21.2. The pH Concept http://www.ck12.org
FIGURE 21.2
How to Measure pH
There are two common ways to measure pH. When only an approximate pH value is needed, an inexpensive test
involves the use ofacid-base indicators, which change color based on the relative acidity of the solution. One type
of common indicator is litmus paper. For example, if a piece of litmus paper is dipped into an acidic solution (pH
below ~4.5), it will turn red, and dipping it into a basic solution (pH above ~8.5) will turn it blue. Solutions of
intermediate pH will result in various shades of purple. Because different indicators change color at different pH
values, the use of multiple indicators can generally narrow down the possible pH of a solution to a range of about
0.5-1 pH units.Figure21.3 shows the colors exhibited by various indicators at different pH values.
More precise measurements can be obtained by using a digital pH meter, which can detect the concentration of H+
based on small changes in electrical potential. Although a bit more expensive and cumbersome to use, such a device
is essential for determining the exact pH of a solution.
Lesson Summary
- Water can react with itself to produce H 3 O+and OH−. The equilibrium constant for this reaction (Kw) is
equal to 1.0× 10 −^14 at 25°C. - When talking about acids in an aqueous solution, H+is often used as a shorthand for H 3 O+.