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Topic: Copper(II) Acetate(l) + Ascorbic Acid(l) -> Cu2O + green solution?  (Read 1603 times)

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Offline CopperPiggy94

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Hello everyone,
for the last couple of weeks I've tried to gain a better understanding of chemistry, as I was unfortunate enough to not have chemistry classes back in school.
At the moment, I am trying to convert elemental copper into different copper compounds to be used as pigments for paint.
For that, I have started with copper(II) acetate and CuO, both of which were successful.

Then, I saw a video on using a solution of ascorbic acid in a copper-salt solution to convert copper II ions to copper I ions, resulting in Cu2O. I have also succeeded with that it seems, as the bottom sediment of my solution seems to be the color of Cu2O.
However, the remaining solution is somewhat green. I wonder what this solution contains?

The solution I used as my copper II acetate source was made from elemental copper, acetic acid and H2O2. The ascorbic acid used should be pretty pure - I have used a supplement, that supposedly contains nothing but ascorbic acid powder.

I have added an access of ascorbic acid to convert as much copper acetate into copper I oxide.
Attached you will find an image of the beaker the solution is currently stored in, along with the Cu2O sediment.
If anyone has a suggestion on what this might be, I would be very grateful!


Offline Hunter2

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Re: Copper(II) Acetate(l) + Ascorbic Acid(l) -> Cu2O + green solution?
« Reply #1 on: August 21, 2023, 10:08:30 AM »
Its complex Cu I,II compounds. To get a better reduction use alcaline pH. Like the basic Fehling reaction. Use sodium tatrate and copper II salt. And add sodium hydroxide to  get alcaline deep blue solution. Reducer can be alco Ascorbic acid or even glucose.

https://en.m.wikipedia.org/wiki/Fehling%27s_solution

Offline CopperPiggy94

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Re: Copper(II) Acetate(l) + Ascorbic Acid(l) -> Cu2O + green solution?
« Reply #2 on: August 21, 2023, 10:21:41 AM »
That seemed to do the trick! I may need to add more sodium hydroxide eventually, since the solution is still slightly green. I will do that once I reduced the volume of the solution a bit - thank you so much for your response!

Offline rolnor

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Re: Copper(II) Acetate(l) + Ascorbic Acid(l) -> Cu2O + green solution?
« Reply #3 on: August 22, 2023, 01:46:34 AM »
Great to see a beaker on this forum!

Offline CopperPiggy94

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Re: Copper(II) Acetate(l) + Ascorbic Acid(l) -> Cu2O + green solution?
« Reply #4 on: August 22, 2023, 05:13:23 AM »
Of course! Borosilicate glass is something my clumsy self needs.

Alright, here's an update: I have boiled down the solution by quite a bit and added aqueous solutions of sodium hydroxide until the yellow precipitate stopped re-dissolving into solution. After that, I had an opaque, orange-colored solution which i boiled down some more.
However, at some point, the precipitate seemed to re-dissolve, and the solution turned a very nice looking dark-red, almost wine color.
There was some Cu2O that precipitated out, which I have filtered out, but it is much less than anticipated. I suppose a lot has somehow re-dissolved into solution. I tried crashing it out by taking small samples and adding acetic acid. Nothing happened, so I took a new sample and tried H2O2 - nothing, and sodium hydroxide also didn't appear to do anything. Adding more ascorbic acid similarly did nothing to the solution.
Does anyone have an idea what has happened to the solution?

/edit:
It might be worth noting that the intensity of the solutions color did not increase after reducing its volume, nor did any precipitate fall out of solution.


« Last Edit: August 22, 2023, 08:02:43 AM by CopperPiggy94 »

Offline Corribus

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Re: Copper(II) Acetate(l) + Ascorbic Acid(l) -> Cu2O + green solution?
« Reply #5 on: August 23, 2023, 05:03:26 PM »
Copper ion and ascorbic acid undergo some complex redox reactions that evoke Fenton chemistry.

See, e.g., Zhou et al. RSC Adv., 2016,6, 38541-38547  (Link)

They identify dehydroascorbic acid, 2,3-diketogulonic acid, and L-xylosone as downstream ascorbic acid oxidation products.

I note that copper (II) ascorbate was long ago considered as a food preservative, but with a drawback:

"Dehydroascorbic acid may become oxidized further to CO2 and other byproducts [probably those mentioned in the more recent article]. The formation of these secondary metabolites renders the use of copper(II) ascorbate unsuitable for clear beverages, since these types of products assume a slight reddish-brown hue after extended shelf life." (emphasis mine)

(See, Graf, E. J. Agric. Food Chem. 1994, 42, 1616-1619, Link)

"Reddish brown hue" seems pretty consistent with what you're getting. So, I'm guessing the copper ions in your treatment are catalyzing the extensive oxidation of ascorbic acid, rendering a mixture of organocopper complexes that add up to some reddish brown muck.
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline rolnor

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Re: Copper(II) Acetate(l) + Ascorbic Acid(l) -> Cu2O + green solution?
« Reply #6 on: August 23, 2023, 10:19:59 PM »
It seems you dont follow a recipe and thats fine, but maybe you should get a recipe and repeat the reaction, so you learn what you did wrong the first time?

Offline CopperPiggy94

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Re: Copper(II) Acetate(l) + Ascorbic Acid(l) -> Cu2O + green solution?
« Reply #7 on: August 24, 2023, 03:36:43 PM »
What @Corribus said seems to be the case. I have since given up on the solution, as there is too many variables in it for me to try and recover copper, as its more effort than what it's worth.
@rolnor, I have sort of used a recipe, although it was adapted from another copper compound using the same ascorbic acid method. Of course, I only added sodium hydroxide after the discussion in this thread. I will likely revisit this topic in the future, but I will stick to methods that are known to work well.

Offline rolnor

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Re: Copper(II) Acetate(l) + Ascorbic Acid(l) -> Cu2O + green solution?
« Reply #8 on: August 27, 2023, 10:26:37 AM »
I gave you a "mole snack".

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