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 (10^0, 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).

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