The dependence of the water ionization on temperature and pressure has been investigated thoroughly. The value of pK decreases as temperature increases from the melting point of ice to a minimum at c. 250 °C, after which it increases up to the critical point of water c. 374 °C. It decreases with increasing pressure.
With electrolyte solutions, the value of pK is dependent on ionic strength of the electrolyte. Values for sodium chloride are typical for a 1:1 electrolyte. With 1:2 electrolytes, MX, pK decreases with increasing ionic strength.
The value of K is usually of interest in the liquid phase. Example values for superheated steam (gas) and supercritical water fluid are given in the table.
:Notes to the table. The values are for supercritical fluid except those marked:  at saturation pressure corresponding to 350 °C.  superheated steam.  compressed or subcooled liquid.