Dialysis
Definition and meaning of Dialysis in chemistry.
Dialysis is a separation technique that uses a semipermeable membrane to remove small dissolved ions and molecules from a solution while retaining larger colloidal particles, such as proteins or other macromolecules.
In more detail
The membrane's pores are large enough to let small solutes diffuse through into a surrounding wash solution but too small to pass colloidal particles, which are much larger than true solution species. Diffusion continues, driven by the concentration gradient, until small-solute levels equalize across the membrane, so the outer solution is periodically replaced or continuously flowed to keep drawing impurities out. This principle is central to purifying biological macromolecules in the lab and underlies hemodialysis, which removes urea, excess ions, and other small waste solutes from the blood of patients with kidney failure.
Key facts
| Field | Physical Chemistry |
|---|---|
| Membrane pore size | ~1-10 nm (excludes colloids, passes small ions/molecules) |
| Driving force | Diffusion down a concentration gradient |
| Key application | Hemodialysis (blood purification in kidney failure) |
To desalt a protein solution after ammonium sulfate precipitation, a biochemist seals it inside semipermeable dialysis tubing and submerges the bag in a large volume of buffer; sulfate and ammonium ions diffuse out through the membrane over several hours while the much larger protein molecules remain trapped inside.
Frequently asked questions
How does dialysis differ from filtration?
Filtration uses pressure to force a mixture through a filter that physically blocks particles above a size cutoff, whereas dialysis relies solely on diffusion of small solutes across a semipermeable membrane down a concentration gradient, with no applied pressure.
Why do colloidal particles stay behind during dialysis?
Colloidal particles are far larger than true solution-phase ions or small molecules, so they cannot pass through the membrane's pores and remain on the original side while small solutes diffuse out.