Element Collection

Element Collection

Friday, May 20, 2011

Copper Compound Conundrum, Part 2

Copper Compound Conundrum Part 2: Testing the Solution

If you remember from my last post, my reaction scheme for this was the following:

1) CuSO4 + 2NaHCO3 == CuCO3 + Na2SO4 + CO2 + H2O
2) CuCO3 + 2HCl == CuCl2 + CO2 + H2O
3) CuCl2 + 2NaOH == Cu(OH)2 + 2NaCl
4) Cu(OH)2 + H2SO4 == CuSO4 + 2H2O

After performing step 3 and filtering and drying the precipitate, I ended up with a dark green powder which was likely a mix of copper(II) hydroxide, oxide, and carbonate. All three of these compounds should react with sulfuric acid to form copper sulfate, so I figured I'd go ahead with the last step in my video's reaction scheme anyway. I should have ended up dissolving all the solids into a nice blue solution of copper sulfate like I started with. Here's what actually happened.


The original solution is in the beaker on the right. You can see that I ended up with an emerald green solution, and a blue precipitate!

This was very puzzling, so I decided to perform some experiments. You can see one of these in the test tube on the left. The solution looks very similar to my copper chloride step from above, so I wanted to see if this is what it was. Copper chloride solutions show different colors based on the concentration of chloride ions - concentrated solutions are green, and on dilution become blue. So, I transferred a few mL of the green solution into the test tube and diluted it with a few mL of distilled water. Sure enough, as you can see in the photo, the color changed to blue! This was a strong indication that I was correct.

To test this further, I performed the standard test for the chloride ion. This is done by simply adding a solution of silver nitrate. If chloride is present, a white precipitate of silver chloride will form. This is exactly what happened:

 The first photo is while dropping in the AgNO3 solution, which immediately forms the white precipitate. The second photo is after some had settled. Leaving it standing for a while longer, more settled and the turbidity disappeared with the color remaining the same. This to me suggests the presence of copper(II) nitrate in solution, as it should be.

The green liquid of my mystery product is indeed copper(II) chloride.

But how did this happen? Where did the chloride come from? There may have been some salt left over from reaction 3, but I washed the precipitate a number of times with water so that should have been mostly removed. Certainly there wasn't enough around to make such a concentrated solution of chloride. These are interesting questions that I am currently searching for an answer for. I'll update this post when I have it figured out.

Stay tuned for Part 3, where I investigate the identity of the blue precipitate!

Part 3: Testing the Precipitate 

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