Chemistry - Copper acetate solution turning clear

Wikipedia states that upon heating, copper(II) acetate reacts with metallic copper to make copper(I) acetate, which would be colorless. While this is no guarantee of what happened with your solution, you can mix a sample of the solution with saltwater (chloride ions). White precipitate = copper(I).

In light of a comment, I am compelled to expand on the possibility of copper(I) in solution. According to the Wiley Online Library, this salt is soluble in water with hydrolysis to cuprous oxide. At the same time we have the copper Pourbaix diagram reported from Ref. 1, indicating that the oxide is stable only above a pH of 4.

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This means any significant leftover acetic acid could prevent the hydrolysis and keep the copper(I) in solution, apparently stabilized by complexing with the acetate ion given that disproportionation is not reported. (Such a stabilization is not included in the above diagram but would contract the Cu2O region further.)

Since this is only indirect evidence, again testing for a copper(I) chloride precipitate by mixing a sample with saltwater is recommended.


1. Oh, Youn-Jin & Park, Gyung-Soon & Chung, Chan-Hwa. (2006). Planarization of Copper Layer for Damascene Interconnection by Electrochemical Polishing in Alkali-Based Solution. Journal of The Electrochemical Society - J ELECTROCHEM SOC. 153. 10.1149/1.2200288.