Exchange Between Counter Ions

Despite very efficient stirring during the addition of R OD to 2, not all molecules of 2 will be deuterated at the same time. Deprotonation of 3 by 2 leads to the formation of some di-deuterated compound 5. More polar conditions (K+ as a counter ion HMPT as a co-solvent) are likely to favour such processes. These can be minimized by inverse addition a solution of the organometallic compound is added to an excess of R OD in the organic solvent (for an example see Chap. V, Exp. 44). 

In a number of derivatization reactions it is advisable to replace the original counter ion by another one. As mentioned in Sects. II-6 and II-9, organolithium compounds give much better results than potassium compounds in reactions with enolizable carbonyl compounds and with sulfur, selenium and tellurium. On the other hand, alkylations with alkyl halides and with oxiranes proceed more smoothly with the potassium intermediates. Although HMPT may be used as a co-solvent, simple replacement of lithium by potassium may give similar results combination of the counter ion and solvent effects may be even better. The replacements Li+ K + and K+ Li+ are generally fast reactions in a wide temperature range (compare e.g. [161,241])  

Since it is extremely difficult to remove traces of water from commercial zinc salts, it is desirable to prepare the salts in Et2Q. ZnCl2 and ZnBr2 are obtained by reacting 

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Exchange between counter-ions A in beds of resin and counter-ions B in a well-stirred solution may be represented by the Nemst-Planck equation as ... 

For an ion-exchange SMB process with three components (Nc ,p=3), consider 15 columns involving 5 sections such as wash water (3 columns), production (3 columns), slip water I (4 columns), regeneration (4 columns) and slip water II (1 column) sections. See Lim Jorgensen (2002) for the column configuration and operating conditions. A cation exchange between counter-ions C" and B" takes place in the columns ... 

Exchange of counter-ions (and solvent) between the polymer and the solution in order to keep the electroneutrality in the film. In a compacted or stressed film, these kinetics are under conformational relaxation control while the structure relaxes. After the initial relaxation, the polymer swells, and conformational changes continue under counter-ion diffusion control in the gel film from the solution. 

These methods for achieving slow diffusion of reactant solutions are not just applicable in the initial chemical synthesis of a desired product. For ionic coordination compounds, they can also be used when it is desirable to exchange the counter-ion to obtain potentially better quality crystals or to reduce the solubility by better size-matching of the ions. The initially obtained complex salt is in one solution, and a simple salt of the new counter-ion is in the other. Slow mixing of these two may be carried out by any of the techniques described here, or by simple layering of one solution over the other as in Section 1.28.4 obviously, vapor diffusion is not appropriate for this purpose, as it does not transfer the solutes between the two liquids. 

One way of probing the interactions between solvated ions is to study aqueous bulk electrolyte activity coefficients, y. For the most common choice of reference state, the mean activity coefficient is a measure of the excess free energy, fM = ksT In y, of transferring a solvated salt pair from an infinite dilution to a solution -with a finite salt concentration. In the present context, we are interested in the ion specijkity and therefore — for a fixed salt concentration — define a free energy of exchanging one counter ion -with another. For example,... 

Ion-exchange methods are based essentially on a reversible exchange of ions between an external liquid phase and an ionic solid phase. The solid phase consists of a polymeric matrix, insoluble, but permeable, which contains fixed charge groups and mobile counter ions of opposite charge. These counter ions can be exchanged for other ions in the external liquid phase. Enrichment of one or several of the components is obtained if selective exchange forces are operative. The method is limited to substances at least partially in ionized form. 

Exchange Between Ions of Manganese. Metallic managanese was bombarded for forty-five minutes with a 4 micro-ampere beam of 5.8 million volt deuterons and the radioactive layer was dissolved. A special device was used with a Geiger counter for determining the amount of manganese, tagged by means of its radio-... 

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