An example of the Henderson-Hasselbalch equation is to determine the pH of a buffered solution containing acetic acid and its salt, sodium acetate. Another example is to determine the ratio between the concentration of sodium acetate and acetic acid necessary to create a buffer with a pH of 5.0. All examples of the Henderson-Hasselbach equation are concerned with determining how to compose a buffered solution with a set pH value.
A buffer is a solution consisting of an acid or a base along with its salt. A buffer's purpose is to resist changes in its pH by neutralizing small amounts of other acids or bases that are added to the solution. This is possible because if the solution is prepared using an acid, then its salt acts as a base and vice versa.
Previously, the calculations used to prepare buffered solutions were complex and inefficient. By rearranging the expression that describes the rate that a weak acid or base dissociates into its components, the Henderson-Hasselbalch equation created a simplified method for the approximation of the pH of buffered solutions. Its general form is that the pH of the solution is equal to the negative logarithm of the acid dissociation constant plus the logarithm of the ratio between the acid concentration and the salt concentration.