READ: Indicators

READ: Indicators

Indicators

An indicator (a substance that changes color at a specific pH) is used to indicate the pH of the solution. One example of an indicator is litmus paper. Litmus paper is paper that has been dipped in a substance that will undergo a color change when it is exposed to either an acid or a base. If red litmus paper turns blue, the solution is basic (pH>7), and if blue litmus turns red the solution is acidic (pH<7). There are many more indicators that can be used.

The color of phenol red, an indicator, changes depending on the pH.

There are many more indicators than are shown in Table above, but these are ones that you may find in common chemistry classroom laboratories. One example of an indicator not found in the table is known as the universal indicator. The universal indicator is a solution that has a different color for each pH from 0 – 14. Universal indicator is produced by creatively mixing many of the individual indicators together so that a different color is achieved for each different pH. It is used for many types of experiments to determine if solutions are acids or bases and where on the pH scale the substance belongs. The chart below indicates the colors of universal indicator for different pH values.

Example 1

A solution found in the laboratory was tested with a number of indicators. These were the results:

• Phenolphthalein was colorless.
• Methyl red was yellow.
• Phenol red was yellow.

What was the pH of the solution?

Solution:

• Phenolphthalein was colorless, pH<8.0.
• pH>5.4.
• Methyl red was yellow, pH>6.0.
• Phenol red was yellow, pH<6.4.

Therefore, the pH of the solution must be between 6.0 and 6.4.

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