What is pH?
pH measures how acidic or basic a solution is. It's defined as the negative base-10 logarithm of the hydrogen-ion concentration:
pH = −log₁₀[H⁺]
where [H⁺] is the concentration of hydrogen ions in moles per liter (mol/L).
A pH of 7 is neutral (pure water at 25 °C). Below 7 is acidic, above 7 is basic. Each whole pH unit represents a 10× change in [H⁺] — pH 4 has ten times more hydrogen ions than pH 5.
How to use
- Enter the hydrogen-ion concentration in mol/L. Scientific notation works (e.g. 1e-7 for 10⁻⁷).
- The pH is calculated instantly.
For [H⁺] = 10⁻⁷ mol/L: pH = −log(10⁻⁷) = 7 (neutral water).
Reference values
| Solution | [H⁺] (mol/L) | pH | |---|---|---| | Battery acid | 1 | 0 | | Stomach acid | 0.01 | 2 | | Vinegar | 10⁻³ | 3 | | Black coffee | 10⁻⁵ | 5 | | Pure water | 10⁻⁷ | 7 | | Sea water | 10⁻⁸ | 8 | | Baking soda | 10⁻⁹ | 9 | | Bleach | 10⁻¹³ | 13 |
FAQ
How do I go from pH back to concentration?
Reverse the formula: [H⁺] = 10⁻ᵖᴴ. A pH of 4 means [H⁺] = 10⁻⁴ = 0.0001 mol/L.
What about pOH?
For aqueous solutions at 25 °C: pH + pOH = 14. So pOH = 14 − pH.
Why is pH only meaningful between 0 and 14?
The 0–14 range is the practical scale for dilute aqueous solutions where [H⁺][OH⁻] = 10⁻¹⁴. Concentrated strong acids/bases can technically go outside this range, but the formula still works — just less common.