dialysis (dīăl´ĬsĬs), in chemistry, transfer of solute (dissolved solids) across a semipermeable membrane. Strictly speaking, dialysis refers only to the transfer of the solute; transfer of the solvent is called osmosis. Dialysis is frequently used to separate different components of a solution. For example, a solution of starch and sodium chloride in water can be separated by placing the mixture in a vessel on one side of a semipermeable membrane and placing pure water on the other side. The smaller particles of sodium chloride (which dissolve in water to form sodium and chloride ions) will diffuse across the membrane; diffusion of the much larger starch particles (which are not truly in solution but are in colloidal suspension) is hindered and may be completely prevented. By continuously or periodically replacing the solvent with fresh solvent, almost all of the sodium chloride can be removed. The method was originated by Thomas Graham, who termed the substance that remained within the membrane a colloid and the substance that diffused a crystalloid.
An extension of the method makes possible the separation of mixed colloids by the use of a semipermeable membrane (usually synthetic) of known selectivity, i.e., one that will permit the diffusion of one colloid and hinder the diffusion of others. Mixed macromolecules, such as proteins, may be similarly separated. By the use of graded semipermeable membranes chosen to allow successively smaller molecules to pass, mixtures can be separated into components of graded ranges of molecular weight.
Artificial kidney machines have been developed that make use of dialysis to purify the blood of persons whose kidneys have ceased to function. Known as hemodialysis, this procedure has saved the lives of many persons suffering from renal failure. In such machines, blood is circulated on one side of a semipermeable membrane (often cellophane) while a special dialysis fluid is circulated on the other side. The dialysis fluid must be a solution that closely matches the chemical composition of the blood. Metabolic waste products such as urea and creatinine diffuse through the membrane into the dialysis fluid and are discarded, while loss by diffusion of substances necessary to the body (such as sodium chloride) is prevented by their presence in the dialysis fluid.
In peritoneal hemodialysis, the dialysis fluid is introduced into the abdominal cavity. Waste products leach from the blood vessels into the fluid, which is later drained from the patient. Home peritoneal dialysis machines that release patients from dependence on hospital dialysis (usually three 4-hr visits weekly) have been available since the 1980s. See diffusion.