Anion exchange dissociation constant

Aggett and Kadwani [13] report the development and application of a relatively simple anion exchange method for the speciation of arsenate, arsenite, monomethylarsonic acid and dimethylarsinic acid. As these four arsenic species are weak acids the dissociation constants of which are quite different it seemed that separation by anion exchange chromatography was both logical and possible. 

An especially challenging task is maintaining the selectivity of the method for separation of compounds whose elution time is very short, close to the dead time. In such cases, it is necessary to perform a preliminary review of the planned chromatographic conditions, including the composition of the analyzed material. For example, a typical eluent employed in anion-exchange chromatography (with pH of 8.5) is intended to facilitate the dissociation of separated compounds. Neglecting the time necessary to achieve acid/base equilibrium of substances loaded into the column in a neutral solution can result in their elution in the dead volume. The phenomenon is observed, for example, for MMA(V), whose consecutive dissociation constants are p/sTi 3.6 and p/sT2 8.22 [164]. 

Comparison of the gel filtration and the anion exchange methods The binding of ADP alone is quite identical when measured by the gel filtration method or the anion exchange separation (fig.l). The total number of sites (respectively 6.0A and 5.87) and the dissociation constant of low affinity sites (A5 and 52 pM) are similar in both cases (fig.2). That means that the binding of CFl on the DEAE column does not perturb the nucleotide fixation on CFl. 

Fig.2 Scatchard plots of ADP binding on CFl obtained by gel filtration ( ) or by anion exchange chromatography (o). f=number of moles of ADP bound per mole of CFl, A=free ligand molar concentration. The dissociation constants for low affinity sites, measured by the two methods are respectively A5 and 52 p Moles, and the total number of sites are 6.0A (5.3, extrapolated from the Scatchard plot + 0.7A, endogenous ADP) and 5.87 (A.72 + 1.15).

The same comparison between gel filtration and anion exchange has been performed with ATP. Scattering of the data precludes a precise determination of the dissociation constant but it is clear that it is greater than that corresponding to ADP (/xl25 pM). 

The subscripts ieS and ieM refer to the ion-exchange equilibria at the solution-stationary phase and the solution-micelle interface, respectively. The [C] concentrations are the counter anion concentration in the aqueous phase, aq, including added salts, and the one on the stationary phase, s. is the micellar counterion dissociation constant, (j) is the column phase ratio, [M] is the micellar concentration and k is the anion retention fector. Equation 13.6 obtained from ion-exchange equilibria resembles the classical Armstrong-Nome equation. The Pwm Partition coefficient of eq. 13.6 can be related to the KjeM constant by [30] ... 

Functional ISEs based on charged carriers can be fabricated with membranes that contain just the salt of a charged ionophore, since the ionophore has both ionophoric and ion-exchanger properties. However, it has been shown that the corresponding sensing selectivities are then often less than ideal [40]. Consider, for example, a membrane with a charged ionophore selective for a monovalent anion. The concentration of uncomplexed ionophore in the membrane is ordinarily small and dictated by the dissociation constant of the complex ... 

In general, the following model parameters need to be independently determined to develop a thorough understanding of LLPTC kinetics dissociation constant in aqueous phase mass transfer coefficient for QX and QY and intrinsic kinetics of the ion-exchange and organic phase reactions and anion selectivity ratio of the quat. 

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