Diffusive gradient

One procedure makes use of a box on whose silk screen bottom powdered desiccant has been placed, usually lithium chloride. The box is positioned 1-2 mm above the surface, and the rate of gain in weight is measured for the film-free and the film-covered surface. The rate of water uptake is reported as u = m/fA, or in g/sec cm. This is taken to be proportional to - Cd)/R, where Ch, and Cd are the concentrations of water vapor in equilibrium with water and with the desiccant, respectively, and R is the diffusional resistance across the gap between the surface and the screen. Qualitatively, R can be regarded as actually being the sum of a series of resistances corresponding to the various diffusion gradients present ... 

The e.m.f. of a thermogalvanic cell is the result of four main effects (a) electrode temperature, (b) thermal liquid junction potential, (c) metallic thermocouple and (d) thermal diffusion gradient or Soret. 

Thermal diffusivity Gradient Contact angle Kinematic viscosity Density Surface tension... 

Meylan S, Odzak N, Behra R, Sigg L (2004) Speciation of copper and zinc in natural freshwater comparison of voltammetric measurements, diffusive gradients in thin Aims (DGT) and chemical equilibrium models. An Chim Acta 510 91... 

Upon formation of a metal chelate or complex, the next rate-limiting step in delivering iron to the cell is the diffusion of iron complexes through the. soil in response to diffusion gradients. In the vicinity of plant roots, metal chelates and complexes may also move by bulk flow in the transpiration stream as water moves from the soil into the plant. However, depending on their charge characteristics and hydrophobicity, metal chelators and complexes can become adsorbed to clay and organic matter, which may then decrease their mobility and bioavail-... 

Van Moorleghem C, Six L, Degryse F, Smolders E, Merckx R. Effect of organic P forms and P present in inorganic colloids on the determination of dissolved P in environmental samples by the diffusive gradient in thin films technique, ion chromatography, and colorimetry. Anal. Chem. 2011 83 5317-5323. 

Soil Gas The minmum 02 concentration that can support aerobic metabolism in unsaturated soil is approximately 1%. 02 diffuses into soil because of pressure gradients, and CO 2 moves out of soil because of diffusivity gradients. Excess water restricts the movement of 02 into and through the soil. A minimum air-filled pore volume of 10% is considered adequate for aeration. Soil gas surveys using a mobile geoprobe unit have become a valuable tool to demonstrate a zone of enhanced microbial metabolism in the subsurface. 

In response to stimulation, the neuronal membrane becomes more permeable to Na+, which is drawn into the neuron down the electrical and chemical diffusion gradients. This depolarizes the neuron and makes the membrane further permeable to Na+. If sufficiently depolarized, a threshold of excitation is reached (approximately -60 mV), where voltage-gated Na+ channels open and the membrane becomes highly permeable... 

The solution of Example 7.3 will be compared with an analytical solution of a diffusive front moving at velocity U, with D = 1/2U Az. First, we must derive the analytical solution. This problem is similar to Example 2.10, with these exceptions (1) convection must be added through a moving coordinate system, similar to that described in developing equation (2.36), and (2) a diffusion gradient will develop in both the +z-and -z-directions. 

Boundary condition 1 is chosen because of similarity in diffusion gradients in the +z-and -z-directions. Similar to Example 2.10, we will assign... 

The physical meaning of Equation (17) is simply this if the specific rate constant goes up, say nine times, because of an increase of temperature, the concentration profile must be steeper in order to diffuse in the extra amount of reactant gas. Consequently, the penetration into the carbon decreases. Obviously, equilibrium is reached when the concentration gradient increa.scs threefold, the penetration distance decreases threefold, and the over-all reaction rate (proportional to fc times penetration distance) increases threefold, where 3, in this example, is the square root of the factor of specific-rate-constant increase. For these conditions, the over-all reaction rate has increased threefold, but so has the diffusion gradient that is, equilibrium has been reached. 

Bvlk Diffusion Occurring But Pores Too Fine to Allow Poiseuille Flow. For bulk diffusion, Dett l /P), where P is the total pressure. If Poiseuille flow is negligible, then the concentration profiles of CO and CO through the rod can be shown as in Fig. A2. For the reaction C + CO —> 2CO, the increase in volume of CO over CO is two and at any point, the diffusion gradient for CO must be double that for CO. If C is the concentration of CO at a point where the concentration of CO is C,... 

It is said that the CV response has a peak because the electroactive substance is depleted faster than diffusion can replace it near the electrode. The relationship between the current at the electrode and the diffusion gradient at the surface is... 

The diffusion potential of an atom at the surface is proportional to local surface curvature as demonstrated in Section 3.4. The curvature can be determined from Eq. 14.1 and is a function of x. The local diffusion potential produces boundary conditions for diffusion through the bulk or transport via the vapor phase. For surface diffusion, gradients in the diffusion potential produce fluxes along the surface. 

Paleolimnological Conditions. Because of the interplay between primary production, oxygen content of bottom waters, and the sulfur content and speciation of sediments, sediment profiles of S probably preserve records of paleolimnological conditions. Several studies (23-25, 205) point to increased S content of sediments as a result of eutrophication. Mechanisms involve both rates of S supply to sediments (seston deposition and diffusive gradients) and rates of S reduction and oxidation. The relative S enrichment... 

In the absence of catalysis on the surface, similarity of the concentration and temperature fields is achieved precisely at the ignition limit if the coefficients of diffusion and thermal diffusivity are equal, since in this case both the diffusion gradient and the temperature gradient at the igniting surface are equal to zero, and the equations of diffusion and thermal conductivity with the chemical reaction may be reduced to the form of an identity (see our work on flame propagation [3]). 

Examples of dimensionless groups that specify ratios of transport mechanisms are listed next in Table II and depend on the size and shape of the domain. The Peclet numbers for heat (Pet) and solute (Pes) and momentum (Re) transport are ratios of scales for convective to diffusive transport and depend on the magnitudes of the velocity field and the length scale for the diffusion gradient. Boundary layers form at large Peclet numbers (Pet or Pes) or Reynolds numbers (Re). The fonnation of a boundary layer at a large Re is particularly important in crystal growth from the melt, because the low... 

There is a linear diffusion gradient inside the enzyme layer. 

The hydrated layer has finite thickness, therefore the exchanging ions can diffuse inside this layer, although their mobility is quite low compared to that in water (n 10-11cm2s-1 V-1). As we have seen in the liquid junction, diffusion of ions with different velocities results in charge separation and formation of the potential. In this case, the potential is called the diffusion potential and it is synonymous with the junction potential discussed earlier. It can be described by the equation developed for the linear diffusion gradient, that is, by the Henderson equation (6.24). Because we are dealing with uni-univalent electrolytes, the multiplier cancels out and this diffusion potential can be written as... 

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