Effusion measurements after

Knudsen121 Effusion Gauge. To measure the vapor pressure of solids or liquids indirectly, a Knudsen cell is a cylindrical cell containing the sample. A small opening at the top of the cell allows molecules to evaporate at a fixed rate, proportional to the vapor pressure inside the cell. The mass loss of the cell is proportional to the pressure and is measured after a fixed time, for several temperatures. 

In summary, Raman backscattering measurements showed the presence of C-Hx and N-H local vibrational modes in single crystal ZnO. Heating the specimens to temperatures of up to 950 °C caused hydrogen out diffusion. After dehydrogenation the local vibrational modes disappeared indicating that they are related to the presence of H. From H effusion measurements the... 

The apparent inapplicability of the method to measurements (by the Arrhenius approach) of rate constants k, which at first glance are not related to the absolute value of equilibrium pressure of the primary product Pgqp-However, none of the above reasons can account for the fact that this method is ignored in measurements of the vaporization rate from a free surface (after Langmuir), while it is employed widely in effusion studies (after Knudsen). 

Figure 1. Raman backscattering spectra of single crystal ZnO before (a) and after hydrogen effusion (b). The measurements were performed using the 488 nm line of an Ar laser and a laser power of 190 mW. Background data were subtracted.

Thermal Stability. Uranyl metaborate is one of the most stable (thermally) uranyl salts the pure salt begins to show evidence of decomposition only when heated above 750°C. Even at 800°C., only 3% decomposition is observed after two days. The reverse reaction occurs to 1100°C. in air when excess B2O3 is present, thus showing that this instability is occasioned by loss of B2O3 from the crystal. The equilibrium pressure of boric oxide over the metaborate between 800° and 1100°C. must be less than that of pure B2O3. An equation derived by Nesmeya-nov and Firsova (16) from effusion data on boric oxide gives a vapor pressure of approximately 0.0002 mm. at 1000°C. and 0.002 mm. at 1100 °C. these values must constitute the upper limit for the uranyl metaborate equilibrium decomposition pressure. No measurable decomposition of U02(B02)2 was detected after two hours at 925°C. in a nitrogen atmosphere, but substantial decomposition occurred after two hours at 1000 °C. 

After the system has reached equilibrium, a tiny hole is drilled in the side of the flask allowing gaseous compounds to effuse out of the flask. Measurements of the effusing gas show that it contains 33.0 times as much H2(g) as CH30H(g). Calculate K for this reaction at 250.°C. 

In an in vitro percutaneous study performed on rodent and pig skin with DEBT (diethyl-m-toluamide CAS no. 134-62-3), permethrin and carbaryl results were that no permethrin was absorbed. It was believed by the authors that DEBT inhibited the absorption of permethrin (Baynes et al. 1997). Bast et al. (1997) studied the percutaneous absorption of permethrin through the isolated perfused rabbit ear. Permethrin was applied in isopropyl myristate (reference ointment) or in ethanol. The ethanol was evaporated off skin, and the skin was covered with 1.5 (w/w) methyl cellulose in water. The authors measured the appearance rates (pmol min cm ) of 3-phenoxy benzene methanol (CAS no. 13826-35-2) and 3-phenoxybenzoic acid (CAS no. 3739-38-6) in the effusate after dermal application (3.61 pmol cm of skin). No permethrin per se was found in the effusate. The metabolites were believed to be from impurities in the permethrin. From the concentration that permethrin constitutes in a pharmaceutical brand (Ambush ) and from detection limits. Bast et al. (1997) calculated a Kp of 2.63 x 10 cm h . ... 

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