pH and pOH Explained
By the Periodixy Editorial Team · Last reviewed July 10, 2026
pH measures how acidic or basic a solution is by tracking hydrogen ion concentration on a logarithmic scale from about 0 to 14. pOH does the same for hydroxide ions, and at 25 °C the two always add up to 14.
The scale hides an important surprise: each single step of pH is a factor of ten in acidity.
The formulas
pH
pH = −log₁₀[H⁺]
pOH
pOH = −log₁₀[OH⁻]
The 25 °C link
pH + pOH = 14
Square brackets mean “concentration in mol/L”. Pure water at 25 °C has [H⁺] = 1 × 10⁻⁷ M, giving pH = 7 — the neutral point.
Reading the scale
| pH | Meaning | Everyday example |
|---|---|---|
| 0–2 | strongly acidic | stomach acid, lemon juice |
| 3–6 | weakly acidic | vinegar (≈3), coffee (≈5), rain (≈5.6) |
| 7 | neutral | pure water |
| 8–11 | weakly basic | baking soda solution (≈8–9), soap |
| 12–14 | strongly basic | household ammonia (≈11–12), drain cleaner (≈14) |
Worked examples
pH from concentration
A solution has [H⁺] = 3.2 × 10⁻⁵ M. Find its pH.
- pH = −log(3.2 × 10⁻⁵)
- = −(log 3.2 + log 10⁻⁵) = −(0.51 − 5)
Answer: pH ≈ 4.5 — weakly acidic.
pOH to pH
A solution has pOH = 3. What is its pH, and is it acidic or basic?
- pH = 14 − pOH = 14 − 3 = 11
Answer: pH 11 — basic. (High pOH would mean acidic; low pOH means basic.)
Check any of these with the pH & pOH Calculator, which shows each conversion step and a visual scale.
Assumptions worth knowing
- pH + pOH = 14 holds at 25 °C; at other temperatures the sum shifts slightly (Kw changes with temperature).
- These formulas treat concentration and “activity” as equal — fine for dilute school solutions, adjusted in advanced work.
- Strong acids are assumed fully dissociated; weak acids (like acetic acid) need equilibrium calculations beyond this scale.
Summary
- pH = −log[H⁺]; pOH = −log[OH⁻]; at 25 °C, pH + pOH = 14.
- pH < 7 acidic, pH = 7 neutral, pH > 7 basic.
- Each pH unit is a 10× change in H⁺ concentration.
- The simple formulas assume 25 °C and dilute solutions.