Experiment with copper (II) and copper (I)

This page describes a dark green/brown compound, which is formed, when copper (I) and copper (II) are put together in concentrated hydochloric acid. What this dark brown compound is, is not clear (at least not to me, the writer of this). Another experiment is described at http://www.woelen.nl/chem/experiment_with_cucl2.htm, which also shows formation of the dark brown compound. The experiments, given here, support the idea that the dark brown compound is NOT simply copper (II) at very high concentration.

For this purpose, two liquids are prepared:

1) A very small amount of CuCl2.2H2O is dissolved in 30% HCl. This yields a lime green/yellow liquid. To this liquid a very small amount of red powdered copper (I) oxide (Cu2O) is added. This oxide quickly dissolves and the liquid becomes dark green/brown. This shows that a very small amount of copper (I) and copper (II) yields a dark compound in concentrated HCl. If only copper (I) is present, then the liquid is colorless, if only copper (II) is present, then the liquid is yellow.

2) A saturated solution of CuCl2.2H2O in 30% HCl is prepared. This yields a dark green liquid, with a little undissolved CuCl2.2H2O remaining, sticking on the glass. It is believed that the dark green species is the CuCl4(2-) cation.

Now we have two liquids, looking very similar, but definitely different. Liquid (1) is very dilute in Cu, liquid (2), on the other hand, is saturated in Cu. This shows that the compound, consisting of both copper (I) and copper (II) has a much more intense color than the CuCl4(2-) complex. Below some images are given, each time comparing liquids (1) and (2). Liquid (1) at the left and liquid (2) at the right.

The first image, just below, shows both liquids, which served as a starting point. They look very similar, but on what is sticking on the glass, one can see that the right liquid is much more concentrated.

Left: copper (I) and copper (II), very dilute, right: copper (II), concentrated

The figure below shows the same test tubes, lying on a slightly tilted piece of paper. This picture shows that the copper (I) / copper (II) liquid is somewhat more brown than the concentrated CuCl2-solution.

                Left: copper (I) and copper (II), very dilute, right: copper (II), concentrated

Now both liquids are diluted approximately four times with water. This dilution shows that they are really different. The left contains a small amount of copper. This liquid only has a very pale green color after the dilution. The right one has a green color. It does not show plain blue aqueous Cu(2+), still some copper is coordinated to chloride.

               Both liquids four times diluted, left with copper (I), right with concentrated copper (II)

Finally, the liquids are diluted another three times. This results in an almost colorless liquid on the left and a light blue liquid on the right, with all copper present as aqueous Cu(2+).

            Both liquids three times diluted again, left with copper (I), right with concentrated copper (II)

This experiment shows that the dark green/brown color, formed on the left is not simply a copper (II) complex with chloride, as is shown by the reference test tube on the right side of the pictures. The copper (I) / copper (II) solution has a much more intense color and its hue also is somewhat different.

For comparison, an almost saturated solution of CuCl2.2H2O was prepared again, but now some Cu2O was added to this liquid. This results in formation of the dark brown compound, but now at higher concentration. This liquid is totally opaque and even after minutes of standing, still a brown layer is sticking to the glass, showing that it is concentrated. Now one can see, that this is really darker than just saturated CuCl2.2H2O solution in concentrated HCl. Below, the more concentrated copper (I)/copper (II) solution is shown, next to the liquids (1) and (2), allowing easy comparison.

                Comparison: dilute copper (I)/copper (II); concentrated copper (II); concentrated copper (II) /copper (I), 2 times

NOTE: The experiments can also be repeated with copper sulfate instead of copper chloride. They can also be done with copper metal instead of copper (I) oxide. Use of copper metal results in in-situ formation of copper (I).