Pair Equilibria

Ion Pair Equilibria.—The above situation is merely one example of the gradation in structure or electron distribution which can occur in any chemical bond, A—B. Nowadays it is generally recognized that any organic salt, may 

In the case of cationic polymerization A+ would represent the initiating or propagating species and B would be the counterion, whereas in anionic polymerization B would be the reactive entity and A+ would be the metallic 

Ledwith and D. C. Sherrington, in Reactivity, Mechanism and Structure in Polymer Chemistry , ed. A. D. Jenkins and A. Ledwith, Wiley, London, 1974, p. 383. 

From a mechanistic point of view considerable interest has been shown in evaluating the contribution of each of these species, especially free ion and the various ion-paired entities in propagation reactions, particularly since the realization that each may have a quite different reactivity. The degree of association of the counterion can also have a profound effect on the stereochemistry of the propagation reaction, and hence the involvement of the gegenion will constitute an important common theme in the discussion of each type of polymerization reaction. 

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Ledwith, A. and Sherrington, D. C. Stable Organic Cation Salts Ion Pair Equilibria and Use in Cationic Polymerization. Vol. 19, pp. 1 — 56. 

On the other hand, optionally added co-ions of the eluent may also interfere with the ion-exchange process through competitive ion-pairing equilibria in the mobile phase. The effect of various amines added as co-ions to the polar-organic mobile phase was systematically studied by Xiong et al. [47]. While retention factors of 9-fluorenylmethoxycarbonyl (FMOC)-amino acids were indeed affected by the type of co-ion, enantioselectivities a and resolution values Rs remained nearly constant. For example, retention factors k for FMOC-Met decreased from 17.4 to 9.8 in the order... 

Even with activity coefficients, we are always limited by chemistry that we do not know. In the mixture of sodium hydrogen tartrate (Na+HT ), pyridinium chloride (PyH+Cl ), and KOH, several possible ion-pair equilibria are... 

From the ion-pair equilibria, we write NaT ] = A-NaT Na 11T2 1 and [NaHT] = ArNaHT[Na+l[HT-]. With these substitutions for [NaT ] and [NaHT] in the mass balance for sodium, we find an expression for [Na+] ... 

The new fractional composition equations are somewhat messy, so we reserve this case for Problem 13-18. The end result is that adding the ion-pair equilibria 13-12 and 13-13 changes the calculated pH from 4.30 to 4.26. This change is not large, so neglecting ion pairs with small equilibrium constants does not lead to serious error. We find that 7% of sodium is tied up in ion pairs. Our ability to compute the distribution of species in a solution is limited by our knowledge of relevant equilibria. 

Even in such a simple solution as KH2P04 plus Na2HP04, for which we were justifiably proud of computing an accurate pH, we have overlooked numerous ion-pair equilibria ... 

Ion pairing in acid-base systems. This problem incorporates ion-pair equilibria 13-12 and 13-13 into the acid-base chemistry of Section 13-1. 

A. Ledwith and D. C. Sherrington C. Conductance and Ion Pair Equilibria C.l. Theoretical Considerations... 

Stable Organic Cation Salts Ion Pair Equilibria and Use in... 

Acid-base equilibria in aqueous solutions are a source of enormous possibilities in terms of altered (increased or decreased) reactivity for practical purposes or as a tool in the mechanistic toolbox. The same is true for coordination and ion-pairing equilibria involving typically cationic metal complexes and anions, although oppositely charged reactants can also be involved. 

The BMREP and SDM currently use the Davies technique for activity coefficient prediction. The Davies technique is a combination of the extended Debye-Huckel equation (6) and the Davies equation (7). The Davies technique (and hence both equilibrium models) is accurate up to ionic strengths of 0.2 molal and may be used for practical calculations up to ionic strengths of 1 molal (8). Ion-pair equilibria are incorporated for species that associate (e.g., 1-2 and 2-2 electrolytes). The activity coefficients (y ) are calculated as a simple function of ionic strength (I) and are represented as ... 

The LCM is a semi-theoretical model with a minimum number of adjustable parameters and is based on the Non-Random Two Liquid (NRTL) model for nonelectrolytes (20). The LCM does not have the inherent drawbacks of virial-expansion type equations as the modified Pitzer, and it proved to be more accurate than the Bromley method. Some advantages of the LCM are that the binary parameters are well defined, have weak temperature dependence, and can be regressed from various thermodynamic data sources. Additionally, the LCM does not require ion-pair equilibria to correct for activity coefficient prediction at higher ionic strengths. Thus, the LCM avoids defining, and ultimately solving, ion-pair activity coefficients and equilibrium expressions necessary in the Davies technique. Overall, the LCM appears to be the most suitable activity coefficient technique for aqueous solutions used in FGD hence, a data base and methods to use the LCM were developed. 

Figure 1 demonstrates the necessity of ion-pair equilibria to insure an accurate activity coefficient prediction in the low concentration range (0-0.2 molal) for salts that associate. 

The importance of solvent parameters such as DN and AN and the advantage of their use over physical-electrostatic parameters was further demonstrated by Mayer et al. [21], who studied correlations between the DN and AN of solvents and redox potentials and their chemical equilibrium and ion pair equilibria. According to the Born theory, redox potentials should depend linearly on the reciprocal of the solvent s dielectric constant. Plotting Em values of a redox such as Cd/Cd2+ versus 1/e of the solvents in which it is measured results in a very scattered picture. In contrast, it has been clearly shown by Mayer et al. [15] that redox potentials of metals (e.g., Zn/Zn2+, Cd/Cd2+, Eu/Eu2+) can be nicely... 

This article reviews the experimental evidence, accumulated in our laboratory, relating to different mechanisms involved in reactions of trityl and tropylium ions with reactive olefins and cyclic ethers and outlines the probable effect of ion pair equilibria on these reactions. 

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