— pH
pH is defined as −log₁₀[H+], the negative base-10 logarithm of the hydrogen-ion activity (approximated by concentration in dilute solutions). In water at 25 °C the scale runs 0–14: below 7 is acidic, 7 is neutral, above 7 is basic. Each pH unit is a tenfold change in [H+].
SI unit: unitless
Used in
- Acid-base equilibria
- Buffers
- Titrations