an effective scattering coefficient, overall thermal conductivities can be actually derived by means of standard techniques dealing with energy transfer in scattering media. The physical basis for relating cellular structure and net heat flow in plastic foams is thereby established

Heterogeneous decomposition of trichlorofluoromethane on carbonaceous surfaces

The interaction of trichlorofluoromethane with activated charcoal has been investigated by dynamic mass spectrometry up to 750 K. Prior physical adsorption, revealed in programmed desorption experiments, is followed by irreversible first-order decay with formation of nearly equimolar amounts of HCl above 550 K. This unexpectedly fast process has an apparent activation energy of only 59.4 kJ mol^(–1) and is demonstrably catalytic. The mechanism of Cl_3CF decomposition on carbon surfaces and its possible impact on atmospheric chemistry are discussed

Sonolytic Decomposition of Aqueous Bioxalate in the Presence of Ozone

Ultrasonic irradiation in the presence of ozone is demonstrated to be effective for the rapid oxidation of oxalic acid, bioxalate, and oxalate (H_(2)C_(2)O_(4)/HC_(2)O_(4)−/C_(2)O_(4)^2−) in aqueous solution to CO_2 and H_(2)O. The degradation rate of bioxalate exposed to “sonozone” (i.e., simultaneous ultrasonication and ozonolysis) was found to be 16-times faster than predicted by the linear addition of ozonolysis and ultrasonic irradiation rates. The hydroxyl radical (•OH) is the only oxy-radical produced that can oxidize oxalate on a relevant time-scale. Thus, plausible •OH production mechanisms are evaluated to explain the observed kinetic synergism of ultrasonication and ozonolysis toward bioxalate decomposition. •OH production via decomposition of O_3 in the cavitating bubble vapor and via the reaction of O_3 and H_(2)O_2 are considered, but kinetic estimations and experimental evidence indicate neither to be a sufficient source of •OH. A free-radical chain mechanism is proposed in which the HC_(2)O_(4)− + •OH reaction functions as a primary propagation step, while the termination occurs through the O_3 + CO_(2)•− reaction via an O-atom transfer mechanism. Kinetic simulations confirm that ozone reacts efficiently with the superoxide (O_(2)•−) ion that is produced by the reaction of O_2 and CO_(2)•− to form •OH radical, and that the reaction of O_3 + CO_(2)•− must be chain terminating. Oxalate is also readily oxidized by “peroxone” treatment (i.e., H_(2)O_2 and O_3). However, the addition of H_(2)O_2 during the course of the sonolytic ozonation of oxalic acid does not appear to increase the observed degradation rate and decreases rates at millimolar levels

Entropic and enthalpic effects of 4-methoxy substitution in phenoxyl radicals

Values of ΔH_3=(–12.4 ± 1.6) kJ mol^(–1), ΔS_3=(–18.5 ± 5.6) J K^(–1) mol^(–1) for reaction (3) (see text), corresponding to an O–H bond energy of 322.2 kJ mol^(–1) in 1, and to a 14.5 J K^(–1) mol^(–1) entropy loss for the CH_3O–Ar (Ar = aromatic) libration in 2 relative to 1, are derived from the temperature dependence of the equilibrium constant K_3, determined by EPR spectrometry in benzene–toluene media, between 251 and 304 K. These results allow, for the first time, discrimination between enthalpic and entropic effects on the rates of (O)H-atom abstraction by peroxyl radicals from 4-methoxyphenols and related species

The Role of Nitrogen Dioxide in the Production of Sulfate during Chinese Haze-Aerosol Episodes

Haze events in China megacities involve the rapid oxidation of SO_2 to sulfate aerosol. Given the weak photochemistry that takes place in these optically thick hazes, it has been hypothesized that SO_2 is mostly oxidized by NO_2 emissions in the bulk of pH > 5.5 aerosols. Because NO_2(g) dissolution in water is very slow and aerosols are more acidic, we decided to test such a hypothesis. Herein, we report that > 95% of NO_2(g) disproportionates [2NO_2(g) + H_2O(l) = H+ + NO_3–(aq) + HONO (R1)] upon hitting the surface of NaHSO_3 aqueous microjets for < 50 μs, thereby giving rise to strong NO_3– (m/z 62) signals detected by online electrospray mass spectrometry, rather than oxidizing HSO_3– (m/z 81) to HSO_4– (m/z 97) in the relevant pH 3–6 range. Because NO_2(g) will be consumed via R1 on the surface of typical aerosols, the oxidation of S(IV) may in fact be driven by the HONO/NO_2– generated therein. S(IV) heterogeneous oxidation rates are expected to primarily depend on the surface density and liquid water content of the aerosol, which are enhanced by fine aerosol and high humidity. Whether aerosol acidity affects the oxidation of S(IV) by HONO/NO_2– remains to be elucidated

Conversion of gaseous nitrogen dioxide to nitrate and nitrite on aqueous surfactants

The hydrolytic disproportionation of gaseous NO2 on water's surface (2 NO_2 + H_2O → HONO + NO_3- + H+) (R1) has long been deemed to play a key, albeit unquantifiable role in tropospheric chemistry. We recently found that (R1) is dramatically accelerated by anions in experiments performed on aqueous microjets monitored by online electrospray mass spectrometry. This finding let us rationalize unresolved discrepancies among previous laboratory results and suggested that under realistic environmental conditions (R1) should be affected by everpresent surfactants. Herein, we report that NO_2(g) uptake is significantly enhanced by cationic surfactants, weakly inhibited by fulvic acid (FA, a natural polycarboxylic acid) and anionic surfactants, and unaffected by 1-octanol. Surfactants appear to modulate interfacial anion coverage via electrostatic interactions with charged headgroups. We show that (R1) should be the dominant mechanism for the heterogeneous conversion of NO_2(g) to HONO under typical atmospheric conditions throughout the day. The photoinduced reduction of NO_2 into HONO on airborne soot might play a limited role during daytime