Dark copper (I) / copper (II) complex and NaOH
This series of images shows that copper (I) chloride, dissolved in conc. HCl (30%) is almost colorless and that on oxidation by air, it becomes dark brown again. The brown complex is formed by adding copper wire to a solution of CuCl2 in conc. HCl. For more on this, see previous experiments with copper and HCl:
The copper slowly dissolves and the liquid becomes very dark brown, as shown in the experiments, described in the URL's above. If excess copper is used, then at a certain point, the liquid becomes lighter again. In this experiment a test tube is shown, with the copper wire left in the liquid and the test tube stoppered, so that no oxygen could enter the system.
The following picture shows the contents of the stoppered test tube.
The liquid is almost colorless, one can see the copper wire in the test tube.
Now, the test tube is opened and fresh air is allowed to come in. The following sequence shows the contents of the test tube, with a few tens of seconds between each image taken, and sometimes a little shaking. The most right picture is approximately 3 minutes after the left picture. So the liquid becomes darker quickly. The liquid, sticking to the glass, almost immediately gives a strong yellow/brown tinge to the glass.
Next step is to learn a little more about the dark brown compound formed. The copper wire was removed from the liquid and the liquid was allowed to stand much longer in contact with fresh air. It has become dark brown, almost black at that time. Probably it contains quite a lot of copper (II) at this moment, together with copper (I). A dilute solution of NaOH was prepared in the mean time, as shown in the next picture.
The contents of the test tube is poured in the dilute (but excess) solution of NaOH. The result is a yellow/brown slimy and voluminous precipitate.
When shaken, it changes to small particles.
Part of the liquid with precipitate is poured in a test tube and allowed to settle at the bottom. The solid seems to become more yellow. A brighter color is obtained after a few minutes.
What is the yellow compound? Or is it a mix of compounds?