Element Collection

Element Collection

Sunday, October 13, 2013

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!


  1. Sweet. I have worked with a lot of physicists (I'm a chemist) and didn't meet a lot of them with your enthusiasm for chemistry. On the other hand, I was a physics undergrad, and haven't met that many chemists who like physics or math. It's all science to me, and much of it can be super cool. Just subscribed to your youtube channel. Keep it going, and stay safe! Good stuff, man.

  2. Hello! do you know what is the overall reaction?

  3. what exactly is the green solution? do you know the green compounds name?

    1. No idea! Presumably it's some sort of copper-sulfite complex. There are a few copper complexes that are similar in color.

  4. Have you tried to heat the red salt to see if it can easily melt?

    1. I haven't tried it, but I'm nearly certain that it will decompose before melting. My guess is it will release SO2 and end up as CuO.

  5. Do you happen to know what this salt is soluble in(in order to obtain larger crystals)?