Strength of Acids

The ability of an acid to release hydrogen ions is specified by its strength. A strong acid (for example, sulfuric or hydrochloric acid) dissociates (breaks up) completely, or nearly completely, into positive and negative ions in a dilute (nonconcentrated) water solution. Only a small percentage of a weak acid (for example, acetic acid), on the other hand, forms ions in solution, the major portion of the acid remaining in the form of molecules. As a general rule, inorganic acids are stronger than organic acids. The formation of positive and negative ions is the reason that an acid in a water solution will conduct electricity.

The strength of an acid is denoted by its pH. The pH value of a dilute solution is given approximately by the following expression:

pH = log 1/[H+]

In this expression [H+] is a number whose value is determined by the quantity of hydrogen ions per unit volume. The pH of a solution can range from 0 to 14. Pure water, which is neutral (that is, neither an acid nor a base), has a pH of 7. Substances with pH values of less than 7 are acids, and substances with pH values of more than 7 are bases. Strong acids have pH values near 0, while strong bases have pH values near 14.

Certain indicators, such as methyl orange and bromphenol blue, which will change color at a known pH, can be used to find the pH of a solution. Electric meters that give a direct reading of the pH of a solution are also used by chemists.