ACIDS, BASES, SALTS, AND pH
DOE-HDBK-1015/1-93
Fundamentals of Chemistry
CH-01
Rev. 0
Page 50
Example:
What is the hydrogen ion concentration [H ] and the hydroxyl concentration
+
[OH ] in a solution with a pH of 5.5?
-
Solution:
Dissociation Constant
The product of ionic concentrations, K = [H ] [OH ], is called the Ion Product Constant for
w
+
-
water, or more frequently, the Ionization Constant or Dissociation Constant. At 25?C, Kw
equals 1 x 10 . K varies with temperature and, at 37?C (body temperature), the value is
-14
w
about 3.4 x 10 . Figure 17 illustrates the relationship of the Dissociation Constant with water
-14
temperature. As indicated earlier, a change in temperature causes a change in equilibrium in
a chemical reaction. Because the ionization of water is an equilibrium process, it is also affected
by temperature. The Dissociation (or Ionization) Constant, K , for water increases with
w
increasing temperature up to about 500?F and then decreases. Experimental values of K at
w
various temperatures are listed in Table 5 and graphed in Figure 16. Notice in Table 5 that the
pH 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).
