Begging help on a electrochemistry problem!!! Urgent!!!

[Wener]

I encountered an strange electrochemical problem in my recent
experiments, which really drove me crazy.

Since I do not know how to attach a image with my post, the following
URL will links to the figure of my problem.
Loading Image...


As shown in left part of the attached figure, I am using an cubic
acrylic tank of 10cm by 10cm by 10cm, containing a saline solution of
conductivity 1mS/cm.

1 pair of surface platinum electrodes (A1-A2) is mounted through the
wall of the tank, at midpoints of two opposite walls.

By using a voltage-controlled current source shown by the right part
of the attached figure, a 10 kHz sinusoidal signal was converted into
a sinusoidal current of fixed peak amplitude 0.25 mA.

No air bubble was observed.

Then, another platinum eletrode pair (B1-B2) was added into this
system, i.e., I applied sinusoidal current signals through both A1-A2
and B1-B2 simultaneously. As shown by left figure, B1-B2 is fixed at
midpoints of other two opposite walls.

Shown by the right figure, the current signal between B1-B2 was
generated by the same function generation and similar conversion
circuit.

HOWEVER, now things became totally different, since tons of air
bubbles can always be observed at A1 and B1.

After I read some references on 'water electrolysis', I become even
more confused:
(1) Since B1-B2 is at the symmetric positions with A1-A2, which means
the equivalent resistance between A1-A2 is identical to that between
B1-B2, no large voltage difference between A1 and B1 should be
expected.
(2) Even though there is such a voltage difference, since the
frequency is as high as 10KHz, the eletrolysis phenomenon should not
be so rapid.
(3) If a 10KHz current signal is enough to induce such a terrible
electrolysis, Why did not it arise when I had only A1-A2? The
frequency is always 10KHz, and the voltage difference between A1-A2
should be much larger than between A1-B1.

Can anybode help me explain this phenomenon, and give me some
suggestion about how to avoid the bubble generation during my multi-
electrodepair experiment????

Millions of thanks!!!

P.S. (1), after a calucation with conductivity value, the equivalent
resistance of the saline between A1-A2 (or B1-B2) is approximately
given by 6.4 Kohm.
P.S. (2), when I measured the potentials at each electrodes, I found
DC voltage difference between A1 and A2, between B1 and B2, between A1-
B1. And the DC voltage difference ranges from 0.x Volts to x Volts. I
still cannot figure the source of this DC component:
(i) the DC offset of my function generator is as low as uV
level;
(ii) this DC difference was also observed when there was
only a single electrode pair A1-A2. Why did not it cause any
electrolysis at that time?
(iii) I am not sure if there does exist such a DC
difference, or it is just becuase the rushing air bubbles disturbed my
voltage measurement.

[Salmon Egg]

On 11/12/07 7:00 PM, in article

Post by Wener
I encountered an strange electrochemical problem in my recent
experiments, which really drove me crazy.
Since I do not know how to attach a image with my post, the following
URL will links to the figure of my problem.
http://web.mit.edu/lvwener/www/Rig.jpg
As shown in left part of the attached figure, I am using an cubic
acrylic tank of 10cm by 10cm by 10cm, containing a saline solution of
conductivity 1mS/cm.
1 pair of surface platinum electrodes (A1-A2) is mounted through the
wall of the tank, at midpoints of two opposite walls.
By using a voltage-controlled current source shown by the right part
of the attached figure, a 10 kHz sinusoidal signal was converted into
a sinusoidal current of fixed peak amplitude 0.25 mA.
No air bubble was observed.
Then, another platinum eletrode pair (B1-B2) was added into this
system, i.e., I applied sinusoidal current signals through both A1-A2
and B1-B2 simultaneously. As shown by left figure, B1-B2 is fixed at
midpoints of other two opposite walls.
Shown by the right figure, the current signal between B1-B2 was
generated by the same function generation and similar conversion
circuit.
HOWEVER, now things became totally different, since tons of air
bubbles can always be observed at A1 and B1.
After I read some references on 'water electrolysis', I become even
(1) Since B1-B2 is at the symmetric positions with A1-A2, which means
the equivalent resistance between A1-A2 is identical to that between
B1-B2, no large voltage difference between A1 and B1 should be
expected.
(2) Even though there is such a voltage difference, since the
frequency is as high as 10KHz, the eletrolysis phenomenon should not
be so rapid.
(3) If a 10KHz current signal is enough to induce such a terrible
electrolysis, Why did not it arise when I had only A1-A2? The
frequency is always 10KHz, and the voltage difference between A1-A2
should be much larger than between A1-B1.
Can anybode help me explain this phenomenon, and give me some
suggestion about how to avoid the bubble generation during my multi-
electrodepair experiment????
Millions of thanks!!!
P.S. (1), after a calucation with conductivity value, the equivalent
resistance of the saline between A1-A2 (or B1-B2) is approximately
given by 6.4 Kohm.
P.S. (2), when I measured the potentials at each electrodes, I found
DC voltage difference between A1 and A2, between B1 and B2, between A1-
B1. And the DC voltage difference ranges from 0.x Volts to x Volts. I
(i) the DC offset of my function generator is as low as uV
level;
(ii) this DC difference was also observed when there was
only a single electrode pair A1-A2. Why did not it cause any
electrolysis at that time?
(iii) I am not sure if there does exist such a DC
difference, or it is just becuase the rushing air bubbles disturbed my
voltage measurement.

I am having difficulty understanding your description. Maybe more diagrams,
a table of conditions with results, and better description would help.

Is this a homework assignment? I ordinarily try to avoid helping out with
such requests.

Whatever else may be going on, A1 and B1 together will act as one electrode,
while A2 and B2 will act like another.

Bill

[Wener]

Nope. It is nothing about the homework, but a part of my on-going
program. I agree with you on the issue that A1 and B1 act as a new
electrode pair. But the voltage difference between them should be
sinusoidal with a frequency 10KHz and an very low amplitude(since A1-
A2 and B1-B2 are symmetric), how can it trigger a water
eletrolysis...

Post by Salmon Egg
On 11/12/07 7:00 PM, in article

Post by Wener
I encountered an strange electrochemical problem in my recent
experiments, which really drove me crazy.
Since I do not know how to attach a image with my post, the following
URL will links to the figure of my problem.
http://web.mit.edu/lvwener/www/Rig.jpg
As shown in left part of the attached figure, I am using an cubic
acrylic tank of 10cm by 10cm by 10cm, containing a saline solution of
conductivity 1mS/cm.
1 pair of surface platinum electrodes (A1-A2) is mounted through the
wall of the tank, at midpoints of two opposite walls.
By using a voltage-controlled current source shown by the right part
of the attached figure, a 10 kHz sinusoidal signal was converted into
a sinusoidal current of fixed peak amplitude 0.25 mA.
No air bubble was observed.
Then, another platinum eletrode pair (B1-B2) was added into this
system, i.e., I applied sinusoidal current signals through both A1-A2
and B1-B2 simultaneously. As shown by left figure, B1-B2 is fixed at
midpoints of other two opposite walls.
Shown by the right figure, the current signal between B1-B2 was
generated by the same function generation and similar conversion
circuit.
HOWEVER, now things became totally different, since tons of air
bubbles can always be observed at A1 and B1.
After I read some references on 'water electrolysis', I become even
(1) Since B1-B2 is at the symmetric positions with A1-A2, which means
the equivalent resistance between A1-A2 is identical to that between
B1-B2, no large voltage difference between A1 and B1 should be
expected.
(2) Even though there is such a voltage difference, since the
frequency is as high as 10KHz, the eletrolysis phenomenon should not
be so rapid.
(3) If a 10KHz current signal is enough to induce such a terrible
electrolysis, Why did not it arise when I had only A1-A2? The
frequency is always 10KHz, and the voltage difference between A1-A2
should be much larger than between A1-B1.
Can anybode help me explain this phenomenon, and give me some
suggestion about how to avoid the bubble generation during my multi-
electrodepair experiment????
Millions of thanks!!!
P.S. (1), after a calucation with conductivity value, the equivalent
resistance of the saline between A1-A2 (or B1-B2) is approximately
given by 6.4 Kohm.
P.S. (2), when I measured the potentials at each electrodes, I found
DC voltage difference between A1 and A2, between B1 and B2, between A1-
B1. And the DC voltage difference ranges from 0.x Volts to x Volts. I
(i) the DC offset of my function generator is as low as uV
level;
(ii) this DC difference was also observed when there was
only a single electrode pair A1-A2. Why did not it cause any
electrolysis at that time?
(iii) I am not sure if there does exist such a DC
difference, or it is just becuase the rushing air bubbles disturbed my
voltage measurement.

I am having difficulty understanding your description. Maybe more diagrams,
a table of conditions with results, and better description would help.
Is this a homework assignment? I ordinarily try to avoid helping out with
such requests.
Whatever else may be going on, A1 and B1 together will act as one electrode,
while A2 and B2 will act like another.
Bill- Hide quoted text -
- Show quoted text -

[Bob M]

Funny things can happen in electrolytes. It is possible to polarise an
electrode is such a way that it becomes unidirectional and rectifys AC
which passes through.
There is of course the possibility that your applied AC current is
superimposed on top of a small DC component.
Even more funny things. Traces of surfactants can do even more funny
things.

How clean are your electrodes

Bob M

[Bob M]

To confuse you even more I seem to remember in the dim and very
distant past the first time a transistor was demonstrated it was an
electrolytic one with three electrodes in a solution. Definitely pre
germanium times. I think look in an old scientific american late1940s.
Electrolytic diodes were quite common in the 1930s they could handle
quite high current.
Aluminium rod and a copper rod immersed in sodium bicarb solution.
Real simple.

Bob M

[Dieter Britz]

Post by Wener
I encountered an strange electrochemical problem in my recent
experiments, which really drove me crazy.
Since I do not know how to attach a image with my post, the following
URL will links to the figure of my problem.
http://web.mit.edu/lvwener/www/Rig.jpg
As shown in left part of the attached figure, I am using an cubic
acrylic tank of 10cm by 10cm by 10cm, containing a saline solution of
conductivity 1mS/cm.
1 pair of surface platinum electrodes (A1-A2) is mounted through the
wall of the tank, at midpoints of two opposite walls.
By using a voltage-controlled current source shown by the right part
of the attached figure, a 10 kHz sinusoidal signal was converted into
a sinusoidal current of fixed peak amplitude 0.25 mA.
No air bubble was observed.
Then, another platinum eletrode pair (B1-B2) was added into this
system, i.e., I applied sinusoidal current signals through both A1-A2
and B1-B2 simultaneously. As shown by left figure, B1-B2 is fixed at
midpoints of other two opposite walls.
Shown by the right figure, the current signal between B1-B2 was
generated by the same function generation and similar conversion
circuit.
HOWEVER, now things became totally different, since tons of air
bubbles can always be observed at A1 and B1.
After I read some references on 'water electrolysis', I become even
(1) Since B1-B2 is at the symmetric positions with A1-A2, which means
the equivalent resistance between A1-A2 is identical to that between
B1-B2, no large voltage difference between A1 and B1 should be
expected.
(2) Even though there is such a voltage difference, since the
frequency is as high as 10KHz, the eletrolysis phenomenon should not
be so rapid.
(3) If a 10KHz current signal is enough to induce such a terrible
electrolysis, Why did not it arise when I had only A1-A2? The
frequency is always 10KHz, and the voltage difference between A1-A2
should be much larger than between A1-B1.
Can anybode help me explain this phenomenon, and give me some
suggestion about how to avoid the bubble generation during my multi-
electrodepair experiment????
Millions of thanks!!!
P.S. (1), after a calucation with conductivity value, the equivalent
resistance of the saline between A1-A2 (or B1-B2) is approximately
given by 6.4 Kohm.
P.S. (2), when I measured the potentials at each electrodes, I found
DC voltage difference between A1 and A2, between B1 and B2, between A1-
B1. And the DC voltage difference ranges from 0.x Volts to x Volts. I
(i) the DC offset of my function generator is as low as uV
level;
(ii) this DC difference was also observed when there was
only a single electrode pair A1-A2. Why did not it cause any
electrolysis at that time?
(iii) I am not sure if there does exist such a DC
difference, or it is just becuase the rushing air bubbles disturbed my
voltage measurement.

Someone suggested contaminated electrodes, which I don't go for as
the cause of this, and someone correctly pointed out that you simply
have a new electrode geometry by adding the second input pair, so you
should not get any DC. Forget about faradaic rectification before
someone else suggests it.

I would be putting an oscilloscope on both inputs, to see exactly what
you are applying to the input terminals. I suspect that there is an
error in the circuit you actually have set up and that you are in
fact putting in some dc, inadvertently.

[Richard Schultz]

In sci.chem Wener <***@gmail.com> wrote:

: Since I do not know how to attach a image with my post

FYI, posting images to non-binary newsgroups is a serious breach
of netiquette.

-----
Richard Schultz ***@mail.biu.ac.il
Department of Chemistry, Bar-Ilan University, Ramat-Gan, Israel
Opinions expressed are mine alone, and not those of Bar-Ilan University
-----
"an optimist is a guy/ that has never had/ much experience"

[Bill Penrose]

Post by Wener
I encountered an strange electrochemical problem in my recent
experiments, which really drove me crazy.

If this isn't a homework problem, I'll eat my shorts.

Try looking at it in terms of current density at the electrodes. Up
to a point, the gas will remain adsorbed until destroyed by the next
phase of the sine wave. Above that, gas will form at a rate faster
than it can be destroyed.

It you can't get it now, choose a less tough school.

Dangerous Bill

[Wener]

I swear you will be much happy if you have delicious shorts.

Anyway, thanks for your suggestion. The current flows through either
electrode pair is 0.25mA, and I made my electrodes with 99.9%
platinum wires, with a diameter ~1.5mm. Since the section area of
electrode is small, I do have a huge current density(0.14 mA/mm^2) on
it. However, if that is exactly what caused a rapid water
electrolysis, I am now still confused why that did not happen when
there was only a single electrode pair A1-A2.

Post by Bill Penrose

Post by Wener
I encountered an strange electrochemical problem in my recent
experiments, which really drove me crazy.

If this isn't a homework problem, I'll eat my shorts.
Try looking at it in terms of current density at the electrodes. Up
to a point, the gas will remain adsorbed until destroyed by the next
phase of the sine wave. Above that, gas will form at a rate faster
than it can be destroyed.
It you can't get it now, choose a less tough school.
Dangerous Bill