- Buffers are chemicals that can reversibly bind free hydrogen ions in fluid solutions and in doing so prevent, or "buffer", against changes in the fluid's pH.
- Buffers exist in both hydrogen-bound and hydrogen unbound forms. The hydrogen bound forms of buffers, notated as "H-B", are typically uncharged and termed "Weak Acids" due to their capacity to release a free hydrogen ion into solution. The hydrogen unbound forms of buffers, notated as "B-", are typically negatively charged and termed "Weak Bases" due to their capacity to absorb a free hydrogen ion from solution. Given these features the typical chemical equation for a buffer absorbing or releasing a free hydrogen ion in solution can be written as follows:
- B- + H+ ↔ H-B
- Buffers prevent large swings in the pH of solutions by absorbing or releasing free hydrogen ions as needed. For example, if free hydrogen ions are added to the solution, say by injection of a strong acid, the B- form of the buffer will simply absorb most of the added free H+ ions, generating more H-B form, and thus preventing a sharp rise in the free H+ concentration (i.e. drop in pH).
- In contrast, if free hydrogen ions are removed from the solution, say by injecting a strong base, the H-B form of the buffer will dissociate, releasing free H+ ions and generating more of the B- form, thus preventing a sharp drop in the free H+ concentration (i.e. rise in pH).
- Given these properties of buffers, it should be intuitively clear that buffers are most capable of preventing changes in the pH of a solution, in either direction, when there is an equal ratio of the B- (Weak Base) and H-B forms (Weak Acid) of the buffer in the solution. The pH of a solution at which buffers display equal ratios of their Weak Acid and Weak Base forms is termed the "pK" of the buffer and is dependent on the specific chemical features of the buffer. This property of buffers is discussed further on the Henderson-Hasselbalch Equation page.