The fourth colligative
property is osmotic
pressure, and it is useful in molecular-weight
determinations when freezing point depressions are not.
Many membranes have pores large enough to let some molecules
pass through, but too small to pass others. These are known
membranes. Some will permit water to pass, but not
ions or salts. Others, with larger pores, will be permeable
to water, salts, and small molecules, but not to protein molecules
with molecular weights in the thousands.
This selective passage of ions
and small molecules but not proteins is called dialysis,
and is a common biochemical method of separation and purification.
Our kidneys essentially are fine networks of dialysis tubing,
excreting liquids, salts, and small waste molecules, but at
the same time preventing the loss of proteins from body fluids.
Artificial kidney machines simulate this blood-purification
process with man-made dialysis tubing, in which the blood
from the patient flows across one side of semi-permeable membranes,
and wash fluid flows across the other.
Picture of a dialysis machine