Dialysis Donnan effect

In experiments where the ligand and macromolecule are charged, the equilibrium may be affected by the Donnan potential of the charged macromolecule, which is confined to one compartment. Donnan effects can be minimised by carrying out the dialysis in sufficiently high ionic strength media, such as in the presence of 0.1 M NaCl or KC1. 

Dialysis, or ordinary dialysis as discussed in this section, is referred to as the diffusion of neutral molecules. If electrolytes are separated with neutral membranes or with charged membranes, then Donnan effects arising from the unequal distribution of ions, interfere with the normal dialysis process. This type of dialysis is called Donnan dialysis or diffusion dialysis and these processes will be described in section VI.4.6. 

When a nondiffusible component (e.g., protein) is at dialysis equilibrium in a medium containing two or more diffusible components, then the molalities of the diffusible components on the two sides of the dialysis membrane are not necessarily the same (e.g., due to Donnan effects, preferential binding, etc.). But, though at equilibrium the mass ratios of the diffusible components may differ, the chemical potentials, p, of each component are equal in the two phases. Then, a plot of the densities of a series of solutions vs. C2 yields another limiting slope, namely (Bp/Bci). ... 

Winzor and coworkers have employed measurements of the Donnan distribution of small ions in dialysis equilibrium [14] to reinforce earlier evidence of charge-screening effects in polysaccharide anions [164,165]. These researchers used the absorption optical system of a Beckman XL-1 ultracentrifuge to monitor the distribution of ions in polysaccharide solutions... 

Mapleson WW (1987) Computation of the effect of Donnan equilibrium on pH in equilibrium dialysis. J Pharmacol Methods 17(3) 231-242... 

Donnan dialysis is a membrane separation process that uses ion-selective membranes to prevent the flow of certain ions from one solution to another. A schematic of the process is presented in Figure 29.8. When a salt solution is separated from its corresponding acid by a cation-exchange membrane, the anions are excluded from the membrane, whUe the cations are redistributed across the membrane to attain Donnan equilibrium. By changing the salt solution periodically, it would be possible to shift the equilibrium favorably to effect simultaneous neutralization of acid on one side of the membrane (feed compartment) and acid recovery on the other side (receiver compartment). The driving force for ion migration is the chemical potential gradient for the cation across the membranes. 

Donnan dialysis has been exploited in flow systems with flame atomic absorption spectrometry and ICP-OES, for which remarkable analytical figures of merit, including high enrichment factors, have been reported [283—285]. This type of dialysis is less compatible with flow-based spectrophotometric determinations, as the inherent concentration gradients can give rise to the occurrence of the Schlieren effect. 

This method has been widely and traditionally used for the study of the interaction of proteins in solution with smaller molecules and ions including surfactants. The principle rests on the fact that a membrane can be chosen such that, while the macromolecule is contained in the dialysis bag made from the membrane, the smaller molecule (and water) can move freely between the bag and the solution in the outer container. When equilibrium is established, analysis reveals the degree of binding of the smaller molecule by the macromolecule. [One complication is the Donnan membrane effect (11,12) but this can be overcome if the measurements are done in the presence of salt, e.g., 0.1 M NaCL]... 

In cases where steady-state conditions are achieved, Donnan dialysis can be effectively integrated with Hg(II) bioreduction to Hg(0) using mixed microbial cultures. While many organisms have been shown to perform Hg(II) bioreduction in pure culture, mixed microbial cultures offer the advantage of not requiring aseptic operational conditions, and can thus be much more readily integrated with the lEMB process, where aseptic conditions would involve additional operational costs and complicate the process unnecessarily. Efficient Hg(II) bioreducing mixed cultures have... 

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