Ionic exchange, competitive

In the presence of competitive ionic exchange are used exchange or selectivity coefficients provided they may be expressed through the partition coefficient. Equations for switching from one adsorption coefficients to the others reviewed in detail in the section Equilibrium of ionic exchange . 

Specific-ion electrodes are expensive, temperamental and seem to have a depressingly short life when exposed to aqueous surfactants. They are also not sensitive to some mechanistically interesting ions. Other methods do not have these shortcomings, but they too are not applicable to all ions. Most workers have followed the approach developed by Romsted who noted that counterions bind specifically to ionic micelles, and that qualitatively the binding parallels that to ion exchange resins (Romsted 1977, 1984). In considering the development of Romsted s ideas it will be useful to note that many micellar reactions involving hydrophilic ions are carried out in solutions which contain a mixture of anions for example, there will be the chemically inert counterion of the surfactant plus the added reactive ion. Competition between these ions for the micelle is of key importance and merits detailed consideration. In some cases the solution also contains buffers and the effect of buffer ions has to be considered (Quina et al., 1980). 

Increasing the ionic strength of the mobile phase generally decreases the retention of most analytes on ion exchange. This seems to be primarily due to increased competition of ions for the ion-exchange sites [29]. 

In the geological and soil science literature, ion exchange and precipitation are frequently considered as adsorption and thermodynamically described by adsorption equations, or isotherms. This is not correct because, as shown previously, the processes are principally different adsorption is directed by the decrease of surface energy, and it takes place on the free surface sites ion exchange is just a competitive process on an already covered surface, determined by the ionic composition of the liquid phase. Precipitation, including colloid formation, is governed by the composition of the liquid phase, the crystal structure (coprecipitation), or primary chemical forces. 

Inner-sphere complexes are relatively stable in comparison to outer-sphere complexes under equivalent solution conditions (i.e. pH, ionic strength), and in a competitive situation will tend to displace less stable adsorbates. This is a fundamental property of coordination reactions, and explains the observed trends in metal uptake preference observed in lichen studies (Puckett et al., 1973). Metal sorption results previously attributed to ion exchange reactions are more precisely described as resulting from competitive surface complexation reactions involving multiple cation types. Strictly speaking, each metal adsorption reaction can be described using a discrete mass law relation, such as... 

The much more common case of hydrolysis reactions, which seem to correspond to the intermediate case of Eq. (XVI.1.4) with feCX ) = kz(H20)j is difficult to classify with certainty because, with the increase in salt content of the system, there is an increased tendency toward ionization (i.e., increase in Xion = ki/k2 due to increased ionic strength), which tends to compensate for the mass-law retardation. Because of these ambiguities, other approaches have been employed to throw light on the mechanism. One of these is to study stereochemical changes of RX during reaction, while another is to study competitive reaction of the intermediate R+. Thus f-butyl chloride in formic acid solution exchanges with radio-... 

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