pH & pOH — Quick answer
At 25 °C the two add to 14, and each is the negative log of an ion concentration.
pH + pOH = 14 · [H⁺] = 10⁻ᵖᴴ · [OH⁻] = 10⁻ᵖᴼᴴ
Worked example: pH 3 → pOH 11, [H⁺] = 10⁻³ M → acidic.
Examples
| pH | pOH | Type |
|---|
| 3 | 11 | acidic |
| 7 | 7 | neutral |
| 8.5 | 5.5 | basic |
The sum of 14 holds at 25 °C (Kw = 1×10⁻¹⁴).
Standards & method
✓ Independently verified 12 July 2026- Basis
- First principles
- Method
- Definition of pH and the ion product of water. Kw = 1.0 × 10⁻¹⁴ at 25 °C — it is TEMPERATURE-DEPENDENT.
- Core formula
pH = −log₁₀[H⁺] · pH + pOH = 14 (at 25 °C only) · Kw = [H⁺][OH⁻]- Why this matters
- pH + pOH = 14 holds ONLY at 25 °C. At 100 °C, Kw rises and neutral pH is about 6.14, not 7. Hot process water is not "acidic" just because its pH reads below 7.
- Independently verified
- 12 July 2026 — Formula re-derived from first principles and verified numerically against hand-computed reference cases, including edge cases and unit handling.
Results are for guidance. Verify against the current edition of the governing standard and have a licensed engineer review before construction or installation.
pH and pOH measure acidity and basicity on a log scale. At 25 °C they satisfy pH + pOH = 14, with [H⁺] = 10⁻ᵖᴴ and [OH⁻] = 10⁻ᵖᴼᴴ. Enter any one of the four and this calculator returns the rest and tells you whether the solution is acidic, neutral or basic.
Reviewed: June 20, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: Kw and the log definitions, recomputed in code.
The relationships
The "p" means "−log₁₀ of," so each unit of pH is a tenfold change in [H⁺]. Because water's ion product Kw is 10⁻¹⁴ at 25 °C, the two logs always sum to 14. Acidic solutions have more H⁺ than OH⁻ (pH < 7); basic solutions have more OH⁻ (pH > 7); pure water is neutral with both at 10⁻⁷ M.
Worked examples
pH 3:
[H⁺] = 0.01 M:
pOH 4:
A pH of 8.5 gives pOH 5.5, with [H⁺] ≈ 3.16×10⁻⁹ M and [OH⁻] ≈ 3.16×10⁻⁶ M — basic, since there's a thousand times more hydroxide than hydrogen ion.
Frequently Asked Questions
What is the relationship between pH and pOH?⌄
At 25 °C, pH + pOH = 14, from Kw = [H⁺][OH⁻] = 10⁻¹⁴.
How do I convert pH to [H⁺]?⌄
[H⁺] = 10^(−pH). pH 3 → 0.001 M. Reverse: pH = −log₁₀[H⁺].
How do I find pOH from [OH⁻]?⌄
pOH = −log₁₀[OH⁻]. [OH⁻] = 10⁻⁴ → pOH 4, pH 10.
What pH is acidic or basic?⌄
Below 7 acidic, 7 neutral, above 7 basic (at 25 °C).
Why is the sum 14 only at 25 °C?⌄
Kw changes with temperature, so neutral pH and the sum shift away from 7 and 14.