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The potential of hydrogen, or pH, points to a solution's acidity or basicity— properties based on the concentration of hydronium ions, formed when water molecules gain an extra proton.

The pH of a solution is the negative logarithm of its hydronium ion concentration. For example, a solution with 1 × 10⁻⁵ M hydronium concentration has a pH of 5.

pH values commonly range from zero to fourteen and are typically depicted in decreasing order of hydronium ion concentration on a pH scale at a standard temperature of 25°C.

Since pH is on a logarithmic scale, a solution of pH 4 has ten times more hydronium ions than a solution with pH 5, which has ten times more hydronium ions than pH 6, and so on.

pH seven is neutral since the water molecules autoionize through proton transfer between two water molecules, producing equal concentrations of hydroxide and hydronium ions.

Coffee has an acidic pH due to the high concentration of hydronium ions.

On the other hand, a low hydronium concentration, as in milk of magnesia, results in higher pH values, indicating a basic solution.

Substances above a pH of seven are basic, while those below are acidic.