What is (approximately) the minimum exchange current for a potentiometric measurement? Is there a rule of thumb for what should be the minimum exchange current to detect a certain redox couple in an aqueous solution? 

I would like to make a rough calculations to see whether any of my hypothesis is feasible but I have no idea about the order of magnitude an exchange current should have in water. I could find a few examples for "normal" exchange currents for pH measurements or $\ce{Fe^{3+}/Fe^{2+}}$ but no minimum.

Background: I am trying to figure out, why one can measure a negative ORP in fermentations. There has been a lot of work done on the role of the microorganisms and their exofacial sulfhydryl groups. However, the question of WHAT is actually measured/happening at the electrode has not been answered. 
I want to tackle this as part of my thesis and I can think of two mechanisms: 
A) Direct interaction of sulfhydryl groups with the electrode surface
B) Mediation, e.g. via Fe or Mn ions.
My concerns about A are that cells are pretty big compared to ions so I am not sure whether enough cells could ever interact with the electrode to make this scenario realistic. In the case of B I am not sure if the concentrations are high enough.

Apologies if my language is too sloppy.
Pfeiffer at al. defines sensor effective (SERC) and sensor ineffective redox couples (SIRC) in relation to the biggest net exchange current found in the system $\text{i}_{net}$.

$\text{SERC} \geq 0.01~\text{i}_{net}$

$\text{SIRC} &lt; 0.01~\text{i}_{net}$

DOI: 10.1016/0169-7722(92)90041-C