The concentration of hydrogen ions (H+, or protons) in aqueous solution is measured in pH units.

The H+ concentration at 25°C ranges from 1 Molar (100, or pH=0) to 10-14 M (pH=14).

These concentrations are plotted on a negative logarithmic scale: each unit change in pH is a 10-fold change in H+ ion concentration.

An acid has a high concentration of H+ ions (low pH).

A base has a low H+ concentration (high pH).