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Dialysis is the separation of colloids from dissolved ions or molecules of small dimensions, or crystalloid, in a solution. A colloid is any substance that is made of particles that are of an extremely small size: larger than atoms but generally have the size of 10-7 cm ranging to 10-3 cm. A crystalloid is a substance that has some or all of the properties of a crystal or a substance that forms a true solution and diffuses through a membrane by dialysis. Dialysis is a process that is like osmosis. Osmosis is the process in which there is a diffusion of a solvent through a semipermeable membrane.


In 1861, chemist Thomas Graham used the process of dialysis, a process used to separate colloidal particles from dissolved ions or molecules. Thomas Graham is also responsible for developing Graham's Law. Dialysis is possible because of the unequal rates of diffusion through a semipermeable membrane. A semipermeable membrane is a membrane that lets some molecules to pass through it while not letting others. Examples of semipermeable membranes include parchment and cellophane.

Figure 2: Graphic showing the diffusion of solutes across a membrane during dialysis. Image used with permision from wikipedia (Potcherboy)

Another way to think of a semipermeable membrane is to think of a net like object that traps larger objects, but lets smaller object pass through because they can pass through the holes in the net.


Video 1: Osmotic Pressure Dialysis Tubing

When a colloidal mixture is places in a semipermeable membrane, which is then placed in an aqueous solution or pure water, dissolved ions and small molecules are allowed to pass through this membrane. This causes colloidal particles to stay in the membrane, because these particles are unable to pass through the small pores of the membrane.

What is the rate of dialysis ?

Dialysis is not a quick process; the rate of dialysis depends on the speed of the unequal diffusion rates between the crystalloids and the colloids and the differences in particle size. The rate of dialysis can be changed through heating, or if the crystalloids are charged, then applying an electric field, called electrodialysis. Electrodialysis is the type of dialysis in which electrodes are placed on the sides of the membrane. In this way, positive ions can pass through one side of this membrane while the negatively charged ions can pass through the other side of the membrane. This causes acceleration in the process of dialysis.

What is an example of dialysis in our world ?

Hemodialysis is a method in which kidney failure is treated with the process of dialysis. In hemodialysis, blood is removed, purified through dialysis, and returned to the bloodstream. In kidney failure, there is a retention of salts and water, urea, and metabolic acids. The patient is then connected to a dialysis machine, which is also called a hemodialyzer. The blood flows through small channels made of semipermeable membranes. The dissolved substances like urea and salts pass through a sterile solution. Compounds like sugar and amino acids are added to the sterile solution. The dialysis solution is on the other side of the membranes, and the molecules flow through the membranes. The molecules diffuse from a higher concentration to low concentration area. The concentrations of molecules needed to be removed from the blood are zero in the dialysis fluid.


Figure 3: (left) Scheme of semipermeable membrane during hemodialysis, where blood is red, dialysing fluid is blue, and the membrane is yellow. Image used with permission from Wikipedia (Freemesm). (right) Hemodialysis setup. staff. "Blausen gallery 2014". Wikiversity Journal of Medicine. DOI:10.15347/wjm/2014.010. ISSN 20018762. See a full animation of this medical topic.

The process of hemodialysis helps many patients who have kidney failure because a person who suffers from kidney failure are at great risk, because someone who has complete kidney failure will need a kidney transplant within two weeks, or else he/she will face death. Between the time that the person finds a suitable kidney to be transplanted, the hemodialyzer comes into great help in facing the fight against death.


  1. Petrucci, et al. General Chemistry Principles & Modern Applications. 9th ed. Upper Saddle River, NJ: Pearson Prentice Hall, 2007.
  2. Sadava, et al. Life: The Science of Biology. 8th ed. New York, NY. W.H. Freeman and Company, 2007.


  • Katrina Lau (UCD)