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The pH scale

pH measures the acidity of a solution from the concentration of H+\mathrm{H^+} ions: low pH means an acidic solution, high pH means basic. At 25 °C, the sum of pH and pOH is always 14, since water's ion product Kw=10−14K_w = 10^{-14}.

pH=−log⁡[H+]\mathrm{pH} = -\log[\mathrm{H^+}]definition of pH
pH+pOH=14\mathrm{pH} + \mathrm{pOH} = 14at 25 °C

Symbols

[H+][\mathrm{H^+}]hydrogen ion concentrationmol/L

Example

A solution has pH 3, neutral water has pH 7:

[H+][\mathrm{H^+}] is 107−3=10.00010^{7-3} = 10.000 times higher.

The pH scale is logarithmic: each unit down means ten times higher H+\mathrm{H^+} concentration.
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Part of General Chemistry: Acids, bases and equilibrium.