Ingredients: concentrated acid solution, concentrated base solution, phenolphthalein, thymol blue, phenol red

Procedure: A partial recipe follows.

1. Prepare solutions of distilled water and indicator, for each of the indicators.

2. Add drops of concentrated acid or base to each of the solutions.

3. Observe the color changes.

Understanding: The changes in hydrogen ion concentration are quantified using the concept of pH - the negative base ten logarithm of the molarity of the hydrogen ion concentration. A special set of chemical compounds called "indicators" can be used to visually indicate the transition in concentration of hydrogen ion in a solution. The special property of these molecules is that as the concentration of hydrogen ion changes, the molecules change their charged state, the wavelengths of light that they absorb, and the color of the solution. In the titration of solutions, the indicators are particularly helpful in marking the transition between one range of pH and another.

The range of transitions in pH for the various indicators are thymol blue, phenol red, and phenolphthalein. The ranges of transition vary widely.

      thymol blue           red   1.2-2.8  yellow  8.0-9.6   blue
      phenol red                    yellow     6.4-8.0   red     
      phenolphthalein                      clear     8.2     red