Gradients, measurement

Baldi, S. and Yianneskis, M. (2004) On the quantification of energy dissipation in the impeller stream of a stirred vessel from fluctuating velocity gradient measurements. Chem. Eng. Sci., 59 (13), 2659-2671. 

It is essential that the solution be sufficiently dilute to behave ideally, a condition which is difficult to meet in practice. Ordinarily the dilutions required are beyond those at which the concentration gradient measurement by the refractive index method may be applied with accuracy. Corrections for nonideality are particularly difficult to introduce in a satisfactory manner owing to the fact that nonideality terms depend on the molecular weight distribution, and the molecular weight distribution (as well as the concentration) varies over the length of the cell. Largely as a consequence of this circumstance, the sedimentation equilibrium method has been far less successful in application to random-coil polymers than to the comparatively compact proteins, for which deviations from ideality are much less severe. 

This is in excellent agreement with equation 2.70, considering that equation 3.47 was derived from flow rate and pressure gradient measurements rather than from direct measurement of the velocity profiles. 

A method for determining the alteration of the refractive index of a medium as a result of the temperature rise in the path of a beam of coherent light absorbed by the medium. Thermal lensing can also occur with pigmented proteins, and this phenomenon can influence the accuracy of concentration gradient measurements in small aperture flow cuvettes as well as in ultracentrifugation. 

Fig. 3. Thermal gradient measured in the Soultz-sous-Forets wells. (Modified from Pribnow et al. 1999).

A comparison of differential and density gradient measurements. A Differential centrifugation in a medium of unchanging density. B Zonal centrifugation in a prepared density gradient. 

These drugs have thus been used extensively to generate models for the function of V-ATPases. Hydrolysis of ATP by the vesicular H+-ATPase is coupled to movement of H+ into the vesicle lumen, creating a H+ electrochemical gradient. Measurements from cholinergic and aminergic vesicles show a ApH 1.4 units and an electrical potential +39 mV(AT) (Whittaker, 1987 Schuldiner et al., 1998). 

Determination of the flow gradient between two wells (Fig. 4.11) is based on the assumption that the two wells are hydraulically interconnected, as shown in Fig. 4.13a. Flowever, wells may be separated by an impermeable rock bed with no hydraulic interconnection (Fig. 4.13b). Thus the gradient measurement between the two wells in the second case is meaningless. The... 

The situation is more complicated in cases where one wishes to compare the electric field gradients at the sites of different nuclei A and B (belonging to different chemical elements). Here the only reliable way to discuss and compare electric field gradients measured in different substances at the sites of different nuclei supposes an independent knowledge of QAIQn from theory or experiment. 

The chemiosmotic model requires that flow of electrons through the electron-transport chain leads to extrusion of protons from the mitochondrion, thus generating the proton electrochemical-potential gradient. Measurements of the number of H+ ions extruded per O atom reduced by complex IV of the electron-transport chain (the H+/0 ratio) are experimentally important because the ratio can be used to test the validity of mechanistic models of proton translocation (Sec. 14.6). 

Figure 15 shows typical results of axial voidage profile measurements (Li and Kwauk, 1980 Li et al, 1981). Experiments were conducted by using different solids materials in a Type A apparatus, 90 mm i.d. and 8 m high. Voidages were calculated from pressure gradient measurements. Solids circulation rate was controlled by a pneumatically actuated pulse feeder (see Section III in Chapter 7). 

To establish a correlation between the interracial tension gradient measurements given above with three phase foam stability we used the qualitative Barch method for determining foam stability, due to the physicochemical difficulties in providing an accurate measure of three phase foam stability. 

These high stability foams correlate directly with the results based upon interfacial tension gradient measurements, confirming the significance of Marangoni phenomena (31) in three phase foam stability. 

Figure 6 Oxygen fluxes determined from benthic lander measurements (X) and calculated from microelectrode oxygen gradients measured in the top few centimeters of pore waters (O) as a function of latitude and depth on the northwest US continental margin. Error bars are the standard deviation of replicate measurements (after Archer and Devol, 1992).

Hicken, B.L., Sharara, A.I., Abrams, G.A., Eloubeidi, M., Fallon, M.B., Arguedas, M.R. Hepatic venous pressure gradient measurements to assess response to primary prophylaxis in patients with cirrhosis A decision analytical study. Alim. Pharm. Ther. 2003 17 145-153... 

Cowman, L.M. and Ethier, C.R., Concentration and concentration gradient measurements in an ultrafiltration concentration polarization layer. Part I A laser-based refractometric experimental technique. Journal of Membrane Science 131 1997 95-105. 

FIG. 21-25 iFluid" fluidization permeameter, illustrating powder bed supported by distributor plate fluidized at a gas velocity U, with associated pressure taps for multiple pressure gradient measurements dP/dh. Courtesy iPowder Systems, EhG, Associates, Inc.)... 

Density measurements can be used in the calculation of the percentage crystallinity of fibers. Fiber degradation has also been monitored by both sink-float and density gradient measurements. ... 

Beyond this effect, however, the concentration gradients measured in various solid-phase metal fractions at the soil-root interface can also be attributed to the effects of a variety of processes, such as rhizosphere acidification or alka-linization, adsorption or desorption reactions, and precipitation or dissolution phenomena, which are themselves associated with plant uptake and a range of... 

Walchle, S.M. (1996) A potential sampling technique of wheat gluten for stress gradient measurement by infrared microspectroscopy. MS thesis, Kansas State University, Manhattan, KS. 

Robinson and Robbins (1970) think that these processes are effective mainly in the stratosphere. This concept is reasonable since O concentrations in the stratosphere are greater than in the troposphere. Furthermore, the intensity of UV radiations is also greater above the tropopause. According to Ehhalt et al. (1977) the flux of N20 from the troposphere to the stratosphere is between 6 x 108 and 30 x 108 molecules cm 2 s 1 which are equivalent to (7-35) x 1061 yr ". These values are based partly on the N20 concentration gradient measured in the lowest three kilometers of the stratosphere and partly on the net transport coefficient of 1 x 103 cm2 s 1 and... 

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