Buffers are usually made up of a weak acid and its conjugate base or salt, or a weak base and its conjugate acid. A conjugate acid-base pair is a pair of molecules that differ only in having one single hydrogen ion more or less. Buffers made of specific conjugate acid-base pairs serve to buffer solutions at specific pH ranges.
pH is changed by the addition or removal of hydrogen ions--also called protons--or hydroxide ions. When protons are added to a buffered solution, some of the base is automatically converted to the weak acid part of the buffer. This process essentially uses up most of the added protons. Conversely, when hydroxide ions are added, which removes protons, some of the weak acid of the buffer is converted to the base by the release of protons, which replenish the balance of protons in the solution and so again, no major change in pH, the ratio of protons to hydroxide ions in the solution, takes place.
Some examples of common and important buffers are carbonic-acid-bicarbonate, which is the main buffer in human blood that helps to keep pH around 7.4, acid-potassium-phosphate, which helps to maintain consistent pH in the internal fluid of all cells, hydrochloric-acid-potassium-chloride, which is used to buffer solutions with a very acid pH of 1 to 2.2 and glycine-sodium-chloride, used to buffer solutions with an alkali pH of 8.6 to 10.6.