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WayneL
science forum beginner
Joined: 03 May 2005
Posts: 33
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 Posted: Tue May 03, 2005 6:49 pm Post subject:
Help in finding the name of a chem/electrchem feild
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I need to investigate the areas of acid at cathode and basin at anode on a
very simple cell.
As shown in this link, using two pencils.
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/ELECSOL/PAGE1.HTM
Acid is produced at the cathode.
Could someone give me some pointers?
If the polarity was reversed would the acid concertration move to the opsite
electrode and basin vice-versa?
I suspect it will?
Cheers
Wayne
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Dieter Britz
science forum beginner
Joined: 04 May 2005
Posts: 45
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| Posted: Wed May 04, 2005 5:51 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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WayneL wrote:
| Quote: | I need to investigate the areas of acid at cathode and basin at anode on a
very simple cell.
As shown in this link, using two pencils.
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/ELECSOL/PAGE1.HTM
Acid is produced at the cathode.
Could someone give me some pointers?
If the polarity was reversed would the acid concertration move to the opsite
electrode and basin vice-versa?
I suspect it will?
|
They seem to have it in reverse. I assume you are referring to
electrolysis in neutral NaCl. They show the half-reactions in the
pictures, and these show that base OH- is produced at the cathode
and acid H+ at the anode, from the electrolysis of water. But yes,
if you reversed the polarity, the production of acid and base would
also reverse.
--
Dieter Britz, Kemisk Institut, Aarhus Universitet, Danmark. |
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WAYNEL1
science forum beginner
Joined: 04 May 2005
Posts: 25
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| Posted: Wed May 04, 2005 8:33 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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Hi Dieter
What is this field of chem called?
I need to read further on acid/base changes at the electrode on reverse
polarity.
Cheers
Wayne
Dieter Britz wrote:
| Quote: | WayneL wrote:
I need to investigate the areas of acid at cathode and basin at
anode on a
very simple cell.
As shown in this link, using two pencils.
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/ELECSOL/PAGE1.HTM
Acid is produced at the cathode.
Could someone give me some pointers?
If the polarity was reversed would the acid concertration move to
the opsite
electrode and basin vice-versa?
I suspect it will?
They seem to have it in reverse. I assume you are referring to
electrolysis in neutral NaCl. They show the half-reactions in the
pictures, and these show that base OH- is produced at the cathode
and acid H+ at the anode, from the electrolysis of water. But yes,
if you reversed the polarity, the production of acid and base would
also reverse.
--
Dieter Britz, Kemisk Institut, Aarhus Universitet, Danmark. |
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WAYNEL1
science forum beginner
Joined: 04 May 2005
Posts: 25
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| Posted: Wed May 04, 2005 8:33 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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Hi Dieter
What is this field of chem called?
I need to read further on acid/base changes at the electrode on reverse
polarity.
Cheers
Wayne
Dieter Britz wrote:
| Quote: | WayneL wrote:
I need to investigate the areas of acid at cathode and basin at
anode on a
very simple cell.
As shown in this link, using two pencils.
http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/ELECSOL/PAGE1.HTM
Acid is produced at the cathode.
Could someone give me some pointers?
If the polarity was reversed would the acid concertration move to
the opsite
electrode and basin vice-versa?
I suspect it will?
They seem to have it in reverse. I assume you are referring to
electrolysis in neutral NaCl. They show the half-reactions in the
pictures, and these show that base OH- is produced at the cathode
and acid H+ at the anode, from the electrolysis of water. But yes,
if you reversed the polarity, the production of acid and base would
also reverse.
--
Dieter Britz, Kemisk Institut, Aarhus Universitet, Danmark. |
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Dieter Britz
science forum beginner
Joined: 04 May 2005
Posts: 45
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| Posted: Wed May 04, 2005 10:05 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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WAYNEL wrote:
| Quote: | Hi Dieter
What is this field of chem called?
I need to read further on acid/base changes at the electrode on reverse
polarity.
|
It's called electrochemistry. Look up the reactions taking place
at the cathode and anode during the electrolysis of water in a
neutral electrolyte. As I wrote, in fact these are written (in
poorly legible script) as "half-reaction" under the cathode and
anode pencils in the web page you pointed to. When you electrolyse
water, you produce H2 gas plus OH- ions at ther cathode, and O2 gas
plus H+ ions at the anode. Those ions will eventually meet and produce
water again, but before they do, they will cause a pH gradient across
the cell. When you reverse the polarity of the cell, the gradient
will flip.
I don't know who produced the web site you pointed to, but if you know
them, you should maybe point out their error, reversing the acid/base
order.
(PS: In NaCl, at high overpotential, some Cl2 gas will also be
produced, but not much compared with oxygen).
--
Dieter Britz, Kemisk Institut, Aarhus Universitet, Danmark. |
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WAYNEL1
science forum beginner
Joined: 04 May 2005
Posts: 25
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| Posted: Wed May 04, 2005 10:46 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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Hi Dieter
It was the Journal Of Chemical Education. I do not personally know
them I found it on a search.
Thanks
WayneL |
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WAYNEL1
science forum beginner
Joined: 04 May 2005
Posts: 25
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| Posted: Fri May 06, 2005 2:14 pm Post subject:
Re: Help in finding the name of a chem/electrchem feild
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Hi Dieter
I have repeated this experiment and I get similar results.
I placed water between two graphite electrodes and then biased it with
15v.
After a few mins I measured the pH.
I got a pH of apx pH 11 at the cathode and pH 2 at the anode.
As I understand the pH is calculated from -log(H+).
So as the anode is the source of the H+ ion, is that why it is more
acidic?
I thought, that as the H+ ions are moving towards the cathode and
concerntrating towards the cathode the H+ number would increase. Or is
it as they pass the OH- and at some point they neutralise and thus very
few get to the cathode?
Cheers
WayneL |
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Dieter Britz
science forum beginner
Joined: 04 May 2005
Posts: 45
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| Posted: Mon May 09, 2005 9:54 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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WAYNEL wrote:
| Quote: | Hi Dieter
I have repeated this experiment and I get similar results.
I placed water between two graphite electrodes and then biased it with
15v.
After a few mins I measured the pH.
I got a pH of apx pH 11 at the cathode and pH 2 at the anode.
As I understand the pH is calculated from -log(H+).
So as the anode is the source of the H+ ion, is that why it is more
acidic?
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Exactly. At the cathode, you produce hydroxyl ions and hydrogen, so
there the pH goes up, while at the anode you produce hydrogen ions
and oxygen, the pH going down.
| Quote: | I thought, that as the H+ ions are moving towards the cathode and
concerntrating towards the cathode the H+ number would increase. Or is
it as they pass the OH- and at some point they neutralise and thus very
few get to the cathode?
|
The hydrogen and hydroxyl ions will be transported away from where they
are produced and meet each other, producing water. I doubt that many
of these ever make it to the opposite electrode, if any.
--
Dieter Britz, Kemisk Institut, Aarhus Universitet, Danmark. |
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WAYNEL1
science forum beginner
Joined: 04 May 2005
Posts: 25
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| Posted: Mon May 09, 2005 10:48 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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Going back to my pet interest, if I apply an AC bias to this experiment
over a short period of time I do not get any net effect. E.g. no pH
changes at the electrodes.
However, if I think about what is happening, H+ ion are lighter that
OH- ion - in terms of mass. Could there be a net effect over a longer
period of time due to ion kinetics or viscosity etc? Would there be an
electrode that would show a slight pH difference?
As in say some technique in DC Electrokinetic used in DNA analysis. Or
would the wavelength of the ion dictate a frequency up in the GHz
(lambda = c/f)?
Cheers
WayneL |
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Dieter Britz
science forum beginner
Joined: 04 May 2005
Posts: 45
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| Posted: Tue May 10, 2005 5:19 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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WAYNEL wrote:
| Quote: | Going back to my pet interest, if I apply an AC bias to this experiment
over a short period of time I do not get any net effect. E.g. no pH
changes at the electrodes.
|
If you think about it, this is what you'd expect. An AC polarisation
means that each electrode is alternately a cathode and then an anode,
so each would produce H+ ions, then OH- ions, and the two lots react
to form water, right at each electrode.
| Quote: | However, if I think about what is happening, H+ ion are lighter that
OH- ion - in terms of mass. Could there be a net effect over a longer
period of time due to ion kinetics or viscosity etc? Would there be an
electrode that would show a slight pH difference?
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Viscosity or "ion kinetics" (whatever that means) have nohting to do
with it. But H+ ions are indeed a bit more mobile than OH- ions, so
you might get some very local pH gradients. I doubt that you would be
able to see them though.
| Quote: | As in say some technique in DC Electrokinetic used in DNA analysis. Or
would the wavelength of the ion dictate a frequency up in the GHz
(lambda = c/f)?
|
If you are thinking electrophoresis, forget it, it doesn't apply here.
As for the rest, Neddy Seagoon would say, "It's not as simple as that!".
Or maybe that should be Henry Crunn.
--
Dieter Britz, Kemisk Institut, Aarhus Universitet, Danmark. |
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WAYNEL1
science forum beginner
Joined: 04 May 2005
Posts: 25
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| Posted: Wed May 11, 2005 5:39 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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Hi Dieter
This small experiment using DC and now I have tried it using AC seems
to have "thrown a cold bucket of water all over my work"!
I have images of residue on several printed circuit boards that bridge
an AC signal (no DC bias). These residues are the after effect of some
electrochemical reaction. They are bluish/white in colour, powdery in
appearance, slightly conductive and it is believed that they are formed
from contaminated water.
If it was a DC bias then it would be easier to explain. However, as I
mentioned it is true AC (symmetrical at either side of the cycles).
All I have is the images and no analysis of the composition of the
residue left over or how long it took to create. Thus I am trying to
recreate it to analysis the composition.
The tracks at each side are copper and the contamination is certain to
be water but what else is mix in I am unsure.
One of the ideas I had was that they is some form of salt created from
the electrodes alternating through pH4 to pH10 to pH 4 and so on,
overtime creating a salt/carbonate composition.
However, when I run the Universal Indicator experiment over two days I
get no pH gradients at the electrodes and therefore demonstrating your
point that the H+ and OH- ion reform back into water - solution
staying at pH 7.
The other idea I have is that the residues have been formed through a
fierce reaction cause by the availability of higher voltages/currents
say 50v/4amps. But for something like that to happen the conductivity
of the original contaminate must have been very high.
Do you have any thoughts on the matter?
Cheers
WayneL |
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Dieter Britz
science forum beginner
Joined: 04 May 2005
Posts: 45
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| Posted: Thu May 12, 2005 4:50 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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WAYNEL wrote:
| Quote: | Hi Dieter
This small experiment using DC and now I have tried it using AC seems
to have "thrown a cold bucket of water all over my work"!
I have images of residue on several printed circuit boards that bridge
an AC signal (no DC bias). These residues are the after effect of some
electrochemical reaction. They are bluish/white in colour, powdery in
appearance, slightly conductive and it is believed that they are formed
from contaminated water.
If it was a DC bias then it would be easier to explain. However, as I
mentioned it is true AC (symmetrical at either side of the cycles).
All I have is the images and no analysis of the composition of the
residue left over or how long it took to create. Thus I am trying to
recreate it to analysis the composition.
The tracks at each side are copper and the contamination is certain to
be water but what else is mix in I am unsure.
One of the ideas I had was that they is some form of salt created from
the electrodes alternating through pH4 to pH10 to pH 4 and so on,
overtime creating a salt/carbonate composition.
However, when I run the Universal Indicator experiment over two days I
get no pH gradients at the electrodes and therefore demonstrating your
point that the H+ and OH- ion reform back into water - solution
staying at pH 7.
The other idea I have is that the residues have been formed through a
fierce reaction cause by the availability of higher voltages/currents
say 50v/4amps. But for something like that to happen the conductivity
of the original contaminate must have been very high.
Do you have any thoughts on the matter?
|
You are not telling us much. Was there an electrolyte? What was
the AC amplitude, what current flowed? My guess is this (and I
am guessing that the electrodes are copper): at the anode cycle,
instead of producing H+ ions and oxygen, you are (also) producing
Cu++ ions. Then, when you go cathodic, producing OH- ions and hydrogen,
some of the Cu++ is replated, but some forms Cu(OH)2, some of which
settles on the electrode. This may be what your bluish white stuff is.
Scrape it off and see if it dissolves in an ammonia solution, producing
a strong blue colour.
--
Dieter Britz, Kemisk Institut, Aarhus Universitet, Danmark. |
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WAYNEL1
science forum beginner
Joined: 04 May 2005
Posts: 25
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| Posted: Thu May 12, 2005 5:35 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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Hi Dieter
Sorry, the electrodes are copper (both sides) and the voltage is apx
20v. The electrolyte is contaminated water but that is all I know.
I have to try and repeat the test to try and get some of the
bluish/white stuff. Is Cu(OH)2 (copper hydroly?) soluable in water and
is it conductive? Would Cu(OH)2 be produced by an AC field or would it
be produced with a DC field?
Cheers
WayneL |
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Dieter Britz
science forum beginner
Joined: 04 May 2005
Posts: 45
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| Posted: Fri May 13, 2005 5:10 am Post subject:
Re: Help in finding the name of a chem/electrchem feild
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WAYNEL wrote:
| Quote: | Hi Dieter
Sorry, the electrodes are copper (both sides) and the voltage is apx
20v. The electrolyte is contaminated water but that is all I know.
I have to try and repeat the test to try and get some of the
bluish/white stuff. Is Cu(OH)2 (copper hydroly?) soluable in water and
is it conductive? Would Cu(OH)2 be produced by an AC field or would it
be produced with a DC field?
|
Cu(OH)2 is insoluble in water (you can find out its solubility
product yourself from thermodynamic data), and would be nonconductive
as a film. It is however soluble in a solution of ammonia, forming a
complex with a deep blue colour. It would be formed both by DC or AC
polarisation, as I have explained, at the anode, where Cu++ would go
into solution, and eventually meets up with OH- ions. This would be
easier with AC polarisation, however, because the OH- ions are formed
in the cathodic phase at the same electrode where the Cu++ was made
in the anodic phase, whereas with DC, OH- needs to be transported
across the cell from the cathode, where it then combines with the H+
and/or the Cu++ formed there.
--
Dieter Britz, Kemisk Institut, Aarhus Universitet, Danmark. |
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