electrolysis of copper in CH3COOH (acetic acid)

[urgr8estfear]

Hi all!

 

I'm a bit of a fan of electrolysis, there is something compelling about those bubbles rising from the metal while it falls apart.

 

Anyway, I gave up on messing with NaCl solutions for this 'hobby' because I find the Cl2(g) is a bit to aggressive to toy with.

 

So I tried some CH3COOH this time, concentration around 8%.

Very nice stuff to remove oxidation from copper wires and I wanted to try electrolysis with it.

So I used two copper 2.5mm^2 wires and a variable power supply set at 12V DC.

Fired it up, waited a day and I wanted to share the results with you because I have some questions....

 

Here we see the kathode after 24 hours, which I tapped every few hours to make the brown oxide fall off because it hampers the electrolysis :

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Some verdigris just above the surface of the liquid, lotsa brown stuff on the copper wire which I believe is copper oxide, and some kind of deposit on the bottom which I'm not sure of what it is.

 

The anode (or whats left of it) looks like this :

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Obviously dissolved for the most part and again some mysterious deposit right below the copper wire.

 

From above you can see the deposits do not move around but just sit at the bottom.

Also, I get a nice blue colored liquid which I hope is loaded with Cu2+ ions.

 

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And here you can see how the copper wires look after I pull them from the liquid, the colors look a bit different from when they are still submerged:

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Some verdigris (green), lotsa brown copper oxide (I think) and some pink/brown stuff at the bottom of the kathode.

The pink brown stuff is the only one that is firmly attached to the copper wire.

The verdigris and brown oxide (I think) comes of easily.

 

I know this is kind of a hefty first-post but I'm just dying to get some answers:

 

1. Why do I get sediments at both the kathode and anode?

2. Is what I describe as brown oxide really brown oxide? And if so is it CuO or Cu2O?

3. Whats that pink/brown stuff I only see at the kathode in sediment form and stuck to the wire?

4. If the liquid has such a nice blue color from the copper ions, why don't they stick to the kathode? I expected something in the line of Cu2+ and 2e --> 2 Cu??

5. Is there any way I can make those (assumed) copper ions stick to another copper wire to make a thicker copper wire? I'm actually looking for a way to make thicker wires myself, or some bigger chunk of pure copper in any shape.

Preferrably using electricity and bubbles included!

6. Is there any way to 'recycle' the copper oxides which have formed?

 

I hope I'm not discouraging anybody from answering because I have so many questions....please try!

Thanks in advance!

[YT2095]

the pinky brown will be copper metal itself, the blue soln should be copper acetate (crystalise some of that, the color and crystal forms are Fantastic!).

 

you`ll find the Anode will be the most eroded, and the Cathode will have copper deposits on it.

 

CuO is brown sometimes but pure black when calcined.

 

with the correct voltage and current per cm^3 you can make thicker copper wire at the cathode sure, PCR will be the best technique to employ though.

 

hope that helps a little

[urgr8estfear]

Indeed it does help YT2095! Thanks.

 

But it also leads to new questions. Damned Wikipedia.

Copper acetate is listed as Cu2(CH3COO)4, supposedly a dark green crystal solid.

Obviously whats in my jar is not solid, and blue instead of dark green.

Does this mean that what I have in the jar is the hydrated version : Cu2(CH3COO)4(H2O)2? (which supposedly is blueish green).

 

http://en.wikipedia.org/wiki/Copper(II)_acetate

 

I'm crystalizing some of it as you read....to see if it turns dark green.

 

I checked out the PCR (periodical current reversal) thingie:

http://www.freepatentsonline.com/6511591.html

Looks cute but kinda difficult to make a DIY version.

(unless I can somehow abuse a audio amplifier for this in combination with my computer:D )

 

So I'll try the less complicated method of DC current first.

Is it safe to assume that in my case, the copper did not stick to the kathode because of excess hydrogen gas bubbling?

I'm guessing so because the pink/brown bottom of the kathode was sitting in a pile of copperoxide sediment.

Maybe tapping the kathode to get rid of the oxide on the copper wire was a bad thing to do.

Time for a new test..........more bubbles!

[YT2095]

PCR can be quite simple, you`ll need an AC transformer, mains in and about 6VAC out.

 

then a single diode to rectify this, so you get 50 forward biased pulses per sec.

but then if you put the correct resistor Across this diode, you`ll get a percentage of Reverse bias current too.

 

1:10 is idea for most applications.

 

and yes, do the maths correctly and you can even charge ordinary Zinc Carbon batts this way too without the plates going spongey!

[urgr8estfear]

Note : initial Ohmic resistance 27KOhm.

With 12V DC turned on I measured

 

7:35 PM about 53 mA, which translates to about 226 Ohms.

10:32 PM about 58 mA, lotsa oxide at the kathode and far less hydrogen bubbles.

01:14 AM around 61 mA, bubbles can be counted individually.

That's funny ; more power and less obvious activity.

[YT2095]

ok using that for example, you`de need a 12VAC transformer of the same power rating, and use a Single diode in line, so you get pulsed DC (50Hz since you`re in the Netherlands), then you would use a 1 tenth of that forwards power.

 

in your case of 226 Ohms, then you`de put a 2K2 resistor across the diode.

[urgr8estfear]

Thanks again YT2095!

I understand, seems easy enough, definately worth the try.

Too bad I don't have any ready to be used AC transformers around here.

 

Maybe I could rip one out of an old alarmclock or old small radio or something.

I think I may have something appropriate lying around here somewhere, even with a matching fuse on the 230V AC side.

Rectifiers and resistors are present.

I'm just wondering if I need to add some kind of added safety on the 6V/12V AC side.

A handy guy like me is sure to short it out one day!

But I also know maximum power drawn from the secondary is limited by it's impedance even when shorted, though it may be enough to fry the transformer before any fuses pop.

 

And how did you figure out I'm in the Netherlands?

Are you IP tracing me from the forum's server side?

 

Allright, I think I found a suitable transformer :

 

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No seriously :

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It measured out at 9.6VAC unloaded.

First thing I did was add a 56 Ohms (1) resistor to limit the maximum current to around 160 mA and get down to about 6VAC.

Secondly, just as YT2095 recommended I got a rectifier diode 1N4004 (2) with a 10K resistor (3) attached in parallel.

 

I used a 10K resistor because I found it to be hard to get a stable reading of the solution's resistance. Ions & sediments all alter this value and I have no idea how this will turn out with my primitive PCR 'device'.

So for starters I used fresh acetic acid. (still have the copper acetate solution stored)

 

Here look :

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Now after only a few minutes I'm running at ~10 mA, which translates to 600 Ohms DC.

 

So I plan to tweak and fiddle with this thing until I get it right ---> nice clean copper deposits on one of the electrodes.

[urgr8estfear]

UPDATE 28-7:

The 1:16 ratio the 10K resistor as mentioned above produced, gave me bad results ; with one of the electrodes completely dissolved the other was covered in copperoxide which washed off easily.

 

One big difference I found compared to the DC test as decribed above is the resistance of the solution. This does not seem to change at all with primitive AC PCR.

 

So now ,10K Resistor (3) replaced with variable resistor set at 6K Ohms.

Now I'm running at 1:10 exactly.

Precisly what YT2095 recommended.

Lets wait a day or so and see if this improves things.

[YT2095]

10:1 works great for zinc carbon, the figures may shift with a different metal (I never went that deep into it).

 

and no I knew netherlands as your IP resolved to .nl

I watch whois online (mostly for spam traffic so I can stop it before it makes a mess of our site).

 

Acetic acid isn`t the Best electrolyte either and you also have to factor in the concentration as well as the current density in cm^3.

 

I don`t do the maths thing myself, and our best guy for that (woelen) has left.

[bob000555]

Odd that you should generate any copper oxide at all; what your doing is basically a crude Kolbe Reaction whereby acetic acid is converted into ethane and carbon dioxide. I could see copper carbonate, but oxide? Anyway....Woelen left, when did this happen?!?!

 

http://en.wikipedia.org/wiki/Kolbe_electrolysis

[urgr8estfear]

I figure the math on this thing is going to be hell.

 

I'm measuring DC mA here, my multimeter won't do AC mA and somehow the AC amp reading always reads zero allthough 0.01A would fit nicely on the display.

 

Even if it did, it would make calculations next to impossible.

The DC Ohm reading of the solution keeps on changing and so will its capacitance.

Not to mention the mutilated AC waves the diode/resistor combo produces.

 

So I'll stick with the DC mA reading for now.

Funny thing is, when I halve the distance between the electrodes in the solution the mA reading does not double.

Not even close, 7 cm distance reads 10 mA current through the solution and 1 cm reads 20 mA.

 

So I'll skip the math.

 

The reason for the CuO is not so strange I think.

8% acetic acid contains over 90% water.

Meaning, I should get both H2(g) and O2(g) at the electrodes.

(aside from ethane and CO2 as Bob said) thanks bob!

If I electrolise 1 H2O(l) I'd get 1 O2- ion that fits nicely onto a Cu2+ ion.

Right?

I think the O2- is competing with the e- for the available Cu2+.

 

Now I've read about the Kolbe electrolysis I'm totally confused.

If I understand the reaction correctly (why I probably don't) this would eventualy lead to a depletion of the acetic acid.

Wiki reads :

CH3COOH → CH3COO− → CH3COO· → CH3· + CO2

2CH3· → CH3CH3

This implies that both CO2 and C2H6 would become (g) and simply fly out of the solution?

Which should mean the solution should get less acidic.

 

Problem is.... I did DC electrolysis of this solution....for a day or two...while dissolving 2 electrodes completely.

When I inserted the 3rd fresh electrode the reactions became more violent after about one hour or so after insertion.

Suddenly I had bubbles on both the kathode and the anode and it started making a funny sound like a welding job under water.

This is where I turned off the power, because I don't want to walk away from violent reactions which I do not understand at all.

 

Anyway, you know what bugs me most of all?

That both the DC and AC version of electrolysis both result on copperoxide.

I figured, that the kathode would be the biggest attractor of Cu2+ ions once I've got those in the solution.

I hoped for the anode producing O2(g) and the kathode producing H2(g), then both could just fly out the window leaving the kathode to absorb Cu2+ ions once they were in the solution.

Appearantly I've underestimated the nastyness of Oxygen.

 

Maybe I it's time to try another electrolyte.

Recommendations anyone?

Ofcourse I prefer cheap, easy to obtain and harmless electrolytes.

[bob000555]

I think what’s happening when u put in a third electrode is that you starting to use copper II acetate as an electrolyte which is where it starts getting far too complex for me as copper acetate is a complex salt (no pun intended).