B07 Shifting Copper Redox Equilibria

Description
A copper(II) aqueous solution is stirred over a methylene chloride layer. As iodide is added to the solution, a precipitate of Copper(I) Iodide and a violet(iodine) methylene chloride layer form. The potential is measured with respect to a copper/copper(II) ammonia electrode. Next, ammonia is added to the mixture. The violet color fades and an intense blue copper(II) ammonia complex is observed. The potential is measured again. Nitric acid is added to reverse the reaction, and iodine is observed again.

Equation
2 Cu²⁺ + 4 I^- --> 2 CuI + I₂

Cu²⁺ + 4 NH₃ --> Cu(NH₃)₄²⁺

4 I^- + 2 Cu(NH₃)₄²⁺ + 8 H⁺--> 2 CuI + I₂ + 8 NH₄⁺

Reference
L. R. Summerlin, C. L. Borgford, and J. B. Ealy,"Chemical Demonstrations A Sourcebook for Teachers",Vol. 2, American Chemical Society, Washington, DC, (1987) p.179.