Dissociation Constant:

The product of ionic concentrations, K_w  = [H⁺] [OH^-], is known as the Ion Product Constant for water, or more often, the Ionization Constant or Dissociation Constant.  At 25ºC, K_w equals 1 x 10^-14.  K_w varies along with temperature and, at 37ºC (body temperature), a value is about 3.4 x 10^-14. Figure describes the relationship of the Dissociation Constant along with water temperature. As denoted previously, a modification in temperature causes modify in equilibrium in a chemical reaction. Since the ionization of water is an equilibrium procedure, it is also affected through temperature. A Dissociation (or Ionization) Constant, K_w, for water increases along with increasing temperature up to about 500ºF and then reduce.  Experimental values of K_w at several temperatures are listed in Table 5 and graphed in Figure. Remember in Table 5 in which the pH of pure water changes along with temperature.  For pure water at any temperature, therefore, [H⁺] = [OH^-]. It should be remember that the equation pH + pOH = 14 is true just at or near 25ºC (77ºF).