Section IV. pH and pOH
9-10. CONCEPT OF pH
Pure water contains an equal concentration of hydrogen and hydroxide ion.
Thus, it is a neutral substance, neither acidic nor basic. If a solution has an excess of
hydrogen ions, the solution will be acidic. If a solution has an excess of hydroxide ions,
the solution will be alkaline. It is customary to use the hydrogen ion concentration (pH)
as a measure of the acidity or alkalinity of a solution. pH is a unit of concentration that
allows the technician to work with very dilute concentrations of hydrogen and hydroxide
ion in a convenient form. This scale permits the representation of the enormous range
of the possible [H ] concentrations from a 1.0 mol/L [H ] to a 1.0 X 10-14 mol/L [H ].
9-11. APPLICATION OF CONCEPT OF pH
The notation p denotes "negative log of." Logarithms are discussed in lesson 1,
and there is a four-place logarithm table in Appendix B. pH is mathematically defined
-log [H ] or log 1/[H ]
a. Example 1. What is the pH of a 0.133 mol/L HCl solution?
Solution. Select the expression that allows you to solve for the unknown
pH = -log [H ]
Substitute the given information, and solve for the unknown quantity.
pH = -log 0.133 = --log [1.33 X 10--1] = --(0.1239 -- 1)
pH = -(-0.876)
pH = 0.876
b. Example 2. What is the pH of a 0.020 mol/L acid solution that is ionized 1.8
Solution. From previous discussion, hydrogen or hydroxide does not
contribute to the acidity or alkalinity of a solution unless ionized. So in this problem
consider only that hydrogen ion that is ionized.
pH = -log [H ]
pH = -log [(0.020 mol/L) (0.018)] = --log [3.6 x 10--4] = --(.5563 --4)
pH = 3.4