Convert between pH and pOH.
pH and pOH are complements, but not always to 14. They sum to pKw, the negative logarithm of the ion product of water, and that quantity is strongly temperature-dependent: 14.94 at 0 C, 14.00 at 25 C and only 12.26 at 100 C. The consequence catches people out — neutral water at 50 C has a pH of about 6.63, not 7, and it is not acidic, because hydrogen and hydroxide concentrations remain equal. Only at 25 C is the familiar pH + pOH = 14 exactly right, so this interpolates pKw from measured values and reports the true neutral point alongside the conversion.
pH and pOH complementarity
pH + pOH = pKw; [H+] = 10^-pH; [OH-] = 10^-pOH; neutral pH = pKw / 2, where pKw is interpolated from measured values between 14.94 at 0 C and 12.26 at 100 C
Only at 25 C. Water autoionises more readily when hot, so neutral pH falls to about 6.63 at 50 C and 6.13 at 100 C. Hot water at pH 6.5 is alkaline relative to its own neutral point.
Autoionisation of water is endothermic, so Le Chatelier's principle pushes the equilibrium towards ions as temperature rises. Kw grows by nearly two orders of magnitude between 0 C and 100 C.
It corrects the electrode slope for temperature, which is a different effect. Whether it also reports the temperature-adjusted neutral point depends on the instrument — check before interpreting readings taken far from 25 C.