Concentration-depth gradient determination

Assessing the depth by determining the protein amount removed per strip, Mueller et al. noted a nonlinear steady-state concentration gradient which they ascribed to an increased permeability of the cornified envelope within the... 

Figure 5. Data from the literature (56, 80, 99, 164, 195, 220, 222, 223, 243) indicate that diffusive fluxes of sulfate (calculated from 40 pore-water profiles measured with pore-water equilibrators) are linearly related to concentrations of sulfate in the overlying lake water. The correlation is significant (p < 0.05) both with (r2 = 0.991) and without (r2 = 0.42) the two lakes with high sulfate concentrations. The strong correlation suggests that variations in the depth interval within which sulfate is consumed and in the minimum sulfate concentration defining the gradient are relatively unimportant in determining the flux, compared to variations in sulfate concentrations defining the upper end...

In the EHD impedance method, modulation of the flow velocity causes a modulation of the velocity gradient at the interface which, in turn, causes a modulation in the concentration boundary layer thickness. As demonstrated previously in Section 10.3.3 and Fig. 10.3 the experiment shows a relaxation time determined solely by the time for diffusion across the concentration boundary layer. Although there is a characteristic penetration depth, 8hm, of the velocity oscillation above the surface, and at sufficiently high modulation frequencies this is smaller than the concentration boundary layer thickness, any information associated with the variation of hm with w is generally lost, unless the solution is very viscous. The reason is simply that, at sufficiently high modulation frequencies, the amplitude of the transfer function between flow modulation and current density is small. So, in contrast to the AC impedance experiment, the depth into the solution probed by the EHD experiment is not a function... 

Two of the key assumptions of the thin-film model (see Section 6.03.2.1.1) are that the main bodies of air and water are well mixed, i.e., that the concentration of gas at the interface between the thin film and the bulk fluid is the same as in the bulk fluid itself, and that any production or removal processes in the thin film are slow compared to transport across it. It is quite likely that there are near-surface gradients in concentrations of many photochemically active gases. Little research has been published, although the presence of near-surface gradients (10 cm to 2.5 m) in levels of CO during the summer in the Scheldt estuary has been reported (Law et al., 2002). Gradients may well exist for other compounds either produced or removed photochemically, e.g., di-iodomethane, nitric oxide, or carbonyl sulfide (COS). Hence, a key assumption made in most flux calculations that concentrations determined from a typical sampling depth of 4-8 m are the same as immediately below the microlayer may well often be incorrect. 

A quantity of interest in mass diffusion processes is the depth of diffusion at a given time. This is usually characterized by the penetration depth defined as the location where the tangent to the concentration profile at the surface (x = Oj intercepts the Q, line, as shown in Figure 14-27. Obtaining the concentration gradient at. r = 0 by differentiating Eq. 14-36, the penetration deptli is determined to be... 

As a result of carried out calculations it was shown, that the HCl accumulation in pore space results in occurrence of gradient both of the concentration, and functionality of vanadium-oxide groups on the depth of separate grain. The value of a given gradient is determined by boundary conditions in equation (7). In Figure 7 the dependencies of... 

From a more general standpoint, if we consider a polyphase sample with a concentration gradient that extends over a depth of a few tens of micrometers, then X-ray diffraction at a fixed incidence can be used for producing concentration profiles based on measnrements of diffracted intensities. Quantitatively determining concentrations leqnires accurately taking into account the irradiated volumes. Relations inferred from the equations given in section 2.3.1.2 have been laid out and are nowadays relatively often used [SCA 93]. 

Fig. 3.1 Concentration profiles in the pore water fractions of sediments obtained off the estuary of the River Congo, at a depth of approximately 4000 m. The sediments contain a relatively high amount of TOC. Values ranging from 1 to 3.5 wt. % indicate that this sediment is characterized by the high reaction rates of various early diagenesis processes. These processes are reflected by diffusion fluxes over gradients and by reaction rates determined by gradient changes (after Schulz et al. 1994).

Irrespective of the differences between in situ and ex situ measurements (see below and Chapter 6), there are generally two ways to determine the transport rates of dissolved substances across the water/sediment boundary determination of the concentration differences along the depth profiles (gradient approach) and measurement of the time-dependent concentration changes of dissolved species within a closed... 

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