Help using potentiometer in research project

[kris]

Hello EEV, newbie here, so thanks for your patience.
I'm trying to build a circuit that uses feedback from a DC voltage reading to control a potentiometer for a university research project. My thinking is that a potentiometer can vary a resistance so that the voltage between my reference electrode (Ag/AgCl) and cathode remains at a relatively constant 500 mV. Has anyone here done anything like this? Am I way off?

Essentially I have 4 electrodes in water, let's call them A, B, C and ref.
A and B are battery electrodes (anode and cathode, respectively)
C is a cathode
ref is an Ag/AgCl reference electrode
The cell voltage between A and B is approximately 600 mV open circuit, as soon as I start to put resistors and let current flow between them, it drops to less than 100 mV.
What I want to do is then connect electrodes B and C (B becomes the anode, and C is a cathode), but I need the voltage between C and the reference electrode to be maintained at 500 mV SHE. My assumption is that a potentiometer can change the cell resistance between B and C to accomplish this, but I am not certain in the least (I am a chemist, and not great with electronics). Thank-you for any and all help or ideas you may have!!!

[Ian.M]

The only way to change the cell's internal resistance is to physically or chemically change the cell or change its temperature.   If the electrodes are of different materials, sustained current flow can cause chemical changes which can change the internal resistance and electrode potentials.

You could vary the external resistors to maintain a particular potential difference between C and the reference electrode, but there are probably simpler ways to do the job.  More details are required, specifically a schematic of the cell and load circuit, all electrode voltages relative to the reference when unloaded and fully loaded and the current into or out of each electrode when loaded (apart from the reference).

[babysitter]

I think you are looking for a Galvanostat/Potentiostat. Your university might have such units in the biological or chemical department.
If not, bother electrical engineering for a source-measure-unit, might be possible with that, too.

If I understand your problem right, you want a stable potential difference between two measurement electrodes. You can get that by  regulating the voltage between the excitation electrodes. Negative feedback would make the excitation voltage go up in case the 500mV drop, if the 500 mV rise, excitation voltage will go down.

You can do that by hand (turn your excitation voltage up or down depending on your reference measurement), or get a programmable multimeter and programmable power supply and give them a few lines of code on a PC - or you get the right tools(tm).

BR
Hendrik

[Keex]

Hello,
as babysitter correctly pointed out you need a Potentiostat that - using a control/feedback loop - adjusts the potential between WE and CE in a way that the potential between WE and RE is of a given value.
Commercial potentiostats are usually quite expensive (talking ~8k+ for the most basic ones), but it is actually not that hard to build one yourself if you don't require currents over say 100 mA.

The book by Bard and Faulkner "Electrochemical Methods: Fundamentals and Applications" (google "Bard Faulkner" for...sources) actually goes into a lot of detail in this regard. A simple potentiostat is relatively easy to build and I have done so myself - as a chemist myself without EE background.
You only need a few resistors and three operational amplifiers. See chapter 15.4, I have built the one in Figure 15.4.5. You can combine this with either a simple DC power supply, or even a simple potentiometer to set the input voltage between WE and RE you need. It works quite nicely. You can measure current with a DMM or include the current measuring circuitry yourself if you e.g. want to log what is going on.
I actually recommend building a potentiostat yourself as an electrochemist. You learn a lot :-)

[kris]

Thanks everyone! Very helpful!
I think everyone suggested I use a potentiostat, which I should have mentioned I already have one (Biologic VSP)
What I am trying to do is convert the energy generated between electrodes A and B to control the potential between electrode C and ref. I am hoping to build an external circuit without using the potentiostat (I think of it like making a potentiostat that is powered by the energy produced between electrodes A and B.

Because the voltage output between electrodes A and B varies, perhaps I should get something like this (but perhaps a step-up voltage converter instead of step-down, and perhaps a lower voltage output).

https://www.banggood.com/DC-DC-8A-300W-Buck-Adjustable-Solar-Charging-LED-Driver-Vehicle-Power-Supply-Module-p-1166654.html?rmmds=search


Then I would use the output of this as the input into the DIY potentiostat system as described by Keex (from Bard), and connect the 3 electrodes (working electrode = electrode C, reference electrode, and counter electrode = electrode B). I guess I am not sure if I can use electrode B as both the cathode (between A and B, delivering the power input into the DC-converter), AND as a counter electrode in the potentiostat. I guess I don't know if pushing electrons to electrode B will interfere with the potentiostat operation...any ideas?

I have a breadboard, electrodes, and I can buy the op-amp and other parts (I have a budget of a few hundred dollars). Is there anything else major I am missing that I don't even realize I need? Thank-you!!!!