How it works
pH measures the concentration of hydrogen ions (H⁺) in a solution, on a logarithmic scale. Taking −log₁₀ of the hydrogen ion concentration compresses a huge range of possible values into the familiar 0–14 scale. pOH and hydroxide ion concentration [OH⁻] follow directly, since pH and pOH always add up to 14 in water at 25°C.
pH = −log₁₀[H⁺]
[H⁺] = 10^(−pH)
pOH = 14 − pH
[OH⁻] = 10^(−pOH)
Worked example
Starting from a known pH of 4.0.
- Use [H⁺] = 10^(−pH): [H⁺] = 10^(−4.0).
- That gives 1.0 × 10⁻⁴ mol/L.
- pOH = 14 − 4.0 = 10.0.
- [OH⁻] = 10^(−10.0) = 1.0 × 10⁻¹⁰ mol/L.
A pH of 4.0 means [H⁺] is 1.0 × 10⁻⁴ mol/L, pOH is 10.0, and [OH⁻] is 1.0 × 10⁻¹⁰ mol/L.
- Start from [H⁺] = 1 × 10⁻³ mol/L instead.
- pH = −log₁₀(0.001) = 3.0.
An [H⁺] of 1 × 10⁻³ mol/L gives a pH of 3.0 — either value gets you to the same place.
Common questions
What does pH actually measure?
pH measures how acidic or basic a solution is, based on the concentration of hydrogen ions (H⁺) dissolved in it. Lower pH means more H⁺ and a more acidic solution; higher pH means fewer H⁺ and a more basic (alkaline) one.
Why is the pH scale logarithmic?
Because hydrogen ion concentrations span many orders of magnitude — from about 1 mol/L in strong acids down to 10⁻¹⁴ mol/L in strong bases. Taking −log₁₀ compresses that huge range into the familiar 0–14 scale.
What's the relationship between pH and pOH?
In water at 25°C, pH + pOH always equals 14. So once you know one, the other follows directly — a high pH (basic) always pairs with a low pOH, and vice versa.