Introduction

An acid-base indicator is a substance that changes color as the pH of the solution changes. Indicators work because they are weak acids which when in solution, exist in equilibrium with their conjugate base. The acid and conjugate base each have different colors and as the equilibrium shifts from one direction to the other, the color of the indicator changes. As an example, the equilibrium of a weak acid can be represented but the equation below where HA is a weak acid and A- is its conjugate base.

                        HA →  H+  +  A-

Universal indicator is a wide range indicator and is composed of several different indicators. This allows it to exhibit a wide range of colors at different pH values – visualized here as the “pH rainbow.”

To Conduct Demonstration:  (approx. 5 min)

  1.   Place the white poster board behind the cylinder if a white background is not available.
  2. Add approximately 10 mL universal indicator  and stir so that the water is visibly green.
  3.  Then add 3-4 drops of 6M HCl and stir until the mixture becomes red.
  4. Add 4-6 sodium hydroxide pellets.  As they dissolve at the bottom, the solution surrounding them will become dark blue. Gently stir a few times and wait a few seconds  to observe a rainbow gradient. Eventually the entire mixture will become blue.
  5. Slowly add stepwise 3-4 chunks of dry ice until the solution becomes yellow-orange in color.

Optional:

Bring the solution back to its original uniform color and repeat the demonstration again.

Use a weak acid such as acetic acid and a weak base such as ammonia and after a few times the demonstration will no longer change colors. This variation provides a demonstration for the effect of buffers.

Safety 

Goggles should be worn.  Universal indicator is flammable and slightly toxic if ingested. Hydrochloric acid is corrosive and sodium hydroxide is an irritant.

Reference:

chemmovies.unl.edu/chemistry/beckerdemos/BD024.html

Acknowledgements:

Ditte Thomsen