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pH Calculator

What the calculator does

Four quantities describe the acidity of an aqueous solution at 25 °C, and they are all linked:

  • pH = −log₁₀[H⁺]
  • pOH = −log₁₀[OH⁻]
  • pH + pOH = 14
  • [H⁺] × [OH⁻] = 1.0 × 10⁻¹⁴ (Kw at 25 °C)

Knowing any one fixes the other three. Type any value — pH, pOH, [H⁺], or [OH⁻] — and the calculator inverts the logs and applies the Kw relationship to fill in the rest. A color-coded scale shows where the solution sits between strong acid (red) and strong base (blue).

The non-obvious part: Kw is temperature-dependent

The 14 in pH + pOH = 14 is the −log of Kw at exactly 25 °C. Water’s autoionization shifts with temperature — Kw rises to about 10⁻¹³·⁶ at 37 °C, so neutral pH at body temperature is closer to 6.8 than 7.0. The calculator assumes 25 °C; for biological or high-temperature work you have to adjust Kw yourself.

Worked examples

From pH to everything else. pH = 4.5. [H⁺] = 10⁻⁴·⁵ = 3.16 × 10⁻⁵ M. pOH = 14 − 4.5 = 9.5. [OH⁻] = 10⁻⁹·⁵ = 3.16 × 10⁻¹⁰ M.

From [H⁺] to pH. [H⁺] = 2.5 × 10⁻³ M. pH = −log(2.5 × 10⁻³) = 2.60. pOH = 11.40. [OH⁻] = 3.98 × 10⁻¹² M.

From pOH to pH. pOH = 3.2. pH = 14 − 3.2 = 10.8. [OH⁻] = 6.31 × 10⁻⁴ M. [H⁺] = 1.58 × 10⁻¹¹ M.

From [OH⁻] to everything. [OH⁻] = 0.050 M. pOH = −log(0.050) = 1.30. pH = 12.70. [H⁺] = 2.00 × 10⁻¹³ M.

Familiar pH values

  • Stomach acid: 1.5–3.5
  • Lemon juice: 2.0
  • Vinegar: 2.4
  • Coffee: 5.0
  • Pure water: 7.0 (at 25 °C)
  • Blood: 7.35–7.45 (tightly regulated by carbonate buffer)
  • Baking soda solution: 8.3
  • Household ammonia: 11.0
  • Bleach: 12.5

A one-unit pH change is a tenfold concentration change, so blood pH drifting from 7.4 to 6.4 — which sounds modest — would mean a tenfold rise in [H⁺] and is incompatible with life.

Frequently Asked Questions

What is pH?
pH is the negative base-10 logarithm of the hydrogen ion activity (approximated by molar concentration in dilute solutions): pH = −log10[H+]. The log compresses a concentration range that spans roughly fifteen orders of magnitude in aqueous chemistry into a tidy 0-to-14 scale. Below 7 is acidic, 7 is neutral at 25 °C, above 7 is basic.
What is the relationship between pH and pOH?
At 25 °C, pH + pOH = 14. This comes from the autoionization of water: Kw = [H+][OH−] = 1.0 × 10⁻¹⁴. Take −log10 of both sides and the product becomes a sum, giving the pH/pOH relationship. Kw is temperature-dependent, so the sum equals 14 only at 25 °C — at body temperature it is closer to 13.6.
How do you find [H+] from pH?
Invert the definition: [H+] = 10^(−pH). A pH of 3 means [H+] = 10⁻³ = 0.001 M. A pH of 7.4 (blood) means [H+] = 10^(−7.4) = 4.0 × 10⁻⁸ M. Each whole-number pH change corresponds to a tenfold change in [H+] — a fact that gets buried by the compressed scale.
Can pH be negative or greater than 14?
Yes. Concentrated strong acids like 12 M HCl have [H+] above 1 M, which gives a negative pH. Concentrated strong bases push pH past 14 the same way at the OH− end. The 0–14 range covers ordinary aqueous chemistry, but it is not a hard limit — it is just the range where [H+] sits between 10⁰ and 10⁻¹⁴ M.
What is a buffer and how does it relate to pH?
A buffer is a weak acid paired with its conjugate base (or weak base with its conjugate acid). It resists pH changes because added H+ is consumed by the conjugate base and added OH− is consumed by the weak acid. The Henderson-Hasselbalch equation, pH = pKa + log([A−]/[HA]), predicts the buffer's pH from the ratio — and shows that a 1:1 ratio gives pH = pKa.