Buffer

Common weak acid buffer systems are based upon carbonic acid (H₂CO₃), citric acid (H₃C₆H₅O), and phosphoric acid (H₃PO₄). Common weak base buffer systems are based upon amines (organic bases) or amino acids, which can act as both acids and bases.

A buffer solution based upon a dibasic or tribasic acid (an acid that can produce two or three hydrogen ions per molecule) may be made from two different ions of the same acid, rather than from the acid itself and one salt ion. For example, the phosphoric acid ions H₂PO^- 4 and HPO^2- 4 can form what is known as the phosphate buffer system, which is one of the buffers that control the pH of human blood. In this system the H₂PO ^– 4 ion plays the role of the weak acid and the HPO^2- 4 ion plays the role of its salt. The relevant equilibrium is

The main buffer that is involved in controlling blood pH, however, is the carbonate system, which is based on the following equilibrium:

The carbonic acid in the blood comes from dissolved carbon dioxide:

Our breathing (oxygen in, carbon dioxide out) controls the amount of carbon dioxide that is available to dissolve in the bloodstream. Therefore, our lungs also play an important part in controlling our blood's pH through the carbonate buffer system.