I'm helping my sister out with a science project she had at school. It has to relate to green energy, so I suggested breaking the hydrogen-oxygen
bonds in water, and tying it in with hydrogen fuel cells. Anyway, as I has testing out the setup, which was:
After two or three minutes of everything going smoothly, I accidentally created an arc between the wires, flush with the surface of the water. A deep
yellow chemical was emitted from this point into the water. I have read about how you can crack sodium and chlorine from sodium chloride with
electricity and extreme temperatures, and the arc would have provided both.
So, was I seeing chlorine, or some form of colloidal copper?
Also, I'm not sure about this, but which electrode would oxygen appear on? Positive or negative? I expected one of my wires to rapidly oxidize, but it
never happened.woelen - 18-5-2009 at 12:07
What you have done is not making chlorine, nor oxygen.
At the anode (+) you get this yellow material, and this is hydrous copper(I) oxide. The copper comes from the anode (which erodes) and the oxide is
formed from hydroxide, which is formed at the cathode besides the visible hydrogen.
At the cathode, water is decomposed to hydroxide and hydrogen gas, while electrons are accepted from the cathode.
At the anode, copper metal is oxidized to copper(I) and/or copper(II), while electrons are donated to the anode.
If you want oxygen, then you should not use a solution of sodium chloride. If you use a solution of sodium hydroxide, then you obtain oxygen at the
anode, even with a plain copper wire and under such conditions the copper only very slowly erodes. Be careful though with sodium hydroxide, this is
really caustic and slowly dissolves skin (slippery feeling) and results in irreversible damage, when it comes into the eye.
With a graphite anode, you obtain chlorine gas at the anode and at a later stage you obtain hypochlorite and chlorate, due to the buildup of hydroxide
in the solution.
The spark you had did not do anything at all. The yellow material you observed will be formed anyway, especially if you continue electrolysis of a
salt solution with copper electrodes for a somewhat longer time. Just try it.Hydragyrum - 18-5-2009 at 14:18
There is a large excess of chloride present. Is it not possible that the copper was oxidised to Cu(II) which would then form the
tetrachlorocuprate(II) ion, which can appear green when mixed with aqueous Cu(II) but can also appear yellow when no Cu(II) is present? - as in this
case probably.woelen - 18-5-2009 at 23:59
No, I don't think so. What the OP observed is not a clear solution, but a flocculent yellowish/brown solid floating around in the liquid. I have seen
this stuff myself many times as well, as I have done a lot of electrolysis experiments with salt and copper wire.
Tetrachlorocuprate(II) can appear yellow, but not in aqueous sodium chloride. Even the most concentrated NaCl-solution, when CuCl2 is added, gives a
green solution, like grass, and it is definitely not yellow. Besides that, a copper(II) solution would be clear.
High concentration of chloride ion favors formation of copper(I), copper metal slowly dissolves in a solution of CuCl2 (or CuSO4) in conc. NaCl,
giving a colorless solution, which easily forms yellow/brown flocculent matter due to aerial oxidation.The Fountain of Discordia - 19-5-2009 at 03:11
Actually, I do not know if it was a solid, and it was pure yellow, not yellow brown. Additionally, there is a detail I forgot. From on the wire
attached to the cathodic port on the battery, a qauntity of dark material formed.
To qualify my precise meaning in relation to color, copy these hex values into a color tool of your choice.
Yellow: #F6FF00
Yellow-Brown: #FCD400
I would also like to thank you all for providing insight.woelen - 19-5-2009 at 03:19
The dark material at the cathode most likely is impure copper. From the anode copper dissolves, part giving the yellow material, but some remains in
solution and that can make it to the cathode, where it is reduced to (impure) copper, which looks almost black.
Please try the experiment again and you'll see that the yellow material is a solid and not a clear solution. If the color is F6FF00, then it is
somewhat greenish/yellow and that can be explained as well. Your material then is contaminated with a small quantity of copper(II) and you have a
mixed copper(I)/copper(II) oxide/hydroxide/chloride precipitate. Rather complicated stuff and not a well-determined single chemical entity.
The yellow material looks like this:
[img]http://woelen.homescience.net/science/chem/riddles/copperI+copperII/exp0005/exp0005-14.jpg[/img]
When it is more impure, it might look like this:
[img]http://woelen.homescience.net/science/chem/riddles/copperI+copperII/exp0005/exp0005-10.jpg[/img]
EDIT: The image viewer of the forum does not work anymore ???
If the images do not show up, please copy the URL between the tags (excluding the tags) and put it in the address bar of a browser and press enter.
[Edited on 19-5-09 by woelen]The Fountain of Discordia - 19-5-2009 at 03:41
Will do. The problem is that I produced insufficient qauntities of the substance, just enough to leave a swirl in the water, because if I had cracked
some chlorine, it could have been dangerous. Hydragyrum - 19-5-2009 at 06:11
... because if I had cracked some chlorine, it could have been dangerous.
No too dangerous if you do it outside in a well-ventilated area - as you know, chlorine is a gas so it mostly blows away.Hydragyrum - 19-5-2009 at 06:19
At the anode (+) you get this yellow material, and this is hydrous copper(I) oxide. The copper comes from the anode (which erodes) and the oxide is
formed from hydroxide, which is formed at the cathode besides the visible hydrogen.
woelen, you are right - here is an article (sorry, I do not have full access) which follows the procedure described above.woelen - 19-5-2009 at 07:07
Will do. The problem is that I produced insufficient qauntities of the substance, just enough to leave a swirl in the water, because if I had cracked
some chlorine, it could have been dangerous.
The experiment with copper wires even can be done inside for prolonged time, simply because no chlorine is formed at all when copper wires are used.
Btw, if you use a 6V lantern battery, keep it when it is depleted. Such batteries contain a fantastic set of large graphite electrodes (4 of such
electrodes). It is a somewhat messy job to salvage these electrodes, but it is worth the effort. Such electrodes can be used for many cool
electrolysis experiments.
For more serious electrolysis experiments I suggest you to use a real power supply. Electrolysis is a power hungry process and with batteries (even
large ones) you'll see that you end up with depleted stuff quite soon.The Fountain of Discordia - 19-5-2009 at 08:00
??? 