Back migration

At the cathode, water molecules are discharged yielding gas and hydroxide ions, OH. Some of the caustic generated in the cathode compartment back-migrates to the anode compartment and reacts with dissolved chlorine (Cl2, ) to form chlorate as follows... 

The earliest concerted effort in the research and development of Nafion perfluorosulfonate ionomers was directed toward their use as a permselective membrane separator in electrochemical cells used in the large scale industrial production of NaOH, KOH, and CI2. In short, the membrane in this application, in addition to keeping CI2 and H2 gases separated, prevents the unfavorable back migration of hydrated OH ions from the catholyte (concentrated aqueous NaOH or KOH) chamber, while allowing for the transport of hydrated Na+ ions from the anolyte chamber in which is aqueous NaCl. 

Early experimental versions of Nafion within the context of chlor-alkali cells consisted of SO2F precursor forms that were first reacted on only one side with ethylenediamine (EDA) before the conversion of the remainder of the membrane to the sulfonate form took place. The result was a well-defined stratum of sulfonamide cross-links, that were formed upon heating after reaction, that served to reduce swelling at the catholyte interface, which, in turn, reduced OH back migration. However, these EDA-modified membranes proved inadequate in chlor-alkali cells due to the chemical degradation of these cross-links... 

The diaphragm prevents the diffusion of sodium hydroxide toward the anode. This wall allows for the slow passage of solution and the free passage of sodium ions. It is made of asbestos fibers supported on an iron screen. The anode solution level is maintained higher than in the cathode compartment to retard back migration. If sodium hydroxide built up near the... 

Intramolecular nucleophilic attack on the a-position of the vinylidene complex 148 affords a zwitterionic species (149). Next, a formal 1,3-H-shift must occur before release of the observed product (146). Based on the results of an isotope labeling study, this process is believed to occur via two discrete 1,2-H-migration events (149—>150—>147). Interestingly, no back-migration of heteroatom substituents is observed [50]. 

Validation of the lyophilizer cleaning and sterilization processes should be accomplished. Particular care should be taken to verify that there is no back-migration of contaminants, whether from adjuvant fluids integral to the equipment of by cross-contamination from previous product. Typically, an overkill approach using a sufficient number of thermocouples and biological indicators is the method of choice. Finally, fill testing to verify the adequacy of the sterilization procedure and the aseptic manipulations involved with product filling, transfers, and lyophilization needs to be performed. 

Saturation vapor pressure can be caused by either back streaming or back migration. Although diffusion pump oils have low vapor pressures (10 5-10 8 torr), in use they break down into multiple vapor pressures. The resulting higher... 

Back migration can also be caused by improper cooling around the orifice of the pump. With this problem, pump oil that had condensed begins to re-evaporate. This problem is more significant with ultrahigh-vacuum systems. 

Backflow (also called back diffusion or back migration) occurs when the pressures at the outlet and inlet have established a constant ratio (this is analogous to the compression ratio found in mechanical pumps). At that point, gases can drift either way in the vacuum system. Proper trapping or baffling is an easy mechanism to prevent this problem. 

An intriguing application of this hypothesis may have been observed in the mass spectrometric behavior of MTO.86 Loss of H2 and CO from the parent ion were the major processes observed in both metastable ion and collisional activation mass spectrometry. Both were viewed as unusual the latter, in particular, required a rationale for forming a C—O bond. The authors proposed migration of carbon to oxygen in the parent ion to form a methoxyrhenium complex of the same mass, followed by sequential back migration of the hydrogens ... 

Controlled centrifugal instabilities (called Dean vortices), resulting from flow around a curved channel, were used by MaUubhotla and Belfort [74] to reduce both concentration polarization and the tendency toward membrane fouling. These vortices enhanced back-migration through convective flow away from the membrane-solution interface and allowed for increased membrane permeation rates. 

Pulsation Superimposing an oscillating pressure gradient onto bulk axial flow in a tube produces a velocity profile with two equal maxima nearer the wall than the centerline. The net result of this is increase in the wall shear rate and the back migration of the solute in the bulk flow region, which results in improved per-formance. Beside the improvement in performance. 

The analysis by XPS of the fresh catalysts shows the presence of the chloride ion peak (Cl 2p3/Jt - 198.4 eV). The presence of chlorine species in metal catalysts prepared from metal chlorides has already been reported, indicating the difficulty of eliminating all the Cl by Hs treatment, despite the fact that TPR showed practically complete reduction of the metal (ref. 12). Actually, only a small amount of this Cl modifies the electronic density of metal atoms, leading to the formation of electron-defficient metal species (MT ) on the surface (refs. 11-13), which we have detected by XPS. The rest of the chlorine may remain as HCl or can substitute OH groups attached to the carbon support. These findings can be explained if one accepts that, in the presence of H2, the HCl is held by the carbon carrier, but it car back-migrate to the vicinity of the metal after H2 removal. Table 2 also summarizes the metal/carbon atomic ratios (M/C). As this ratio is an estimate of metal dispersion, a parallelism exists between the experimental M/C XPS ratios and the corresponding values obtained by H2 chemisorption. 

This behavior has also been discussed in terms of alkaline state (to the left of the critical point of MR) and acid state (to the right of the critical point) of the cation-exchange membrane [232], [239]. This difference in internal states of the membrane is connected to the presence in the conductive channels of the membrane of a population of either hydroxyl ions (back migration from the catholyte when the electric current is substantially transported by sodium ions low MR values) or protons (significant portion of the current transported by protons high MR values). The model has remarkable practical consequences, particularly in terms of the sensitivity of the membrane with respect to impurities contained in the feed solutions. 

During the course of these synthesis, the synthesis of 19,20-dehydroyohimbines was also achieved by using the acylated D/E-cis enone, which has a folded ring conformation. On the other hand, the 19,20-unsaturated ketone has a rather planar structure. Therefore, it was expected that the back migration of a double bond at the 18,19-position into the 19,20-position would occur. Actually, acid treatment of the 18,19-enone yielded the 19,20-enone in an excellent yield. Subsequent reductions furnished the first total synthesis of three 19,20-dehydroyohimbines. 

NaOH Concentration. The anodic cell efficiency decreases with increasing NaOH strength in diaphragm cells and with increased back-migration of OH in membrane cells. 

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