Dear Jeroen Nieuwenhuis

Although I don't have any direct experience with this system, I think
you may be forming tetrachloroaurate anions (AuCl4-) in solution.

at the anode (oxidation) the likely reaction is:
         Au + 4Cl- = AuCl4- + 3e-  E0 = -1.002 V

at the cathode (reduction) there are any number of reactions that could
be taking place:
  O2 + 4 H+ + 4 e- = 2 H2O         E0 =  1.229 V
  O2 + 2 H+ + 2 e- = H2O2          E0 =  0.695 V
O2 + 2 H2O + 4 e- = 4 OH-         E0 =  0.401 V
       2 H+ + 2 e- = H2            E0 =  0.000 V
O2 + 2 H2O + 2 e- = H2O2 + 2 OH-  E0 = -0.146 V

The most likely ones, I think, are the third (hydroxyl formation) and
fifth (peroxide and hydroxyl formation) because your solution won't have
much acid. Thus your reaction would be:
4Au- + 16Cl- + 3O2 + 6H2O = 12OH- + 4AuCl4-  E0 = -0.601 V

You could use the Nernst equation to work out what then end
concentrations would be.

Because it's an AC current, both sides would form the soluble gold anion
then diffuse away. If the solution changes to a bright color (yellow,
red, or purple), then you can be fairly certain that's what's happening.

Good luck

Chris

On Wednesday, August 21, 2002, at 05:29 PM, Jeroen Nieuwenhuis wrote:

> Dear all,
>
> I am trying to measure the impedance of a 1% NaCl solution using 50 um
> x 150
> um gold electrodes (thickness 0.6 um) in a microchannel and the
> electrodes
> dissolve in about 40 minutes(!).
>
> I used very pure NaCl and distilled water to make the solution. I use a
> sine-wave generator to make a 50 mV AC voltage (offset oV) and I
> measure the
> current using an transimpedance-amplifier with a 100k resistor. Nor the
> source
> nor the read-out electronics seems to generate a DC value current.
>
> Does anybody have a suggestion what is going on? To the best of my
> knowledge
> nothing should happen when the AC voltage used is less than about a
> volt.
>
> Regards,
> Jeroen Nieuwenhuis
--
Christopher F. Blanford
Inorganic Chemistry Laboratory, South Parks Road, Oxford, OX1 3QR, UK
Phone: (44)/(0)-1865-282603; Fax: (44)/(0)-1865-272690
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