ACIDS, BASES, SALTS, AND pHDOE-HDBK-1015/1-93Fundamentals of ChemistryCH-01Rev. 0Page 50Example:What is the hydrogen ion concentration [H ] and the hydroxyl concentration+[OH ] in a solution with a pH of 5.5?-Solution:DissociationConstantThe product of ionic concentrations, K = [H ] [OH ], is called the Ion Product Constant forw+ -water, or more frequently, the Ionization Constant orDissociation Constant. At 25?C, Kwequals 1 x 10 . K varies with temperature and, at 37?C (body temperature), the value is-14wabout 3.4 x 10 . Figure 17 illustrates the relationship of the Dissociation Constant with water-14temperature. As indicated earlier, a change in temperature causes a change in equilibrium ina chemical reaction. Because the ionization of water is an equilibrium process, it is also affectedby temperature. The Dissociation (or Ionization) Constant, K , for water increases withwincreasing temperature up to about 500?F and then decreases. Experimental values of K atwvarious temperatures are listed in Table 5 and graphed in Figure 16. Notice in Table 5 that thepH of pure water changes with temperature. For pure water at any temperature, however,[H ] = [OH ]. It should be noted that the equation pH + pOH = 14 is true only at or near 25?C+ -(77?F).
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