pH Calculator

In short pH is the negative base-ten logarithm of the hydrogen ion concentration, so pH 3 means 10⁻³ mol/L and each whole unit is a factor of ten. Lemon juice at pH 2 has ten thousand times the hydrogen-ion concentration of coffee at pH 6. Alongside it, pOH is the same logarithm applied to hydroxide, and the two always add to pKw. The part almost every source gets wrong is that pH + pOH = 14 and neutral = 7 are true only at 25 °C. Water ionises more when it is hot, so pKw falls as temperature rises: 14.94 at 0 °C, 14.00 at 25 °C, 12.26 at 100 °C. Neutral means [H⁺] equals [OH⁻], which is pKw ÷ 2 — so pure water is neutral at pH 7.47 when it is near freezing and at pH 6.13 when it is boiling. Boiling water is not acidic; the whole scale moved with it. Blood at 37 °C is neutral around pH 6.81, which is part of why body pH is quoted against its own reference range rather than against 7. For a strong acid, dissociation is complete and pH is simply −log of the concentration, so 0.1 M is pH 1.00. For a weak acid you have to solve x² + Kax − KaC = 0, and this tool solves it exactly rather than using the schoolbook √(Ka·C) shortcut. For 0.1 M acetic acid the two agree closely, at pH 2.88. For a stronger or more dilute acid they do not: 0.001 M of a pKa 2.0 acid is pH 3.04 exactly, while the shortcut says 2.50 — an error of half a pH unit, which is a factor of three and a half in concentration.