2 |. EXPERIMENTAL
Detailed descriptions1 of the apparatus and 10experimental methods employed in the present work are given elsewhere.10-12 Thus, only a brief overview is provided here. The principal component of the discharge flow-electron paramagnetic resonance (DF-EPR) apparatus is a quartz tubular reactor of 1.5 cm i.d. with its internal surface coated with perfluorinated Teflon-like coating (F-46) to reduce wall loss of OH and to prevent heterogeneous reactions. The temperature of the reactor was maintained (±0.3 K) with water (298 and 370 K) or mineral oil (460 K) circulated through its outer jacket. We could not obtain the reliable data at lower temperatures, which is probably due to heterogeneous processes. All reactants were added to the main carrier gas, He, and flow through the reactor at the total pressure of 0.2-0.4 kPa (1.5-3.0 Torr) with the average linear gas flow velocity (vo) ranging from 10 to 27 m/s. Some test experiments described later were performed at increased pressure of 1.1 kPa (8 Torr). The total pressure was measured at both ends of the reactor. OH radicals were generated in the main flow tube near the tip of a movable quartz injector in the fast reaction:
- Die Balling Methode
- In Primary Aldosteronism Acute Potassium Chloride Supplementation Suppresses Abundance and Phosphorylation of the Sodium-Chloride Cotransporter
- C5H8 có bao nhiêu đp ankadien liên hợp
- Cách đọc tên nguyên tố hóa học theo SGK mới khiến giáo viên, học sinh bối rối!
- C6H5ONa + CO2 + H2O → C6H5OH + NaHCO3
Hydrogen atoms were generated by the microwave discharge (2.45 GHz, 10-20 W) in the H2/He mixture flowing through the movable quartz injector. Both NO2 and the reactant under study (HX, where X is Cl, Br, or I) were injected into the flow reactor upstream of the movable injector tip. They were always in large excess over H-atoms. The concentration of NO2 of (3 − 6) × 1013 molecule/cm3 was used thus converting H-atoms into OH radicals in 0.1-0.2 ms. The OH concentration was monitored using EPR spectroscopy at the end of the flow reactor. The initial OH concentration in this study was estimated as (1 − 2) × 1011 molecules/cm3 when the rate constant were measured and up to ≈ 1012 molecules/cm3 in some test experiments at higher reactant concentrations. Pure undiluted reactants were stored in glass bulbs and flew to the reactor through the glass delivery lines equipped with vacuum grease-free Teflon valves. Flow rates of all gases were determined by direct measurements of the pressure change rate in calibrated volumes, which were the parts of gas delivery lines. The overall instrumental uncertainty of measurements was estimated to be ~10%. The decay rate of hydroxyl radical concentration, [OH], was measured by varying the distance, z, from the movable injector tip to the center of the EPR spectrometer cavity between 4 and 30 cm. The OH loss rate coefficients (k′) at any reactant concentration, [HX], were obtained from the equation
where [OH](z) is the concentration of OH (EPR signals) at distance z from the EPR resonator center. The k′ values obtained from experiments with movable injector (expression 5) were slightly corrected for the axial diffusion to obtain the pseudo-first-order rate coefficients, k, using the expression
where D is the OH diffusion coefficient. Finally, the bimolecular rate constant, kHX, at a particular temperature was derived from the slope of a plot of k versus the reactant concentration using the following expression:
where kw is the first-order OH decay rate due to its heterogeneous OH removal in the absence of the reactant and [HX] is the concentration of the reactant (HCl, HBr, or HI, respectively). Note that the length of the OH detection zone (EPR cavity) does not affect results of these measurements.10
Another series of experiments was carried out with a fixed distance, z0 (10-21 cm) between the injector and the detection zone (the center of an EPR cavity). The dependence of the OH concentration (EPR signal) versus the concentration of HX was measured to obtain the kHX value. Under conditions of plug flow, the rate constant can be obtained as
Small corrections to account for the axial diffusion were also applied to these data. Note, that although the physical dimension of an EPR cavity (≈3 cm) is not much smaller than the distance between the injector and the center of the cavity, z0, it does not introduce an additional uncertainty to the measured rate constant. The EPR signal for any [HX] is an integral of the exponentially decreasing [OH] multiplyed by the sensitivity distribution along the EPR cavity. Using thus calculated values instead of [OH] in expression 8 results in ≈0.5% underestimation of the derived rate constant even for the shortest z0 = 10 cm under conditions of our experiments (v0, [HX]).
These complementary experimental approaches are illustrated in Figures 1 and and2,2, which show results obtained for reaction (3) with a movable injector and fixed reaction distance, respectively. Note that the total data scattering is mainly due to difference between results obtained in different series of measurements performed in different days. Data obtained in the single series of measurements (during the same day) are usually less scattered as illustrated in small inserts. Rate constants reported in this paper for each experimental approach were derived from combined fits to the entire data set obtained at the particular temperature.