Tracer exchange experiment

Fisitre 1 Experimental arrangement for NMR tracer exchange experiments. (From Ref. 6.)... 

Figure 2 Typical time dependence observed in NMR tracer exchange experiments (benzene on ZSM-5 crystallites of 55 x 55 x 90 p.m , 4 molecules per unit cell, 386 K. 1 - 7(0 denotes the relative amount of still adsorbed at time t. (From Ref. 6.)...

A plot of the relative intensity of the broad constituent versus the observation time (i.e. the separation between the two field gradient pulses) contains information which is analogous to that of a tracer exchange experiment between a particular crystallite containing e.g. labelled molecules and the unlabelled surroundings. Therefore, this way of analysis of PFG NMR data of zeolitic diffusion has been termed the NMR tracer desorption technique [60]. The first statistical moment ( time constant ) of the NMR tracer desorption curve represents the intracrystalline mean lifetime Tintra of the molecules under study. 

Figure 6 shows a schematic diagram of the reactor system used for tracer-exchange positron-emission profiling (TEX-PEP) experiments. During these tracer exchange experiments, a constant flow of unlabeled hydrocarbons in a hydrogen carrier stream is fed into the reactor. The -hexane/... 

An alternative to the three isotope method is to use an enriched isotope tracer in experiments that are run in parallel to those involving normal isotope compositions under identical run conditions. Use of enriched isotope tracers to evaluate the kinetics of isotopic exchange has a long history (e.g.. Mills and Urey 1940). The extent of exchange toward isotopic equilibrium may be defined as ... 

Table 24.5 Characteristic Data of Chemical Tracers Used for the Gas Exchange Experiment Nondimensional Henry s Law Constant, Ki3/vi, Molecular Diffusion Coefficient in Air, Dia, and Water, Diw, Octanol-Water Partition Coefficient, Kiow, All valid for 4°C (Data from Cirpka et al., 1993)...

A common feature of all models for the upper part of circulating fluidized beds is the description of the mass exchange between dense phase and dilute phase. Analogously to low-velocity fluidized beds, the product of the local specific mass-transfer area a and the mass-transfer coefficient k may be used for this purpose. Many different methods for determining values for these important variables have been reported, such as tracer gas backmixing experiments [112], non-steady-state tracer gas experiments [117], model reactions [115], and theoretical calculations [114],... 

S. Ruben, M.D. Kamen, M.B. Allen, and P.J. Nahinsky, Some Exchange Experiments with Radioactive Tracers, Journal of the American Chemical Society 64, 2297-2298, 1942. 

Notice that in the compilation of Rn and C-determined gas exchange rates (Fig. 10.6) only one measurement has been made at wind speeds above Uio = 10m s. Since this regime is the most important for gas exchange and parameterizations in Fig. 10.6 differ by the most in this range, tracer release experiments have been devised to further refine the gas transfer-wind speed relations. In these experiments two tracers, one of which must be a gas, are released into the surface waters of the ocean (Watson et al, 1991). After injection of the tracers, the ship follows the center of the patch, while the progressive decreases in tracer concentrations are measured. The decreases are caused by dilution from mixing and the gas flux to the atmosphere. Since there are two tracers and two... 

AIR/SEA GAS EXCHANGE STUDIED BY DUAL TRACER RELEASE EXPERIMENTS... 

With the TEX-PEP technique experiments on the diffusion and adsorption of mixture of n-hexane/2-methylpentane in large silicalite-1 crystals have been performed. By modeling the experimental tracer exchange curves values of intracrystalline diffusion coefficient and adsorption constant were obtained. Slight preference for the adsorption of /t-hexane was found. Diffusivity of -hexane sharply decreases with increasing fraction of its isomer, since the last one occupies channel intersections thus blocking zeolite network. 

Eq. 21 with Eq. 23 results as the solution of the corresponding differential equation of normal diffusion with the appropriate initial and boundary conditions. These relations hold with the adequate interpretation of D as a self-diffusivity or a transport diffusivity, respectively, for both tracer exchange between the initially adsorbed species A by species B and the relative uptake in an adsorption experiment. It should be noted that Eq. 21 also describes the molecular uptake by single-file systems, since with respect to adsorption/desorption there are no differences between single-file systems and systems which permit normal diffusion. 

In a tracer experiment, we obtain Dt which is close to the self diffusivity (Dt = f Dr) and then the exchange coefficient should also reflect the tracer exchange coefficient kt = ki and this should be comparable to that calculated as above from measurements of Re the difference should say to what extent the presence of an electrode has changed the kinetics of the rate limiting step of the surface reaction. 

A second fundamental experiment (Fig. 6.17b) consists of exchanging the ambient oxygen surrounding an oxide (which is usually an isotope mixture primarily made up of 02) with a gas enriched with either 02 or 02. The consequence of a difference between the tracer content of the gas and that of the sohd phase is a tracer exchange in the solid. As in conductivity experiments the chemical composition is not altered. Obviously this tracer diffusion is a counterdiffusion of the two oxygen isotopes . ... 

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