I am neutralizing a concentrated solution of copper, nickel, and excess HCl with sodium carbonate. I expected that after the excess HCl was neutralized, I would start to form basic nickel and copper carbonates which would precipitate as blue-green solids, and could determine that I had dropped everything out of solution once the supernatant was colorless.
My starting solution was a very dark yellow-green, remaining so until I had added enough sodium carbonate to neutralize excess acid. The solution then started to turn green as a metal precipitates started to form, this continued until the solution was a light blue-green color. I then let the solution settle over several days. When I came back, the precipitate had settled to the bottom, but the supernatant was a light green which I took to mean that I still had metal-chlorides in solution. With heating and stirring I continued to add carbonate until the solution suddenly turned a light orange-brown color. I let the solution settle and the supernatant was clear, with a small amount of blue-green solid mixed with the orange-brown precipitate.
I think that the precipitate is copper (I) oxide and the blue-green solid is nickel(II) and copper(II) carbonate, but I don't understand why the oxide would form and if anyone has an explanation I would really appreciate it. Thanks.