pH from hydrogen or hydroxide concentration, from pOH, or for a weak acid — with pOH, both ion concentrations and acidity class.
How to use this tool
Hydrogen concentration, hydroxide concentration, a pOH value, or a weak acid with its Ka.
Concentrations in mol/L, mmol/L or µmol/L; Ka values like 1.8 × 10⁻⁵ for acetic acid work directly.
The big number is pH, with pOH, both ion concentrations and the acidity class underneath.
The meter runs from 0 (strongly acidic) through 7 (neutral) to 14 (strongly alkaline).
Formula & worked example
Worked example: a solution with [H+] = 1 × 10⁻⁷ mol/L has pH = −log₁₀(10⁻⁷) = 7 — neutral. And 0.1 mol/L acetic acid (Ka = 1.8 × 10⁻⁵) gives [H+] = (−1.8×10⁻⁵ + √((1.8×10⁻⁵)² + 4 × 1.8×10⁻⁵ × 0.1)) ÷ 2 ≈ 1.33 × 10⁻³, so pH ≈ 2.87.
Limitations — read this before you rely on it
pH + pOH = 14 holds at 25 °C; at other temperatures the neutral point shifts (at 100 °C, neutral is pH 6.14). The weak-acid formula assumes the acid is weak and activity coefficients are 1 — strong acids and concentrated solutions need fuller treatment.
Frequently asked questions
A logarithmic measure of hydrogen ion concentration: pH = −log₁₀[H+]. Each whole-number step is a tenfold change, so pH 3 is ten times more acidic than pH 4 and a hundred times more than pH 5.
Take the negative base-10 logarithm: [H+] = 0.001 mol/L gives pH 3. For hydroxide, find pOH the same way and subtract from 14.
pH 7 at 25 °C, where [H+] equals [OH−] at 10⁻⁷ mol/L. Below 7 is acidic, above 7 is alkaline; the scale runs 0–14 for most everyday solutions.
Weak acids only partly dissociate, so solve [H+] = (−Ka + √(Ka² + 4·Ka·C)) ÷ 2 from the acid constant and concentration, then take −log₁₀. Acetic acid at 0.1 mol/L gives pH ≈ 2.87, far less acidic than a strong acid at the same concentration.
Yes — the pH + pOH = 14 rule is for 25 °C. Warmer water dissociates more, so neutral pH falls (6.14 at 100 °C). A solution that is neutral at room temperature is still neutral when hot; only the number on the scale moves.