Permeation techniques

Beck, et al. have used the permeation technique to study the effect of uniaxial tensile stresses in the elastic region on hydrogen permeation through pure iron, and have shown that it increases with increase in stress. The partial molar volume of hydrogen (cubic centimetres of hydrogen per mole of iron) in ferrous alloys can be evaluated from the variation of permeation with applied stress, and from the relationship... 

The permeation technique is another commonly employed method for determining the mutual diffusion coefficient of a polymer-penetrant system. This technique involves a diffusion apparatus with the polymer membrane placed between two chambers. At time zero, the reservoir chamber is filled with the penetrant at a constant activity while the receptor chamber is maintained at zero activity. Therefore, the upstream surface of the polymer membrane is maintained at a concentration of c f. It is noted that c f is the concentration within the polymer surface layer, and this concentration can be related to the bulk concentration or vapor pressure through a partition coefficient or solubility constant. The amount... 

Figure 25. Schematic illustration of the principle of the electrochemical hydrogen permeation technique.

The electrochemical hydrogen permeation technique has proved to be a valuable tool in the study of these reaction mechanisms. This is mainly due to the ability to estimate the amount of an intermediate (Hads) in the reaction scheme. Such studies have been presented, for example, by Devanathan and Stachurski, by Bockris et and by Iyer et The applicability of the Volmer-Tafel reaction scheme can be evaluated by considering the kinetic expressions for reactions (22) and (23), together with equilibrium in the absorption process (25)... 

The electrochemical hydrogen permeation technique has been used in efforts to establish threshold hydrogen concentrations in steel below which no cracking occurs. The threshold concentration depends largely on the type of failure under investigation, the chemical and physical properties of the steel, and the magnitude of applied and residual stress. 

The electrochemical hydrogen permeation technique can be useful in a variety of investigations. Mechanistic information on the hydrogen evolution reaction can be obtained owing to the ability of the technique to quantify the amount of adsorbed atomic hydrogen intermediary formed in the process. 

This study reports two gel permeation techniques used to evaluate water at various stages of (potable water) treatment. The water samples were taken from a surface-water pilot treatment plant that employed several levels of ozone treatment. Selected fractions of the permeate were subsequently analyzed by pyrolysis gas chromatography-mass spectrometry (GC-MS). 

This apparent contradiction may be related to UV absorbance contributed by mineral forms. The gel permeation technique used in this laboratory has been observed to concentrate mineral components. For example, nitrate in this fraction has been found to exceed 1 g/L. Furthermore, some mineral forms such as ferrous iron have been observed to absorb at 260 nm. Chelates have been found to quench fluorescence. Thus, the concentrations of all minerals and their contributions to UV absorbance or fluorescence quenching should be carefully examined. High concentrations of metallic ions also may play a catalytic role during pyrolysis and further contribute to the absence of pyrolysis products (27). 

The chemical and behavioral aspects of the sex pheromones of several forest defoliating insects of economic importance in eastern Canada are presented, with emphasis on the spruce budworm, Choristoneura fumiferana. Studies conducted over several years in New Brunswick on the use of pheromones as potential control agents, using in particular the air permeation technique to effect mating disruption, are discussed. The identification and the behavioral effects of minor components of the spruce budworm pheromone system are presented and the potential exploitation of their behavioral roles in the mating sequence in terms of control strategies are addressed. 

As in most Lepidoptera, spruce budworm males locate conspecific females by flying upwind along a pheromone plume. The blends and release rates of these pheromone components form an important part of a specific communication system for the species. Once the communication system of an insect is understood, especially the pheromone chemistry as it relates to male behavior, it can be used in a variety of ways. For example, pheromones can be used to detect the presence of an insect in an area, to remove males from a population by trapping or poisoning and in air-permeation techniques in which the controlled and continuous release of pheromone components in the forest can disrupt mating. The latter use of pheromones appears to alter the normal male behavioral responses to the natural pheromone (16). 

The symmetrical reversal of the above chemical polarization experiment is the sudden replacement of the oxygen partial pressure on x = L by the original one Then the concentration profiles decay by chemical depolarization (allowing again for a measurement of Lf). If, however, the gas flow at that gas chamber is just stopped, the gas pressure equilibration takes more time in that it not only affects the sample volume but also the volume of the gas chamber. This directly connects with the permeation technique described in the next section. 

The permeation technique as discussed in the context of permeation membranes in Section II.2, can also be used to determine diffusion coefficients and minority conductivities (in fact steady-state oxygen flux that is given by (see Eq. (34))... 

As described in Section II.2 variants of the permeation techniques use systems in which electronic and ionic pathways are locally separated, e.g., an ion conductor which is internally short-circuited by percolating metallic inclusions (or materials in which grain boundary and bulk possess different conductivity types (e.g., nano- Ce02)). In all these cases the permeation flux is determined by the lowest partial conductivity. 

Procedures to remove the restrictions of the permeation technique, also inherent in the time lag method, have been described by Grachev et al. [13] and Gibilaro et al. [14], As with the Wicke-Kallenbach method, they are based on the application of a carrier gas. Details of these methods may be found in Ref. 1. 

In deciding when to utilize the gel permeation technique, consideration must be given to the configurational differences of the molecules to be separated. This is often in the same ratio as their molecular weight differences. However, compounds with small differences in molecular weight occasionally may have vast differences in molecular conformation in solution. It is important to realize that the separation is based upon molecular size not molecular weight. 

In the light of the poor efficiency of broadcast applications of pheromones in air permeation techniques the alternative practice of scattering or placing a limited number of release points at suitable distances apart over the treated area merits consideration. Since each point will then act as an individual source the treated area will be permeated by a set of individual plumes of pheromone vapor whose direction will vary with airflow through the crop or woodland. Large... 

Molecular Probes Used in Reverse Gel Permeation Techniques for Pore Size Distribution Determination... 

Ono, N. Hirayama, F. Arima, H. Uekama, K. Determination of stability constant of P-cyclodextrin complexes using the membrane permeation technique and the permeation behavior of drug-competing agent-P-cyclodextrin ternary systems. Eur. J. Pharm. Sci. 1999, 8 (2), 133-139. 

Geus et al. [75] reports diffusion data at 21 and 145°C for Hj, N2, CH4, CO2 and CF2CI2 in silicalite membranes on a clay support which are obtained with the similar transient permeation technique as used above by Vroon. The diffusion coefficients for methane are about two orders of magnitude smaller than those obtained by PF-NMR methods. Usually this last technique gives relatively large diffusion coefficient values, which in the case of n-butane are of the same order of magnitude as reported for FR techniques and membrane techniques as reported by Kapteyn. 

Liquid membrane technology has been applied to a great extent for separation of mixtures of saturated and aromatic hydrocarbons. Investigations reveal that the LSM process offers potential for dearomatization of petroleum streams like naphtha and kerosene to meet product specifications for naphtha cracker feedstock and aviation kerosene, respectively [25, 63, 85, 144-146]. The separation is based on a simple permeation technique and occurs due to the difference in solubility and diffusivity of permeating species through the membrane. Kato and Kawasaki [70] conducted studies on the enhancement of hydrocarbon permeation by the use of a polar additive like sulfolane or triethyl glycol. Sharma et al. [147] enhanced the selectivity of the membrane by several orders with the addition of a carrier. Chakraborty et al. [85] used cyclodextrins to enhance the separation factor and removal efficiency of aromatic compound. 

A series of more sophisticated permeation techniques have been recently proposed in order to better characterise the quality of the membrane and to evaluate qualitatively the contribution of non-zeolite pores to the transport. A number of methods derived from permporometry [125] are under study [111,129], The method used in [111] consists in measuring the permeance of an inert gas such as He through a MFI membrane, while increasing the partial pressure (p/pn) of a condensable gas which can enter into the zeolite pores, such as w-hcxanc (Okmaic = 0.43 nm). Zeolite pores arc then blocked first by -hexane adsorption, and when the proportion of n-... 

Of all of the proportioning methods described here, the lowest dosage rates can be achieved with the permeation technique. In many permeation elements, the rates are in the range of less than a few 10 SCCM. Because permeation sources can only contain small quantities of the component to be added, such a calibration gas generator is especially suited for the preparation of gas mixtures in which the constituents are in the range of ppm, ppb or sub-ppb. 

If concentrations in the ppb range are required, this can only be achieved, in one step, with permeation techniques, because only they allow flow rate doses of a few 10 SCCM. Otherwise the concentration of a generated precursory calibration gas can be reduced by one to three orders of magnitude via a second dilution step. 

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