Chevreul's Salt

This post is the Video Companion to this video.

Chevreul's Salt is a little-known copper compound that is quite easy to prepare, and has a few very interesting properties. In this post, I'll go over in detail what I did.

This experiment only requires two materials: copper sulfate and sodium metabisulfite. The former is sold at hardware stores as root killer for plumbing, and the latter is commonly found on eBay (since it is used by gold recovery people).

To start I made two solutions:

• 2.4g CuSO₄ · 5H₂O in 25mL distilled water
• 2.0g Na₂S₂O₅ in 10mL distilled water

This yielded a light blue solution and a colorless one.

Combining the two immediately changes the color to a beautiful emerald green.

Next, the solution needs to be boiled for several minutes. The solution steadily darkens, and once it reaches boiling temperature a brick red solid precipitates. It is a very nice crystalline precipitate that easily and quickly settles to the bottom of the test tube. This is Chevreul's Salt - Cu₃(SO₃)₂ · 2H₂O.

My yield was 1 gram of dark brick red crystalline powder.

Chevreul's salt is interesting for two reasons. One is its color. Just about all copper compounds are green or blue in color, and there are very few exceptions to that. So it's neat to see copper in a color like this. Second is the fact that it is a mixed valence compound. Take a look at the formula for the salt above - you'll notice that the charge only balances if some of the copper atoms are in the +1 state and some are in the +2 state! I.E. there is a +4 charge divided over 3 copper atoms. Perhaps a more descriptive formula for this compound is CuSO₃·Cu₂SO₃·2H₂O.

Now for some testing of this compound.

First, I wanted to show that Chevreul's Salt does indeed have both copper(I) and a copper(II) behaviors. The first test (left picture below) was to take a small spatula-full and add dilute hydrochloric acid. This precipitates copper(I) chloride as a white solid. I used dilute acid here because CuCl can redissolve in excess acid. The second test (right picture below) was to add a similar amount to concentrated ammonia solution. This dissolves the salt and changes color to a deep blue. This is the commonly-seen ammonia complex with copper(II): tetraaminecopper(II).

The next test was to distinguish this compound from the only other red copper salt I am aware of - copper(I) oxide. To do this, I took a small amount of my Chevreul's salt and added it to a test tube (left in the pictures below). To this I added concentrated hydrochloric acid, which releases sulfur dioxide gas. I drew this gas into a pipette and bubbled it through an acidified solution of potassium dichromate prepared in another test tube. (This solution was made by dissolving a tiny amount of K₂Cr₂O₇ into distilled water, and adding a few drops of dilute sulfuric acid.) The solution changed from yellow to green, indicating the presence of SO₂.

This is the specific gas test for SO₂, and proves that this is not Cu₂O (since that has no sulfur).

Here's an awesome picture of the salt under a microscope, at 16x magnification. You can really see the crystalline nature, and some crystals are transparent! Click on the photo for a larger version.

I really enjoyed this experiment because of the range of colors it produced, and how simple it was to conduct!