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.