Buffer

In chemistry, a buffer is a system, usually an aqueous (water) solution, that resists having its pH changed when an acid or a base is added to it.

Normally, the addition of acid to a solution will lower its pH and the addition of a base will raise its pH. If the solution is a buffer, however, its pH will be changed to a much lesser extent than would be expected from the amounts of acid or base that are added. Socalled "buffered aspirin" is not really a buffer, because it does not resist acids and bases. It is simply aspirin combined with a basic compound, such as magnesium carbonate or aluminum hydroxide, which neutralizes some stomach acid.

Almost all chemical reactions that take place in aqueous solution—meaning almost all chemical reactions— are sensitive to the concentrations of hydrogen ions and hydroxide ions, that is, to the pH of the solution. This is because hydrogen and hydroxide ions are the ions of water itself. In particular, many biochemical processes essential to life are quite sensitive to the acidities of various body fluids. A variety of natural buffer systems keep the body's pH values within the limits that are necessary for health. For example, a system of several buffers holds the pH of human blood between 7.33 and 7.43 in a healthy person. A blood pH below 7.0 or above 7.8 can be fatal.

There are two common kinds of buffer solutions: solutions that contain a weak acid plus one of its salts (e.g., acetic acid plus sodium acetate) and solutions that contain a weak base plus one of its salts (e.g., ammonia plus ammonium chloride). Their workings can be understood in terms of LeChâtelier's principle.