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-

Phenolphthalein is typically used in this demo because its change from colorless to bright pink is easy to see in large lecture halls, but universal indicator could also be used.  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.

To Conduct Demonstration:  (approx. 5 min)

  1. Show the students that the HCl solution in the Erlenmeyer is colorless.
  2. Add 10 mls of NaOH to the Erlenmeyer and point out the color change due to the solution becoming basic and the indicator also becoming basic.
  3. Add 10 mls of HCl to the solution in the Erlenmeyer, turning it colorless again.
  4. Continue turning the solution basic and acidic until the concept of the reversibility has been demonstrated adequately.

Safety 

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

Reference:

Dr. Steven George, prior to 2002