Radicals concentration, measured

Figure 6.33 shows a plot of the total peroxy radical concentrations measured at Mace Head, Ireland, as a function of either the square root or the first power of /(O D) (Carpenter et al., 1997). Consistent with Eq. (L), the concentrations vary with the first power under these conditions. 

Altshuller, A. P., Ambient Air Hydroxyl Radical Concentrations Measurements and Model Predictions, JAPCA, 39, 704-708 (1989). 

Absolute Radical Concentration Measurements and Modeling of Low Pressure Flames... 

Hofzumahaus, A., H.-P. Dorn, J. Callies, U. Platt, and D. H. Ehhalt. 1991. Tropospheric hydroxyl radical concentration measurements by laser long-path absorption spectroscopy. Atmos. Environ. 25A 2017-2022. 

Free radicals are formed when most materials are subjected to ionizing radiation, thus ESR can be used to monitor the process. In the food industry such monitoring is extremely important and ESR can assist in direct radical concentration measurement and also in providing a means of measuring the radiation dose. Radiation effects on rocks can be used to measure ages up to one million years. 

The superpositioning of experimental and theoretical curves to evaluate a characteristic time is reminiscent of the time-tefnperature superpositioning described in Sec. 4.10. This parallel is even more apparent if the theoretical curve is drawn on a logarithmic scale, in which case the distance by which the curve has to be shifted measures log r. Note that the limiting values of the ordinate in Fig. 6.6 correspond to the limits described in Eqs. (6.46) and (6.47). Because this method effectively averages over both the buildup and the decay phases of radical concentration, it affords an experimentally less demanding method for the determination of r than alternative methods which utilize either the buildup or the decay portions of the non-stationary-state free-radical concentration. 

Electron spin resonance (esr) (6,44) has had more limited use in coal studies. A rough estimate of the free-radical concentration or unsatisfied chemical bonds in the coal stmcture has been obtained as a function of coal rank and heat treatment. For example, the concentration increases from 2 X 10 radicals/g at 80 wt % carbon to a sharp peak of about 50 x 10 radicals/g at 95 wt % carbon content and drops almost to zero at 97 wt % carbon. The concentration of these radicals is less than that of the common functional groups such as hydroxyl. However, radical existence seems to be intrinsic to the coal molecule and may affect the reactivity of the coal as well as its absorption of ultraviolet radiation. Measurements from room... 

EPR methods that allow a more direct determination of kv have been developed. These enable absolute radical concentrations to be determined as a function of conversion. With especially sensitive instrumentation, this can be done by direct measurement/57 160 An alternative method, applicable at high conversions, involves trapping the propagating species in a frozen matrix361 362 by rapid cooling of the sample to liquid nitrogen temperatures. 

Note that the quasi-steady hypothesis is applied to each free-radical species. This will generate as many algebraic equations as there are types of free radicals. The resulting set of equations is solved to express the free-radical concentrations in terms of the (presumably measurable) concentrations of the long-lived species. For the current example, the solutions for the free radicals are... 

The quasi-steady hypothesis allows the diflficult-to-measure free-radical concentrations to be replaced by the more easily measured concentrations of the long-lived species. The result is... 

Y.C. Chen and R. Bilger 2001, Simultaneous 2-D imaging measurements of reaction progress variable and OF radical concentration in turbulent premixed flames Instantaneous flame front structure. Combust. Sci. Tech. 167 187-222 (more informations through www.infor-maworld.com). 

From the prevailing NO and HONO levels occurring during this period of the irradiation, the HONO photolysis rate (11,14), and the rate constant for the OH + NO reaction (15), we estimate that steady state OH levels of "2 x 1Q7 molecule cm" were present. From this OH radical concentration and assuming an UDMH + OH rate constant similar to those observed for N2Hi and MMH (, j ) we calculate a UDMH decay rate which is in reasonable agreement with what is observed. Thus, the HONO level measured during the initial period is entirely consistent with our assumed mechanism. 

Platt UF, AM Winer, HW Biermann, R Atkinson, JN Pitts, Jr (1984) Measurement of nitrate radical concentrations in continental air. Environ Sci Technol 18 365-369. 

With this setup a series of measurements was taken in which the power was varied from 5 to 30 W. The pressure was low, 0.06 Torr, while the temperature was 250°C. The SiH (0 < jr < 3) radical densities near the surface were measured as a function of effective power delivered to the discharge. It was found that the SiH3 radical is the most abundant one, with a concentration varying between 1.8 x 10" and 3.2 x lO cm. The SiH2 radical concentration was about a factor of 20 lower. Both radical concentrations increase with increasing power. 

The cummulative values of the initiator loading calculated with equations 20a and 28 should agree, betwen themselves and with the measured values. As it is shown in Figure 5 this is not the case, a correction for the Initiator balances seems to be required. Larger efficiency values can be obtained if it Is assumed that all the initiator radicals have the same efficiency. Under this condition, the radical concentration is given by... 

Conversion of coal to benzene or hexane soluble form has been shown to consist of a series of very fast reactions followed by slower reactions (2 3). The fast initial reactions have been proposed to involve only the thermal disruption of the coal structure to produce free radical fragments. Solvents which are present interact with these fragments to stabilize them through hydrogen donation. In fact, Wiser showed that there exists a strong similarity between coal pyrolysis and liquefaction (5). Recent studies by Petrakis have shown that suspensions of coals in various solvents when heated to 450°C produce large quantities of free radicals (. 1 molar solutions ) even when subsequently measured at room temperature. The radical concentration was significantly lower in H-donor solvents (Tetralin) then in non-donor solvents (naphthalene) (6). 

For cationic zeolites Richardson (79) has demonstrated that the radical concentration is a function of the electron affinity of the exchangeable cation and the ionization potential of the hydrocarbon, provided the size of the molecule does not prevent entrance into the zeolite. In a study made on mixed cationic zeolites, such as MgCuY, Richardson used the ability of zeolites to form radicals as a measure of the polarizing effect of one metal cation upon another. He subsequently developed a theory for the catalytic activity of these materials based upon this polarizing ability of various cations. It should be pointed out that infrared and ESR evidence indicate that this same polarizing ability is effective in hydrolyzing water to form acidic sites in cationic zeolites (80, 81). 

In studies comparing measured and modelled HOx radical concentrations, the models usually overestimate [OH] by 20-50%. A detailed review of the comparisons of modelled and measured concentrations of OH and H02 can be found in Heard and Pilling (2003). In particular, several studies have been made in the marine boundary layer. 

The photoelectron either recombines with the photohole or is transferred to the electron accetor, MV2+. Thereby the reduced species MV+ is formed. This MV+ radical exhibits an absorption band with a maximum at 630 nm. The increase of the absorption at 630 nm of the MV+ radical was measured as a function of time (in the microsecond range) after a laser pulse at 355 nm which creates high enough concentrations of photoelectrons and photoholes. In the experiment shown in Fig. 10.7a bare Ti02 particles were examined where the photoholes are trapped by the surface OH groups ... 

Consequently, it is seen, from the measurement of the overall reaction rate and the steady-state approximation, that values of the rate constants of the intermediate radical reactions can be determined without any measurement of radical concentrations. Values k exp and xp evolve from the experimental measurements and the form of Eq. (2.31). Since (ki/k5) is the inverse of the equilibrium constant for Br2 dissociation and this value is known from thermodynamics, k2 can be found from xp. The value of k4 is found from k2 and the equilibrium constant that represents reactions (2.2) and (2.4), as written in the H2 Br2 reaction scheme. From the experimental value of k CX(l and the calculated value of k4, the value k3 can be determined. 

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