the symbol p denotes a cologarithm. The logarithmic form of the equilibrium constant equation is pKw = pH + pOH. Dependence on temperature, pressure and ionic strength The dependence of the water ionization on temperature and pressure has been investigated thoroughly. The value of pKw 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 pKw is dependent on ionic strength of the electrolyte. Values for sodium chloride are typical for a 1:1 electrolyte. With 1:2 electrolytes, MX2, pKw decreases with increasing ionic strength. The value of Kw is usually of interest in the liquid phase. Example values for superheated steam (gas) and supercritical water fluid are given in the table. {| class="wikitable" style="text-align:center" |+Comparison of pKw values for liquid water, superheated steam, and supercritical water. |- ! !! 350 °C !! 400 °C !! 450 °C !! 500 °C !! 600 °C !! 800 °C |- ! scope="row" |0.1 MPa ||| 47.961b || 47.873b || 47.638b || 46.384b ||40.785b |- ! scope="row" |17 MPa |11.920 (liquid)a || || || || || |- ! scope="row" |25 MPa |11.551 (liquid)c ||16.566||18.135||18.758||19.425||20.113 |- ! scope="row" |100 MPa |10.600 (liquid)c ||10.744||11.005||11.381||12.296||13.544 |- ! scope="row" |1000 MPa |8.311 (liquid)c ||8.178||8.084||8.019||7.952||7.957 |} Notes to the table. The values are for supercritical fluid except those marked: a at saturation pressure corresponding to 350 °C. b superheated steam. c compressed or subcooled liquid. Isotope effects Heavy water, D2O, self-ionizes less than normal water, H2O; D2O + D2O D3O+ + OD− This is due to the equilibrium isotope effect, a quantum mechanical effect attributed to oxygen forming a slightly stronger bond to deuterium because the larger mass of deuterium results in a lower zero-point energy. Expressed with activities a, instead of concentrations, the thermodynamic equilibrium constant for the heavy water ionization reaction is: Assuming the activity of the D2O to be 1, and assuming that the activities of the D3O+ and OD− are closely approximated by their concentrations The following table compares the values of pKw for H2O and D2O. {| class="wikitable" style="text-align:center" |+pKw values for pure water |- ! scope="row" |T/°C |10||20|| 25||30|| 40 || 50 |- ! scope="row" |H2O |14.535 || 14.167|| 13.997|| 13.830|| 13.535 ||13.262 |- ! scope="row" |D2O |15.439||15.049||14.869||14.699||14.385|| 14.103 |} Ionization