condennnsa
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Copper hydroxide by electrolysis
I'm electrolyzing a solution of KNO3 with copper anode and stainless steel cathode.
Right when I turned the power on, the cathode produced lots of gas as normal, and the anode started corroding to a nice blue precipitate, but after
about 2 minutes of runtime the SS cathode stopped bubbling completely, and the solution became filled with different hues of green and blue
precipitates, also black. I also noticed that the cathode turned a slight copper colour at the top. My electrolyte is about 20 grams of KNO3 in 200ml
of water. The cell draws a large current, I measured 2.8A at 5 volts.
The solution has a strong smell of ammonia.
What's going on? Why did the cathode stop producing hydrogen? Why did copper plate on the cathode? I thought Cu(OH)2 is insoluble. thanks
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Random
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With KNO3 electrolysis maybe HNO3 and K ions were formed. K ions made the KOH in the solution, but at the another electrode HNO3 made it back into
KNO3. Now everything other could be from corrosion of KOH or HNO3 of electrodes.
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12AX7
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Nitrate attacks the copper anode, that much is straightforward. KOH (from the cathode) reacts with nascent copper nitrate, forming Cu(OH)2
precipitate, which is blue (turning black on mild heating). But that doesn't explain the precipitate. Thing is, the nitrate is also reduced at the
cathode, forming nitrite, N2 and ammonia (how much of each, who knows). I suppose this reaction is going to depend strongly on the type of cathode,
and its overpotential for all these reactions.
Tim
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not_important
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Near the boundary between acidic and alkaline you can get greenish basic salts.
Cu(OH)2 has a low solubility, but it takes very little copper to given a visible copper tint to silvery metals. BTW 5 volts may be a bit high for
this cell unless the electrodes are widely spaced, in which case youe expending muxh of the energy in heating the electrolyte to convert Cu(OH)2 to
CuO.
What current density does this work out to be?
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condennnsa
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I filtered the precipitate and boiled it, most of it turned black, the solution remained colourless. I still don't understand how the ammonia formed.
Is it true that having copper compounds in ammonia and nitrate solutions can lead to the formation of dangerous explosive copper compounds (one of it
is called copper tetramine nitrate, i read)?
Anyway , what would be the best electrolyte for making pure CuO, without unwanted side reactions like in my case making of NH3 from KNO3? Wikipedia
says that sodium bicarbonate is suitable, what do you guys think?
I have also tried to make it with NaCl, it failed totally, the precipitate was not black, but orange-red, i think it was Cu2O.
Would it work with NaOH solution? thanks
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bquirky
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Ive done this most successfully with NaOH but find that the copper anode tends to pacify requiring a higher voltage to run the cell.
ive found that this can be minimised by keeping a very high current density on the anode as the gas generated dislodges the accumulated oxides.
Despite some texts CuOH is solubile in an NaOH solution. Its solubility increases with NaOH concentration.
Because of this it is not uncommon to find metallic copper deposited on the cathode but its appearance (shiny or dark or fuzz) is dependant on
current density.
As for making oxide from the hydroxide. it decomposes very easily Just stick it in the oven for 10 or 20 minutes
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Taoiseach
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>Is it true that having copper compounds in ammonia and nitrate solutions can lead to the formation of dangerous explosive copper compounds (one of
it
>is called copper tetramine nitrate, i read)?
Not easily. Boiling copper in NH4NO3 solution leads to the formation of some TACN tough. Complexation occurs due to some ammonia formed by the
reduction of NH4+. The TACN is relatively harmless btw as it comes in hydrated form -> nearly impossible to detonate.
It IS dangerous to fuse copper with ammonium nitrate tough!
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condennnsa
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Well I tried it today with NaOH(about 20 g in 200ml of water). The cathode produced lots of hydrogen, but to my surprise, the anode was also
bubbling, though it was obviously corroding as well. So I guess that it preferred to take electrons from the oxygen ions, rather than copper. The
current was pretty high, but the copper corroded at a comparatively slow rate, comparing to the NaCl and KNO3 solutions.
As bquirky said, There were small amounts of copper plated on the cathode, but the little CuO that resulted after the run, was a beatiful pitch black,
better than that i got from KNO3 solution.
bquirky, when you tried yours, did your anode also bubble oxygen gas?
I'm still looking for an ideal electrolyte for this process, has anyone here tried other ones that could perhaps share their results? thanks
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12AX7
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In hydroxide solution, you have two reactions at the anode: the lowered oxygen overpotential (hence bubbling O2), and the corrosion of copper,
producing Cu(OH)4(2-), the cuprate ion, which has a deep blue color. Once the solution becomes relatively saturated with cuprate, copper will plate
on the cathode, probably in a spongy deposit because it's disturbed by the bubbles.
I explained the ammonia, read more closely above.
Tim
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