I can't see the attached file, but What do you mean with "current never stabilzes"? If you are performing a chronoamperometry, I vs t curve might be asymptotic aproaching to zero for long times, but capacitive current could be the responsible for the "residual" current. Did you tried to measure the OCP as a function of time?

Remember that a microelectrode behaves different than a macroelectrode, that is a microelectrode (micrometer dimensions) will reach a steady state because current is independent of time, different than a macroelectrode (milimmeter dimensions) which follows a Cottrell behavior,which makes current depending on time. 

Besides that, stabilizing means that the current must reach a steady state and it will not change dramatically as time progresses allowing me to do electroanalytical measurements. 

I'm uploading again the file thx for letting me know.

Hi Cesar, 

Our research group focuses heavily on the fabrication of disk microelectrodes and this behaviour is quite common. The main reason a steady-state is not reached either in CV or chronoamp is typically that the electroactive surface (in your case Au) is not properly sealed or damaged (i.e. cracked). As such, some solution "leaks" into the electrode causing a constant increase in current. 

I hope this helps!

Thanks I appreciate your suggestion and I'm aware of that the strange case is that for an outer sphere reaction like ferrocene methanol I can get to the steady state on CV and CA however when I change my system same electrode to an inner sphere reaction like hydrazine and under migration conditions I cant get a steady current any suggestion ?

Thanks

Hi Cesar,

Are you using a three electrode system with some reference electrode or is there anywhere ref. electrode introduced in your system, as it may have a significant contribution towards stabilization of current values for Chroroamperometry. I would suggest to not overlook the possibility of reference electrode behaviour going problematic.

Cesar, have you tried a good outer electron couple with your Au electrode?

Hydrazine oxidation competes with Au passivation.  So, once the oxide film on Au starts growing it will slow down hydrazine oxidation. Do a good literature search, there are many good articles taking about it and about the difference between Au, Pt and Pd

I think Yuriy is right, although it is not rare to found capacitive currents avoiding to reach steady state for long times...

Thanks to all for you great suggestions, for  Yuriy and Juan, using an outer sphere like ferrocenemethanol gave a very well defined steady state chronoamperogram at any potential I was stepping and know I think is just the system of hydrazine I think based on the experiments on ferrocene methanol. The fact that there's something else happening never crossed my mind and I think that's the reason why the electrode as I move to more positive potentials that steady state is never achieved.

About the literature do you know some authors treating this reaction I've seen Compton's paper of hydrazine on Pt but not anything for Au.

Best 

Cesar

Hi Cesar!

I assume that before U measured the chronoamperogram, U tested the Au electrode in acid solution and run CV till stabilization of the peaks.

Did u try a LSV or CV at slow scan rate? the lim current u record should always be the same for chrono and voltammetry (4nFcDa if your RG is big).

I suppose Yurij can be right, your oxidation reaction can compete with Au passivation, but not sure about Au oxides: just having a quick look at CV of Au (0.5 M H2SO4 vs SCE in doi:10.1038/nprot.2013.007) and fig 4.a of the Compton's PCCP 2011 paper (0.25 M H2SO4 vs SCE and almost same concentration of your anayte), I dont get why the hydrazine (E1/2 around 0.4 V vs SCE) should compete with oxidation of Au (onset at 1.2 V vs SCE) according to the two previous papers. OK, is not the same electrode, so your potential can shift, but so much?

U mentioned FcMtOH... Usually people use that instead ferrocene because ferrocene in acqueous is not very soluble and tends to form a film on the ede surface, decreasing the reversiblity, increasing the hysteresis and lowering the lim current. Could in your system a passivation layer be formed alike the ferrocene system? If U use commercial buffers U never know if there are stabilizers that can be adsorbed on your electrode and therefore change the kinetics of the redox U want to study.

Guessing U wanna use the electrode for analytical purposes as amperometric sensor, why don't U try a slightly more complex waveform in order to electrochemically keep clean the electrode surface?

All the best

Mara

Hi Mara,

Thanks for your answer, I think majorly that indeed the surface is changing along the chronoamperogram and it's funny because the same happens to Pt they tend to "deactivate" in a publication I'm following they show the same trend as mine but in discussions with my adviser we have that question why is this happening. Now, as I mentioned before in ferroceneMtOH the behavior is as expected for a microelectrode, the current measure by CA and CV they match with good agreement (variation of pA). But when I switch to my hydrazine system as I step at positive potentials I see the same behavior as in the publication I'm following.

But, ultimately I think is because something is "deactivating" the surface I'll keep looking for better ways to have a pristine surface. If you know of any treatment can you let me know?

Thx 

Cesar

Hi Cesar.

I meant if u tried cv and ca with hydrazine. I know u cant have problems with fcmetoh if you ede is not broken as previously mentioned. With hydrazine, the cv and ca are consistent?

Furthermore if could give me any further detail on you experiment even in private like solution composition (supporting electroyte, pH) maybe i can try to figure out. I assume again u have a jacket cell with temperature control and only 1 redox active compound. It the problem is not the electrode, it is in the solution.

I am aware of 1 wafeform that could help. It is called sampled current voltammetry. The reason why it was written is different, define precisely the n electrons in a redox, anyway it does the job.

Have a look at the recent paper published by Sam Perry in 2014... i think it is in anal chem but u can ask him directly from Research Gate. Sam is a formal PhD of Guy Denuault who is a very experienced resercher in microelectrodes from the 80'. Another 1 PhD in his group is from Mexico City too.

Hope this can help

Mara

Here is an example of the same phenomenon for slufite.

https://www.researchgate.net/publication/229305760_The_electrochemical_oxidation_of_sulfite_on_gold_UV-Vis_reflectance_spectroscopy_at_a_rotating_disk_electrode

Keep in mind that the oxide onset/offset potential is dynamic and it changes in the presence of a reducer.

Article The electrochemical oxidation of sulfite on gold: UV-Vis ref...