1
$\begingroup$

While testing for formaldehyde (an aldehyde), I added Fehling's solution to the sample and let the test tube simmer in a water bath for some time. I returned ~15 minutes later to see a black/dark grey precipitate awaiting me, which looked very metallic (Almost like Tollen's test, but darker)

I try again and this time heat the tube for only five minutes: I obtain a buff yellow coloured precipitate which, on standing, gives me the same metallic precipitate I observed earlier.

I expected that Fehling's solution will give a red precipitate of $\ce{Cu2O}$ but it looks like the formaldehyde seems to be oxidizing it all the way down to metallic copper. Also, what is the reason for a yellow precipitate to form rather than a red one?

I could not take any pictures: We're not allowed to carry phones/cameras into the lab.

Improve this question
$\endgroup$
  • $\begingroup$ Yeah sometimes you even end up in a blue-green solution, with no precipitate, something you'd confidently conclude to be a negative result. But later I was told by my teacher that it actually is an aldehyde and that turning green is a sign of a reaction undergoing. Net result: I scored poorly in that test -_- $\endgroup$ – Sir Arthur7 Feb 5 '20 at 14:02
-2
$\begingroup$

In the reaction with aldehydes, Fehling's solution produces $\ce{Cu2O}$ as an orange or yellow precipitate. I don't know why the color is not always the same. The oxidation of an aldehyde produces an acid. And usually an acid cannot be more oxidized. There is one exception : methanal $\ce{HCHO}$. Because it can be oxidized in formic acid $\ce{HCOOH}$, and then into $\ce{CO2}$ . That is why Fehling's reagent produce metallic copper $\ce{Cu}$, $\ce{CO2}$ and no acid when reacting with $\ce{HCHO}$.

Improve this answer
$\endgroup$
  • $\begingroup$ Alright that does make sense. I did not consider the further oxidation. However, I don’t think I saw any bubbles of CO2 emerging. Could you please also provide a reference/reaction mechanism? $\endgroup$ – Aniruddha Deb Feb 5 '20 at 11:15
  • 1
    $\begingroup$ By all means, precipitated copper will not appear black. Sorry, the explanation is not correct. Please Google Oxidation of formaldehyde by copper(II) in aqueous solution byJ. J. BYERLEAYN D W. K. TEO. It specifically says that no gas evolution seen and they rule out the oxidation of formaldehyde to CO2. $\endgroup$ – M. Farooq Feb 5 '20 at 12:32

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.