pH
The negative logarithm to base ten of the hydrogen ion concentration, measuring the acidity of a solution.
Formula / equation
pH = −log[H⁺]
Unit
Dimensionless
In depth
Each pH unit represents a tenfold change in acidity, so the logarithmic scale compresses an enormous range into convenient numbers. Values below 7 are acidic and above 7 alkaline at 25 °C, though the neutral point shifts with temperature because Kw does.
Examples in the real world
Gastric acid is near pH 1.5, blood is tightly held at 7.4, and household bleach is about 12.5.