Pure water is both a weak acid and a
weak base. By itself, water forms only a very small number of the H3O+
and OH- ions that characterize aqueous solutions of stronger
acids and bases.
| H2O(l) | + | H2O(l) |  |
H3O+(aq) | + | OH-(aq) | |
| base | acid | acid | base |
The concentrations of the H3O+ and OH- ions in water can be determined by carefully measuring the ability of water to conduct an electric current. At 25oC, the concentrations of these ions in pure water is 1.0 x 10-7 moles per liter.
[H3O+] = [OH-] = 1.0 x 10-7 M (at 25°C)
When we add a strong acid to water, the concentration of the H3O+ ion increases.
HCl(aq) + H2O(l)
H3O+(aq) + Cl-(aq)
At the same time, the OH- ion concentration decreases because the H3O+ ions produced in this reaction neutralize some of the OH- ions in water.
H3O+(aq)
+ OH-(aq)
2 H2O(l)
The product of the concentrations of the H3O+ and OH- ions is constant, no matter how much acid or base is added to water. In pure water at 25oC, the product of the concentration of these ions is 1.0 x 10-14.
[H3O+][OH-] = 1.0 x 10-14
The range of concentrations of the H3O+ and OH- ions in aqueous solution is so large that it is difficult to work with. In 1909 the Danish biochemist S. P. L. Sorenson suggested reporting the concentration of the H3O+ ion on a logarithmic scale, which he named the pH scale. Because the H3O+ ion concentration in water is almost always smaller than 1, the log of these concentrations is a negative number. To avoid having to constantly work with negative numbers, Sorenson defined pH as the negative of the log of the H3O+ ion concentration.
pH = -log [H3O+]
The concept of pH compresses the range of H3O+ ion concentrations into a scale that is much easier to handle. As the H3O+ ion concentration decreases from roughly 100 to 10-14, the pH of the solution increases from 0 to 14.
If the concentration of the H3O+ ion in pure water at 25oC is 1.0 x 10-7 M, the pH of pure water is 7.
pH = -log [H3O+] = -log (1.0 x 10-7) = 7
When the pH of a solution is less than 7, the solution is acidic. When the pH is more than 7, the solution is basic.
| Acidic: | pH < 7 | |
| Basic: | pH > 7 |
Information provided by: http://chemed.chem.purdue.edu