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dialysis (dīălˈĭsĭs), 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.

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The following article is from The Great Soviet Encyclopedia (1979). It might be outdated or ideologically biased.



the removal of impurities of low-molecular-weight substances from colloidal systems and solutions of macromolecular compounds by means of semipermeable membranes, or partitions, which pass small molecules and ions but hold back colloidal particles and macromolecules.

The simplest device for dialysis, a dialyzer, is a pouch or sleeve made of semipermeable material that is filled with the liquid to be purified and immersed in a solvent (a dispersion medium). Cylindrical vessels with a semipermeable membrane instead of a bottom are often used in place of a pouch. The membranes are made of collodion, cellophane, animal and plant membranes, and synthetic materials. Diffusion processes are the basis of dialysis; therefore, it proceeds very slowly. Dialysis may be accelerated by increasing the ratio of the membrane area to the volume of the liquid to be dialyzed, by increasing the temperature, by agitation, by creating a pressure difference across the membrane, and by frequent or continuous replacement of the solvent into which the ions or molecules of the low-molecular-weight substance pass (diffuse) through the membrane.

Dialysis in an electric field (electrodialysis) accelerates the removal of electrolytes from dialyzed systems by dozens of times. A simple electrodialyzer consists of three chambers separated by membranes. The liquid to be purified is poured into the central chamber, and electrodes immersed in solvent are located in the side flow chambers. The ions in the stationary electrical field move directly to the corresponding electrodes, penetrating the membrane from the central chamber into the side chambers. Electrodialysis is particularly efficient when ionite membranes made of ion-exchange materials are used. Depending on the sign of the electrical charge on their surface, the membranes pass either cations or anions. Multichamber electrodialyzers with ionite membranes are used in hydrometallurgy, in the atomic industry (to treat effluents, concentrate saline solutions, and separate elements close in properties), and in the desalinization of seawater.

Dialysis and electrodialysis are used in many industrial processes, in physicochemical and biological research, and in medicine. The dialysis method, which was called vivi-diffusion, was used in 1913 by the American scientist D. Abel to study the components of the blood of a living organism. The animal’s blood passed from an artery into a vein through colloidal tubes placed in a glass cylinder filled with physiologic solution. Abel’s unit was the basis for the design of the artificial kidney, which is used for hemodialysis.


Voiutskii, S. S. Kurs kolloidnoi khimii. Moscow, 1964.
Elektrokineticheskie svoistva kapilliarnykh sistem. Moscow-Leningrad, 1956.
Perry, J. H. Spravochnik inzhenera-khimika, vol. 1. Leningrad, 1969. Page 624. (Translated from English.)
Demineralizatsiia metodom elektrodializa (ionitovye membrany). Moscow, 1963. (Translated from English.)
The Great Soviet Encyclopedia, 3rd Edition (1970-1979). © 2010 The Gale Group, Inc. All rights reserved.


(physical chemistry)
A process of selective diffusion through a membrane; usually used to separate low-molecular-weight solutes which diffuse through the membrane from the colloidal and high-molecular-weight solutes which do not.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


1. the separation of small molecules from large molecules and colloids in a solution by the selective diffusion of the small molecules through a semipermeable membrane
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
Almost all patients on PD at the authors' facility started on CAPD. A few were on other PD modalities for personal or other reasons when starting in the unit; they were not included in the CAPD database but were included in the total PD database.
The creatinine concentrations for the groups of healthy subjects and CAPD and HD patients were respectively 80 [+ or -] 25, 646 [+ or -] 244, and 914 [+ or -] 173 /.mol/L, whereas the As concentrations in the three groups were respectively 0.96 [+ or -] 1.52, 4.67 [+ or -] 5.41, and 6.47 [+ or -] 4.28 [micro]g/L.
Nearly 2,000 surgeons across the United States are already generating reports with the Nuance Surgical CAPD solution today, including North Mississippi Ambulatory Surgical Center in Tupelo, Miss.
CAPD was performed in 45 patients diagnosed with CKD Stage 5 with 27 (60%) boys and 18 (40%) girls (male to female ratio = 1.5:1.0).
Among the patients who were on PD, the HRQOL scores were lower in most domains in the APD group when compared with the CAPD group.
The patient was a 53 year-old Punjabi man with hypertension, hepatitis C and end stage renal disease (ESRD) of unknown cause opted for CAPD. During his follow-up, he was noted to have poor effluent outflow with retention of dialysate about 50 to 700ml per day with progressive dyspnoea.
Only after that guidewire-directed CAPD catheter manipulation failed, did we decide to proceed with an explorative video-laparoscopy.
Nine years of CAPD may have produced structural changes in and loss of function of the peritoneal membrane.
Lastly, PD-specific data extracted was system type (Baxter or Fresenius), length of time on PD, type of PD regimen (CAPD or APD), membrane type, dialysis adequacy, number of episodes of peritonitis, number of episodes of exit-site infection, type and number of PD complications (leaks, hernia, blockage), and reasons for leaving PD.
Media diaria de: Sesiones totales, de sesiones de pacientes agudos, de tecnicas de plasmaferesis, de intercambios de CAPD y de HD en otros servicios del hospital, en ambos periodos.
Meanwhile, the department of Consumer Affairs and Public Distribution (CAPD) says that it's monitoring the rates of all eatables being sold in markets.