22
Hydrogen-Ion Concentration and pH
Chemical reactions in aqueous solutions (including the chemistry of life processes) very often depend on the concentration of hydrogen ion in the solution.
As we shall see, we may deal with hydrogen-ion concentrations varying from
greater than 1 M to less than 10~
14 M. Consequently, it is convenient to express
these concentrations on a logarithmic basis; for this purpose the terms "pH"
and "pOH," which we discuss in this chapter, have been introduced.
HYDROGEN AND HYDROXIDE CONCENTRATIONS IN WATER
Water is a weak electrolyte, ionizing slightly and reversibly as
H 2 O ** H
+
Actually the H
+ is hydrated, forming chiefly H 3 O
+ ion. Just as we ignore the
hydration of all the metal ions (for convenience in writing equations), we also
ignore the hydration of H
+ . You must always remember, however, that a bare
proton (an H
+ ) can never exist in solution by itself.
This dissociation reaction is always at equilibrium, with extremely rapid
formation and recombination of H
+ and OH~. Because it is always at equilib-