pH measures how acidic or basic a solution is, based on its hydrogen-ion concentration. This converts between [H⁺], pH, pOH and [OH⁻].
How it works
pH and pOH
pH = −log₁₀[H⁺] · pH + pOH = 14 (at 25°C)
The 14 comes from water’s ion-product constant, Kw = 1.0×10⁻¹⁴ at 25°C — a temperature-dependent value, not a universal one.
Why “pH + pOH = 14” is specifically a 25°C fact
It follows from Kw, which is itself temperature-dependent — larger at higher temperatures, which shifts neutral pH away from 7. At 25°C (the conventional reference this calculator assumes throughout) neutral pH is exactly 7; at body temperature, 37°C, neutral pH is closer to 6.8. This is the right assumption for the overwhelming majority of introductory chemistry contexts, not a universal physical constant.
How to use this calculator
- Enter either a hydrogen-ion concentration or a pH value directly.
- Read pH, pOH, and both ion concentrations.
Frequently asked questions
Why is pure water’s pH 7, not 0?
Because 7 is the neutral point at 25°C, where [H⁺] and [OH⁻] are exactly equal — not because water has “no” acidity. 0 would represent an extremely concentrated acid, far more acidic than water.
Does this calculator work for weak acids and bases?
Only if you already know the actual [H⁺] — for a weak acid or base, the true [H⁺] depends on the substance’s own dissociation constant and generally isn’t equal to its nominal concentration, a distinction this calculator’s inputs don’t resolve on their own.
Why does pH use a logarithm instead of a plain ratio?
Because hydrogen-ion concentrations span an enormous range — many orders of magnitude between a strong acid and a strong base — and a logarithmic scale compresses that huge range into small, manageable numbers, which is the entire reason Sørensen introduced it in 1909.
Is a pH of 0 or 14 a hard limit?
No — very strong acids or bases can technically have a pH below 0 or above 14, though the simple formula here becomes less physically meaningful at those extremes because ion activity effects (not modelled by this calculator) become significant.
Why does neutral pH shift away from 7 at higher temperatures?
Because Kw itself increases with temperature — more water molecules spontaneously split into H⁺ and OH⁻ ions at higher temperatures, which pushes the neutral point (where the two are equal) to a lower pH number even though the solution is still, by definition, neutral.
