pH = -log [H3O+] (Brønsted-Lowry)
H2O <--> H3O+ + OH-
Example: The oxonium ion (H3O+) concentration of a solution with a pH of 1.0 is 100 times greater than a solution with a pH of 3.0. In pure neutral water the concentration of oxonium ions and hydroxide ions (OH-) are both equal to 10-7 per liter.
When the pH is below 7, the solution is acidic
When the pH is 7, the solution is neutral
When the pH is above 7, the solution is basic (alkaline)