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Author: Subject: Cu reactions with NaOH, NH4OH & (NH4)2SO4
RogueRose
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[*] posted on 3-5-2016 at 20:05
Cu reactions with NaOH, NH4OH & (NH4)2SO4


I was told that NaOH would oxidize Cu to give an oxide. This seemed to happen when I used molten NaOH but a 50/50 w/w of Lye/H2O seems to do nothing as far as oxidation and actually keeps any oxidation from forming on the copper (keeps its very shiny surface).

All pictures are the result of about 3 weeks of uncovered soaking in the solution which has resulted in evaporation of water and ammonium.

I am most confused as to the reaction of the few drops of H2O2 to the NaOH solution (Pic 2)


Here is a picture of 100ml/100g H2O/Lye + 40g Cu wire.
Cu + NaOH.jpg - 60kB

Here is a picture of 200ml/200g H2O/Lye + 40g Cu wire. I did add a couple drops of 3% H2O2 and it immediately gelled somewhat and started to turn the solution blueish.

Cu + NaOH + H2O2.jpg - 80kB

Here is a solution of 50g Ammoium Sulfate / 100g H2O & 40g Cu wire
Cu + AmmSulf.jpg - 28kB

These 2 are "ultra concentrated" Ammonia cleaning solution. It is supposed to be pure and I believe it is. I would guess it is below 10% Ammonium Hydroxide by weight to water. IN the last 4-7 days small flakes have started to appear in the solution. They seem to be copper but I am unsure how these flakes would have appeared. The container has been left uncovered for just under 3 weeks.
Cu + AmmHydrox.jpg - 64kB
Cu + AmmHydrox2.jpg - 210kB
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morsagh
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[*] posted on 4-5-2016 at 04:29


Just NaOH can´t oxidize Cu, there must be some oxygen or something to make galvanic cell. H2O2 can be used as oxidizer instead of O2 so this caused oxidation of copper wire in pic. 2. Your product is most probably Na2[Cu(OH)4].
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solitanze
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[*] posted on 4-5-2016 at 08:18


Indeed, what you have is a solution with cuprate ions. The higher the concentration of hydroxide, the bluer the color of the solution.

Such solutions are easily prepared, but isolating the pure substance isn't easy at all.
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MrHomeScientist
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[*] posted on 4-5-2016 at 09:45


I would think that leaving a container of ammonia (not ammonium!) out for that long, all the ammonia would escape.

You get the nice deep blue color in those beakers because of the ammonia complex with copper ions - it's very dark blue, so a tiny amount of dissolved copper will cause a strong color. None of those things should cause an appreciable reaction with copper, though. You need an oxidizer. In this case it was the hydrogen peroxide you added to one and atmospheric oxygen in the others, like those above have said.
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eanardi
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[*] posted on 19-5-2016 at 08:54


Once I did the reaction between NH4OH and Cu, after adding a few drops of 3% H2O2 the reaction ocurred spontaneously and all the copper dissolved. This stayed in my reagents shelf for some months, when i got it again there was a layer of ammonia and a layer of some orange (not what the copper Orange would look like) compound that i dont know what is, does anyone know what it could be? (After shaking the system everything mixed again and became a blue solution again)
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AJKOER
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[*] posted on 20-5-2016 at 18:27


Here is one possible answer per Atomistry.com which is an historical extract from the literature, to quote:

"Cuprous oxide is produced in the form of an orange-yellow, amorphous gel containing water by the reduction of an alkaline cupric solution with sodium hyposulphite:

2CuO + Na2S2O4 + 2NaOH = Cu2O + 2Na2SO3 + H2O.

A better method is the action of hydroxylamine on a cupric salt in presence of alkali. The initial light-yellow product is probably a hydroxide. In absence of air, the colour quickly changes to orange or brick-red, the phenomenon being probably due to elimination of water. The dry product contains 2 to 3 per cent, of water, but above low red heat this water is expelled, the metastable, yellow, amorphous oxide becoming transformed into the stable, red, crystalline variety. "

Link: http://copper.atomistry.com/cuprous_oxide.html

I suspect with time leaving Cu in the cupric solution produced the Cu2O. It is also possible, per my recollection, that the action of hydroxyl radicals on NH3 forms .NH2 which can further form NH2OH. The hydroxyl radicals could have been created from the action of light on H2O2, or a Fenton-type reaction involving cuprous and HNO2 (a known side product includes NH4NO2, see https://www.sciencemadness.org/whisper/viewthread.php?tid=64... ).

Here are the possible radical paths to NH2OH:

NH3 + OH· → H2O + ·NH2
·NH2 + OH· → NH2OH

See reactions [98] and [96] in reference, "On the aqueous reactions of the aminyl radical with molecular oxygen and the superoxide anion" at https://www.google.com/url?sa=t&source=web&rct=j&... and the presence of nitrite is a photocatalytic source of hydroxyl radicals, see https://books.google.com/books?id=RAJMmvbfyzgC&pg=PA383&... ).

[Edited on 21-5-2016 by AJKOER]
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