Wener wrote:
> 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
> 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 ru****ng 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.
--
Dieter Britz (britz<at>chem.au.dk)
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