Dear Agnivo,

in order to give you an useful input, I would need more information about your set-up. I am missing the following (I am not an electrochemist either):

1. What is the electrolyte you are using ?

2. What is Ag/AgCl electrode (solid ?) ?

3. Which ion transport are you interested in ?

4. What is the nature of the membrane ?

4. the ion current you want to measure, is it ion(which ?) current through the membrane or electrical current through electrode-electrolyte interface (probably controlled by redox reactions at the cathode and anode) ?

The optimal geometry would be the one where the area of the electrodes is "large" (flat discs) and the distance between them (in your figure ~12 mm) small. In such a "sandwich geometry", both the area and the "thickness" of your sample (left electrode plane-electrolyte-membrane-electrolyte-right electrode plane) are well defined and the analysis of your measurements (if your instrument is Gamry instrument, then you are probably measuring electrical impedance as a function of frequency) should be "straight forward". Bulk response (high frequencies) gives you a measure of the ion current through the membrane, while electrode-electrolyte interface response (low frequencies) gives you information about the current across the interface (redox reactions controlled.

I hope this helps

With best regards

Petr

Hello Dr. Braun : actually I have done a few experiments but I think that I am only looking at the oxidation/reduction currents developed at the working electrode. 

Initially I had a WE and RE in one chamber and a CE in the other chamber with the membrane in between. 

I want to measure the current arising from the flow of ions across the membrane when the WE / CE are having a potential difference across them.

Hello Dr. Viscor,

1. I will be using 0.1M KCl. Then I want to try 1X Phosphate Saline buffer.

2. Ag/AgCl electrode in my set up is AgCl coated 1 mm dia silver wire.

3. I want to measure the current due to K+,Cl- ions flowing through the membrane due to the potential difference between the two electrodes.

4. The membrane is Au coated AAO ( anodisc aluminum oxide) nanoporous membrane. It is 25 mm thick and the nanopore is originally 20 nm in diameter but due to Au coating it has reduced near the pore surface. The coating is on one side. In the next stage it will be immobilized with organic molecules (having tail sulfur group), and the experiment will be repeated.

5. I want to measure ion-current through the membrane.

The last portion of your answer is new to me. I was thinking of setting a voltage across the two electrodes (using the set voltage program in Gamry) and read the current. But, i have not thought about the frequency and bulk/interface response. What type of experiment should I perform then to check this bulk response ?

I am thinking of buying Ag sheets for creating flat electrodes and then coating them with the chloride. If you have any suggestion regarding this also then please share.

Thanks.

Never use Ag/AgCl electrode for this experiment (will be destroyed after the first run). Consider 2 Pt electrodes (large area). Use AC current to measure the ionic conductivity.

The membrane must be the  limiting factor for your ionic current (not the solution).

OPTION A:

Use AC current to measure the ionic conductivity. Take the signal from a  conductometer (25…1000 Hz). No electrochemistry (electrolysis) is involved in such an experiment.

OPTION A2 (most recommended)

Adapt  the cell of a conductometer for these measurement (with/without  membrane).

OPTION B:

Use the EIS facility of your potentiostat if available. Connect one Pt to WORK and REF and AUX together to the other Pt. Choose E= ocp, AC signal 10…25 mV, f= 100 kHz….1 Hz. In the Nyquist Plot read R1 (what is called solution resistance) for further comparison. Measure with and without he membrane to see if it is working.

OPTION C (less recommended)

Use cyclic voltammetry (a solution of Fe2+/Fe3+) in a 3 electrode cell with/without membrane).

Think to other models for your experiment. The electronic current in the external circuit is equal to the ionic current in solution (compare results with/without membrane). Maybe the aim of your experiment is not to measure currents (depending on voltages), but measuring directly the conductivity of your membrane (not depending on applied voltages). Think also to osmosys as a starting idea.

"How to measure the ionic current across two electrodes using a potentiostat ?"

Ignoring your practical problem, the general answer is:

Connect WORK to one electrode. Connect REF and AUX together to the other electrode.

WORK, REF and AUX are the labels on potentiostat.

The electronic current in the external circuit is equal to the ionic current in solution (also in membrane).

But pay attention. In any design, if the redox process at electrodes will limit the current, the measured ionic current value is not a property  of the membrane.

Dr Radu,

Thanks for the detailed explanation :

I have used Cyclic Voltammetry (CV) before , where I had Fe2+ / Fe3+ ions in the electrolyte. I have got a set of data which are not satisfactory and can not be explained in line with my hypothesis. I am suspecting that by doing CV I was not measuring the current due to ion flow across the membrane instead I am looking at  "electrical current through electrode-electrolyte interface " as pointed out by Dr. Viscor previously.

In the CV experiments, I have used 0.25 mm diameter Ag/AgCl as the RE and two 1 mm diameter Pt wires for WE and CE. Also, I kept the WE and CE at a distance of about 9 mm with the membrane in between. Distance between WE and RE is 5 mm. 

I tried to replicate the results of a few previously published literature where current due to ionic flow is blocked / reduced due to organic molecules accumulating in the nanopores. However my results were not satisfactory.

Presently, I am looking at the attached papers to understand the technique. The image attached to my question is reproduced from the supplementary information of the paper. I will definitely look at option A , A2 as suggested by you.

Thanks

Dear Agnivo, I agree with Petr Viscor: In the sketch, the distance between your electrodes seems too large. The electrodes should nearby the membrane, and flat electrodes very close to the membrane would be best (but far enough that there is a uniform liquid film between electrode and membrane, avoid touching). Therefore, your intention of buying flat electrodes is a good one; metal foils or a stainless steel mesh might work well. Platinum foils or platinum gauze would be great indeed, but are very costly (for 100x100 mm, the price of platinum gauze can exceed $3900). Stainless steel might work, but it can corrode in concentrated chloride solutions. A sulfate solution would be less aggresive with respect to that. Keep in mind that a potentiostat can be an extremely strong oxidizer, stronger than any chemical agent, i.e. avoid too large potentials. If you want to try steel: watch carefully whether the solution changes its color.

Using Ag electrodes in a chloride solution is indeed problematic, as Cornel Radu has already pointed out. They will be covered with a AgCl precipitate with varying thickness, i.e. with varying resistance, which would make the interpretation of the resistance measured quite problematic and very difficult. (I would not call the electrodes "destroyed" after use, they still would work as a reference and can be cleaned with ammonia solution). One might argue that due to the symmetry of your setup, the mean total AgCl thickness of both electrodes combined might remain quite constant, since the dissolution of AgCl at one side is compensated by the formation of AgCl at the other side. However, this is sketchy and inaccurate since the resistance might still vary due to the fact that the grain size of the AgCl and its distribution varies with time. Furthermore, it might be difficult to make sure that the wetted areas are exactly identical.

If you can't use large flat electrodes due to restrictions of the space, using two electrodes might not the best thing to do if you want to study the properties of the membrane or of one of the electrodes. Whenever possible, use three of four electrodes to separate the effects occurring at the different locations.

Does you potentiostat have an imedance option? If so, measuring impedance as suggested by Petr Viscor would be great.

Good luck with your experiments.

Dear Agnivo,

I can see that you have got a number of very useful comments both from Cornel and Dominik. It should give you a good starting point.

To the points 1. to 5. of your answer, I have the following comments:

And a last comment. Impedance measurement is a two electrode measurement. Third electrode is unnecessary and in fact misleading. The use of third electrode is probably still used for "electrochemistry historical reasons". Like 4PDCR (Four Probe Direct Current Resistance measurement), it introduces systematic errors and it is therefore not an optimal reference electrode. Properly chosen metal electrode (Pt for example) is far better reference (directly related to the vacuum level) as far as the various quantum mechanical energy levels are concerned that are relevant both for the ion transport and the redox reactions at the metal electrode - electrolyte interface.

With best regrads

Petr