Estimation of pH - Acid-Base Indicators

Acid-Base Indicators

One of the easiest ways to determine the pH of a solution is by adding a substance whose color is dependent on the pH. These kinds of compounds are called acid-base indicators, or simply indicators. Litmus is probably the most common acid-base indicator. If placed in relatively acidic solutions (pH < 5), litmus is red. In relatively basic solutions (pH > 8), litmus is blue. Notice that there is a range in which litmus is neither red nor blue (between 5 and 8). This is called the color change region for litmus and is the range of pH's over which litmus changes from one color to the other. Litmus is so commonly used because its color change region is centered around a pH of 7 (a neutral solution).

Some Color Change Regions
indicator color change region color change
methyl green 0.2 - 1.8 yellow to blue
congo red 3.0 - 5.0 blue to red
litmus 5.0 - 8.0 red to blue
phenol red 6.4 - 8.0 yellow to red
thymolphthalien 9.4 - 10.6 colorless to blue

Thymolphthalien's color change region is from 9.4 to 10.6. Knowing this information allows us to say something about the pH of a solution containing a small amount of thymolphthalein. If it is colorless, the pH is less than 9.4. If it is blue, the pH is greater than 10.5. An in-between color means an in-between pH (9.4 < pH < 10.6). You must be careful interpreting data in and around the color change region.

Some indicators have more than one color change region. Thymol blue is an example. At pH's less than about 2.5 it is red. At pH's greater than about 7.5 it is blue. In between these pH's, it is yellow.

By testing a solution with more than one indicator, with different color change regions, we can often narrow the pH to within a unit or so. Consider this example:
A test solution turns blue when a small amount of litmus is added to it and stays colorless when thymolphthalein is added. What is the pH?
Our interpretation should be:
The blue color with litmus indicates that the pH is > 8.0. Remaining colorless with thymolphthalein indicates a pH < 9.4. Therefore, we know the pH must be between 8.0 and 9.4.

Read about the behavior of acid-base indicators in greater detail.