Selectivity in ion exchange

Analyte retention and selectivity in ion-exchange chromatography are strongly dependent on the pH and ionic strength of the mobile phase. Basic principles of the ion-exchange HPLC are discussed in Chapter 4, Section 4.10. 

Graphical integration methods are used to evaluate equations 5.26 and 5.27 which thus establish a scale for selectivity in ion exchangers. A typical selectivity series for some common cations on a sulfonic acid resin exchanging against Li is shown in Table 5.2. 

The first selection in ion-exchange methods development is to choose either cation or anion exchange, which is decided by the charge on the analyte (Fig. 6.8). First, check the pAT of the solute and determine whether it can be a positive, negative, or a neutral species. Examples of several different compounds are shown in Figure 6.8. Compounds such as paraquat or diquat (herbicides) are always protonated thus, they will always bind by cation exchange. On the other hand, the triazine, prometryn, has a pAT of 4.8 thus, the molecule will be 50% protonated at pH 4.8, and totally protonated at pH 2.8. At pH 6.8 it is a neutral species. Thus, it is possible to isolate this compound effectively by cation exchange only when the pH is acidic. 

Brief mention should be made at this point of the approach made by Eisenman [13] to explain observed selectivities in ion exchange systems. He noted that ... 

Selectivity in ion-exchange chromatography may be defined as the relative tendency of sample ions to form an ion pair with the sites of opposite charge within the stationary phase. It is helpful to divide the factors that affect selectivity into two general classes. 

The ability of living organisms to differentiate between the chemically similar sodium and potassium ions must depend upon some difference between these two ions in aqueous solution. Essentially, this difference is one of size of the hydrated ions, which in turn means a difference in the force of electrostatic (coulombic) attraction between the hydrated cation and a negatively-charged site in the cell membrane thus a site may be able to accept the smaller ion Na (aq) and reject the larger K (aq). This same mechanism of selectivity operates in other ion-selection processes, notably in ion-exchange resins. 

Table 2. Selectivity of ion Exchange Resins in Order of Decreasing Preference.

The thermodynamic analysis of the selectivity of ion exchange with the participation of ions of quaternary ammonium bases [56--58] has shown that an increase in bonding selectivity, when metal ions are replaced by organic ions, which is usually accompanied by an increase in entropy of the system (Table 5). It follows from Table 5 that a drastic increase in bonding selectivity upon passing to a triethylbenzylammonium counterion (the most complex ion) is due to a considerable increase in the entropy of the system. 

In accordance with Eq. (3.4) or Eq. (3.6), the concentration selectivity of ion exchange is variable depending on the degree of ideality of the solution and CP phase. For dilute solutions at a constant ionic strength, it is possible to take into account as a variable only the degree of non-ideality of the CP phase. For the systems considered here, it is convenient to study the effect of the molar fraction of organic counterions (NJ on the concentration selectivity constant. Fig. 14 shows the dependences of Ks on the molar fraction of oxytetracycline in CP. For CP... 

Samsanov GV (ed) (1968) Selectivity of Ion-Exchange Sorption of Antibiotics, (in Russian) Transaction of Leningrad Chemico-Pharmaceutical Institute, Leningrad... 

Breadmore, M. C., Macka, M., and Haddad, P. R., Manipulation of separation selectivity for alkali metals and ammonium in ion-exchange capillary electrochromatography using a suspension of cation exchange particles in the electrolyte as a pseudo stationary phase, Electrophoresis, 20, 1987, 1999. 

Electrodialysis units recover plating chemicals differently from the recovery units discussed thus far. In electrodialysis, electromotive forces selectively drive metal ions through an ion-selective membrane (in RO, pressure is the driving force in ion exchange, the driving force is chemical attraction). The membranes are thin sheets of plastic material with either anionic or cationic characteristics.33... 

V.V. Egorov, E.M. Rakhman ko, E.B. Okaev, E.V. Pomelenok, and V.A. Nazarov, Effects of ion association of lipophilic quaternary ammonium salts in ion-exchange and potentiometric selectivity. Talanta 63,119-130 (2004). 

Rinsing, for electroplating, 9 786-787 in ion exchange, 24 412 Rio Declaration, 24 162-163, 185, 188 selected principles from, 24 194-196 Riot control agents, 5 814, 823-824 Ripeners, in photographic crystal growth, 29 182... 

R.R. Kunz, W.F. Dinatale and P. Becotte-Haigh, Comparison of detection selectivity in ion mobility spectrometry proton-attachment versus electron exchange ionization, Int. J. Mass Spectrom., 226 (2003) 379-395. A.M. DeTulleo, P.B. Galat and M.E. Gay, Detecting heroin in the presence of cocaine using ion mobility spectrometry, Int. J. Ion Mobility Spectrom., 1 (2000) 38-42. 

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