pH is related to hydrogen ion concentration. Since the H+ ion concentration in solution is often small, the concentration is generally expressed as the logarithm of its reciprocal, which is called a pH value. Therefore, pH is defined as

pH = -log[H+]

For a ten times increase in H+ ion concentration there is a decrease in the pH value of one unit.

Given the pH of a solution, its H+ concentration can be found:

[H+(aq)] = antilog -pH

[H+(aq)] = 10-pH

See also Ionic Product of Water.