= Dialysis
{disambiguate=biochemistry}
{wiki}
= Dialysis
{synonym}
<Dialysis> exchanges small <solutes> between a sample and an external bath through a <semipermeable membrane>, while retaining suitably large <macromolecules>, such as <proteins>. The transport is passive <diffusion>: random motion carries permeant <molecules> both ways, with net transport down their <concentration> <gradient>. The bath is the <dialysate>. At equal permeant <solute> <concentrations>, microscopic exchange continues but net exchange stops.<Dialysis: an overview, Pierce Biotechnology technical note>[Dialysis: an overview, Tech Tip 20, p. 1, Dialysis is Diffusion Through a Semi-Permeable Membrane]{ref}
For example, a <protein> sample containing excess <sodium chloride> can lose the small <ions> to a low-<salt> <dialysate> without deliberately transferring the <protein>. Conversely, components of a new <buffer solution> can enter from the bath. This is useful before studying <enzyme kinetics>: changing the surrounding <chemical solution> can remove an interfering small <solute> while keeping the <enzyme> available for the next experiment.
\Image[/biochemical-dialysis-membrane-exchange.png]
{title=Small <salt> <ions> exchange across a <semipermeable membrane> while a <protein> is retained}
{description=Original schematic of a sample bag immersed in a <dialysate>, with an enlarged membrane region. Arrows indicate exchange in both directions and an initially outward net <diffusive flux>. Symbols and openings are not to scale; the drawn gap is not a literal <molecular weight cut-off> measurement. Replacing the bath removes escaped <salt> and renews the <concentration> <gradient>.}
The striking feature is that the retained <protein> does not have to be captured and released again to change its surroundings. However, retention is conditional on the <semipermeable membrane> and the particular <macromolecule>, and <dialysis> is poorly suited to separating <proteins> of nearly the same size. The <molecular weight cut-off> is a nominal retention rating, not a sharp geometrical boundary.
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