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Dialysis is a process by which small molecules in a solution are separated from large molecules. The principle behind the process was discovered by the Scottish chemist Thomas Graham in about 1861. Graham found that the rate at which some substances, such as inorganic salts, pass through a semipermeable membrane is up to 50 times as great as the rate at which other substances, such as proteins, do so. We now know that such rate differences depend on the fact that the openings in semipermeable membranes are very nearly the size of atoms, ions, and small molecules. That makes possible the passage of such small particles while greatly restricting the passage of large particles.

In a typical dialysis experiment, a bag made of a semi-permeable membrane is filled with a solution to be dialyzed. The bag is then suspended in a stream of running Man with damaged kidneys on a dialysis machine. The blood is removed from his arm, passed through the machine, and returned to his body. © Giovaux Communication/Phototake NYC Reproduced with permission. water. Small particles in solution within the bag gradually diffuse across the semipermeable membrane and are carried away by the running water. Larger molecules are essentially retained within the bag. By this process, a highly efficient separation of substances can be achieved.

The kidney is a dialyzing organ. By the process described above, it filters waste products such as urea out of the blood and forces them into the urine, in which they are excreted from the body. Proteins and other important large molecules are retained in the blood.

A person whose kidneys have been damaged by disease or physical injury requires some artificial method for cleansing her or his blood. A device for carrying out this task–the artificial kidney machine–was developed in the early 1910s largely through the efforts of John J. Abel and his colleagues at the Johns Hopkins University. In the kidney machine, blood is removed from a person's arm, passed through a dialyzing system, and then returned to the patient. The machine functions much as a natural kidney does with one important exception. A natural kidney has a mechanism known as reverse dialysis for returning to the body certain small molecules (primarily glucose) that should not be excreted. The kidney machine is unable to do so, and glucose that it removes must be replaced by intravenous injection.

Electrodialysis is a form of dialysis in which the separation of ions from larger molecules is accelerated by the presence of an electrical field. In one arrangement, the solution to be dialyzed is placed between two other solutions, each containing an electrode. Cations within the middle solution are attracted to one electrode and anions to the other. Any large molecules in the middle solution remain where they are.

One possible application of electrodialysis is the desalination of water. In this procedure, sodium ions from seawater migrate to the cathode and chloride ions to the anode of an electrodialysis apparatus. Relatively pure water is left behind in the central compartment.

See also Osmosis.

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