E770: Acid/Base – Common Ion Effect

Introduction

The addition of NH₄Cl to a solution of ammonia shows the effect of adding a common ion to a system at equilibrium.

Equations

 NH₃ + H₂O ↔ NH₄⁺ + OH^-

 Addition of NH₄Cl adds NH₄⁺, driving the reaction to the left and consuming OH^-.

To Conduct Demonstration:

1. Add water to each of two beakers.  Add 2 drops of phenolphthalein to each.
2. Add a few drops of 3M NH₃ to each beaker.  The solution will turn pink.
3. Add a scoop of solid NH₄Cl to one of the beakers.  The pink color will become very faint or disappear.  Use the other beaker as a reference.

Variant (courtesy Doug Gin):

1. Add water to one of the beakers.  Add 2 drops of phenolphthalein and a few drops of 3 M aq. NH₃.  This reference solution will turn pink (basic, pH > 7).
2. Add water to the other beaker, and a scoop of solid NH₄Cl.   Add a few drops of 3 M aq. NH₃, stir, and add 2 drops of phenolphthalein. This solution will not shift as much to the right and will not turn pink or as pink (less basic).
3. Use the first solution without added solid NH₄Cl as a reference.

Safety 

Goggles should be worn.