E775: Prep Notes

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.

Supplies

Short List

  • 2 burets
  •  Ring stand
  • Buret clamp
  • 2 x 250 mL beakers
  • water
  • White poster board background
  • Goggles and gloves

Chemicals

  • 0.1 M HCl
  • 0.1 M NaOH
  • Universal indicator

Prior to Lecture  (prep time ~ 5 min.)

  1.         Assemble the ring stand and buret clamp. Secure the two burets in the clamp.
  2.          Set up white poster board (leaning against a ring stand) behind the bottom of the burets where the Erlenmeyer will be positioned.
  3.          Prepare the NaOH and HCl solutions.
  4.          Fill and label one buret with NaOH, and the other buret with HCl.
  5.          Add about 10 mls of NaOH solution plus approximately 100 mL water to a 250 ml Erlenmeyer flask and add 5-7 drops of phenolphthalein indicator.  (universal indicator may also be used)
  6.         Inform the demo performer of the volume of base in the Erlenmeyer and whether the indicator has been added.

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 and Disposal

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

Neutralize waste and pour down the drain.