Dynamic ion exchange

Several theoretical models, such as the ion-pair model [342,360,361,363,380], the dyneuaic ion-exchange model [342,362,363,375] and the electrostatic model [342,369,381-386] have been proposed to describe retention in reversed-phase IPC. The electrostatic model is the most versatile and enjoys the most support but is mathematically complex euid not very intuitive. The ion-pair model emd dynamic ion-exchange model are easier to manipulate and more instructive but are restricted to a narrow range of experimental conditions for trtilch they might reasonably be applied. The ion-pair model assumes that an ion pair is formed in the mobile phase prior to the sorption of the ion-pair complex into the stationary phase. The solute capacity factor is governed by the equilibrium constants for ion-pair formation in the mobile phase, extraction of the ion-pair complex into the stationary phase, and the dissociation of th p ion-pair complex in the... 

Two mechanisms for retention in reversed-phase ion-pair liquid chromatography have been considered. One is the adsorption of the hydrophobic paired ion on the hydrophobic surface of stationary phase material. In the second mechanism, the hydrophobic counter-ion is held on the surface of the hydro-phobic stationary phase, and the analyte ion is retained by ion-ion interactions, as shown in Figure 4.16. In the latter case, of a dynamic ion-exchange... 

On the other hand, the results of several systematic studies of the parameters which govern retention in this type of chromatography were consistent with the predictions of the model for dynamic ion-exchange. Knox and Laird (53) examined the effect of hetaeron concentration on the retention of sulfonic acids and related dyestuffs on a short alkyl silica (SAS) stationary phase. The hetaeron used was cetrimide (cetyltrimeth-... 

The experimental data conformed to Eq. (93) and therefore could be interpreted by either mechanism I or II data analysis showed no linear dependence of the logarithm of parameter C in Eq. (93).on the carbon number of the alkyl sulfate hetaerons. However, in the case of dynamic ion exchange parameter C is the binding constant of the hetaeron to the stationary phase hnd its logarithm should be linearly dependent on the carbon number of the alkyl moiety. Even if the results of this study are not accepted as support for ion-pairing (mechanism I) uniquely, they cannot be used to validate dynamic ion-exchange (mechanism II) either. 

As shown in Fig. 53 for the case of ion-pairing in the eluent and dynamic ligand exchange without expulsion, the addition of organic solvent will increase and decrease the retention factor at relatively low and high hetaeron concentration, and this is shown by points A and B, respectively. The opposite pattern obtains in the case of the dynamic ion-exchange mechanism, of course. At intermediate hetaeron concentrations... 

Solvent-generated (dynamic) ion-exchange chromatography, 242 Solvent gindienl, fomnttion of. 100 linear, 99 shape of, 98... 

Equations (51), (65), (66), and (71) together yield n expression for the re tention factor when mechanism 11, i.e., dynamic ion exchange, governs the chromatographic retention and it is given by... 

A particularly compelling argument for dynamic ion-exchange has put forward the observation that retention of anionic and cationic sample components increases and decreases with increasing concentration of a cationic hetaeron, respectively. Whereas anionic hetaerons are expected to promote the elution of anionic eluites and to enhance the retention of cationic eluites, the quantitative data presented in this regard (226) are not wholly consistent with the model since the hetaeron concentration at which the effect is half-maximal is different for anionic and cationic eluites. If the observed phenomena were due to the presence of bound hetaeron in both cases, the two effects would have identical dependence on the hetaeron concentration in the mobile phase. 

In contradistinction to these investigations the results by Horvath et al. 34) give sufficient support to the proposition that dynamic ion-exchange was not the dominant mechanism under their experimental conditions. 

For dynamic ion-exchange, it is proposed that the ion-pairing reagent, due to the hydrophobic portion, is adsorbed on the surface of the stationary phase originating in ion-exchange sites available for ion-exchange between its counterions and analytes [49,134-136]. 

For the dynamic ion-exchange reactor to work properly, it should satisfy the following features, as described in Section 6.11.1 for the case of adsorption. 

The application of natural zeolites in heavy metal removal is described now. The methodology consists of the development of a process for heavy metal removal from wastewater using dynamic ion exchange in natural zeolite columns [38,53],... 

For designing a canister system for heavy metal removal (see Figure 7.12) [38,53], a simple phenomenological description of dynamic ion exchange in zeolite bed reactors was worked out, which allows for the design of modular canister ion-exchange bed reactors for applications in heavy metal removal from wastewater. 

There are three popular hypotheses. Two models propose extreme situations and each encompasses a substantial amount of chromatographic data. These two proposals are the ion-pair model and the dynamic ion-exchange model. The third view, which is broader in scope than the previous two concepts, accommodates both the extreme views without combining the two models. This proposal is the ion-interaction model. 

An example of the application of dynamic ion-exchange chromatography for the direct separation of rare earths is shown in Fig. 1.22. The sample was a sodium hydroxide leach solution from an aluminium processing operation and contained high concentrations of sodium, iron and aluminium. Due to matrix interference, these solutions could not be accurately analysed by inductively coupled plasma emission spectroscopy. Fig. 1.22 shows the chromatogram when the sample was separated by dynamic ion-exchange... 

Model makers named the technique solvent generated ion exchange [7] and hydrophobic chromatography with dynamically coated stationary phase [8], thereby emphasizing a dynamic ion exchange model. 

Many subsequent stoichiometric mixed mode models are based on various combinations of these ion-pair and dynamic ion exchange extreme mechanisms. The effect of the IPR counter ion [13] and the reduction of available hydrophobic surfaces... 

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