[pantone159]

I did a little investigation of what happens when thiocyanate (as KSCN) is added to a copper (II) solution (as CuSO4).
(This was sort of inspired by a question about an unknown copper compound posted on this board as well as sci.chem.)

The results were more complicated than I expected.  Upon adding small amounts of SCN- ion (less than 1.0 molar ratio compared to Cu(II)), I saw an apple-green solution, with a small amount of pinkish-gray precipitate.  Upon adding more SCN-, up to 1.6 molar ratio or so, more precipitate formed, and the green color of the solution gradually faded.  As even more SCN- was added, up to around 2.2 molar ratio, the solution changed to pale yellow, no longer green at all.

I suspect that the pinkish precipitate is Cu(I)SCN, formed as SCN- reduced Cu(II) to Cu(I), becoming oxidised to (SCN)2.  (Thanks to Borek for this suggestion).  My CRC describes Cu(I)SCN as white, (nearly) insoluble in H2O, insoluble in alcohol, and soluble in NH3(aq).  My precipitate did not dissolve in EtOH, and did dissolve in 2.5 m NH3, forming a deep blue solution that looks like copper tetraamine.  This is consistent with the CRC properties of Cu(I)SCN.

The CRC describes Cu(II)(SCN)2 as a black solid, which decomposes in H2O (to what?), and also describes (SCN)2 as yellow (liquid at room temp) which also decomposes in H2O (to what?).

I wonder what my green and yellow solutions are.  The green might be some complex of Cu(II) and SCN-, or it might be the decomposition products of Cu(II)(SCN)2 with water.  The yellow might be the decomposition products of (SCN)2.  I don't know, however, and ask if aybody here has any insight.

I am attaching a photo of the products, L to R these are the green solution (low SCN amount), pink precipitate (Cu(I)SCN), and yellow solution (large SCN amount).

[Alberto_Kravina]

Watch out! HCN could be formed!

6Cu²⁺ + 7 SCN^- + 4 H₂O ---> 6 CuSCN + HCN + SO₄^2- + 7 H ⁺

[pantone159]

Watch out! HCN could be formed!

It is conceivable that this did indeed happen. When I was adding the larger amounts of SCN-, I smelled something strange.  Fearful of exactly this, I quickly stoppered my test tubes.

I can't describe the odor well, other than it was unpleasant.  I didn't try and sniff further to characterize it (for obvious reasons!).  I haven't ever (knowingly) smelled HCN, so I wouldn't recognize it.  (I have heard that it doesn't actually smell like almonds, contrary to 'conventional wisdom'.)  I don't seem to have got any ill effects.

When I get around to disposing of the solutions, I'll treat them as if they do contain CN-, just in case.  I understand a good disposal method is to add the stuff to an excess of NaOH and NaOCl, let stand for a few hours, then pour down the drain with lots of water.

[Alberto_Kravina]

HCN has an bitter-almond smell

[Borek]

That not everyone smells AFAIR.

[pantone159]

I sort-of remember some thread on sci.chem where a number of people (who said that they were familiar with the odor of HCN) said that it did NOT smell like bitter almonds to them.  Some posters suggested that this supposed 'almond' smell was a dangerous misconception, in that somebody who DID smell HCN might dismiss it since they didn't smell almonds.

Personally, I don't know either way.

[Borek]

That's hardly a reliable source but I recall some forensic documentary on Discovery. You may know the story - woman died in her bathroom, as she took a KCN stuffed pill. Some other woman wanted to kill her husband and she gave him identical pill while putting bottle with the stuffed pill on the shelf in the market to suggest it was some "outside" job, not her own. I am probably not entirelly clear on theat buit my English often weakens on Saturdays evenings

Back to the smell. During body examination at morgue one of the examiners smelled almonds. It was commented that it was quite a coincidence, as most people are not able to smell HCN. Second examiner didn't smelled anything, but they did some test to confirm KCN presence.

[pantone159]

I did a quick search of sci.chem comments about HCN smell, and the general opinion is that, depending on the individual, the smell of HCN is extremely variable.

Some cannot smell it at all.
Some find that it does, indeed, smell like almonds.
Others find that it has a distinct smell, but not at all like almonds.  'Acrid', 'unpleasant', 'metallic' were some of the adjectives used.

To calibrate your own nose, one can (careful!) create a known concentration, much less than a dangerous amount, and see what you smell yourself.

[Alberto_Kravina]

To calibrate your own nose, one can (careful!) create a known concentration, much less than a dangerous amount, and see what you smell yourself.

That is not so safe at all, I think. In my opinion I wouldn't try it, HCN is a violent poison!!!
If you accidentally create a bigger amount then....

[AWK]

Cu2+ + SCN- = [Cu(SCN)]+ (green solution)
 [Cu(SCN)]+ + SCN- + Cu(SCN)2(s) - black precipitate
Cu(SCN)2(s) decomposes spontaneously
2Cu(SCN)2 = 2CuSCN(s) + (CN)2  - almost white precipitate
Both, Cu(I) and Cu(II) form complexes with rodanide excess. Complexes of Cu(I) are more stable

[Borek]

2Cu(SCN)2 = 2CuSCN(s) + (CN)2  - almost white precipitate

Is it (CN)₂ and not (SCN)₂? What happens to sulphur?

I have halfpotential for SCN^-/(SCN)₂ reaction given in tables and thought thiocyanide behave analogously to halides (isn't it classified as pseudohalide?).

[AWK]

Cu2+ + SCN- = [Cu(SCN)]+ (green solution)
 [Cu(SCN)]+ + SCN- + Cu(SCN)2(s) - black precipitate
Cu(SCN)2(s) decomposes spontaneously
2Cu(SCN)2 = 2CuSCN(s) + (SCN)2  - almost white precipitate
Both, Cu(I) and Cu(II) form complexes with rodanide excess. Complexes of Cu(I) are more stable

Correction. As Borek noticed - should be (SCN)2 in the last equation