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 ca. 250 °C, after which it increases up to the critical point of ca. 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.
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pKw values for superheated steam and supercritical water. T/°C 350 400 450 500 600 800 0.1 MPa (abs) 11.920 (liquid at saturation pressure) 47.961 47.873 47.638 46.384 40.785 25 MPa (abs) 11.551 (liquid) 16.566 18.135 18.758 19.425 20.113 100 MPa (abs) 10.600 (liquid) 10.744 11.005 11.381 12.296 13.544
Read more about this topic: Self-ionization Of Water
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