Species Distribution Diagrams in the Copper -- Ammonia System: An Updated and Expanded Demonstration Illustrating Complex Equilibria

The addition of aqueous ammonia to a dilute solution of copper sulfate leads to a complex series of coupled equilibria involving acid–base chemistry, precipitation, and complex ion formation. There are 12 chemical species that must be considered to solve the problem mathematically. Although it is possible to solve the complete system of equations analytically, judicious use of intuitive simplifications and assumptions about the chemical system allow a simpler set of equations to be solved. This provides an opportunity for students to test their chemical intuition. Comparison of the calculated results reveals no major differences between the two mathematical methods. The mathematical results also correspond well with experimental data.