pKw of Water at 25 °C

Take the negative base-10 logarithm of K_w and the awkward exponent becomes a number you can do in your head: pK_w = pH + pOH. The best conductance and EMF measurements give 13.995 at 25 °C, but every textbook and every field procedure rounds it to 14.00, and the 0.005 discrepancy is smaller than the drift on a glass electrode between calibrations, so nothing is lost. A sample at pH 9.3 therefore has pOH 4.7 and [OH⁻] = 2 × 10⁻⁵ mol/L.

What travels badly is the number 14 itself. pK_w reaches 14.94 in ice-cold water and drops to 12.29 at 100 °C, passing a minimum near 11.0 at about 250 °C before climbing again as the density of supercritical water falls away. That is why the pH scale in a geothermal well or a supercritical boiler is not the scale drawn on a classroom poster, and why serious water chemistry software carries a temperature-dependent pK_w rather than the constant 14.