While conducting controlled-potential electrolyses on a zinc metal complex in 0.1 M sulfuric acid, I noticed that my results varied greatly depending upon my selection of the glass frit used to isolate the auxiliary electrode chamber.
If I used a course glass frit, my current vs. time curve was an exponential decay as expected but some of the analyte diffused into the auxiliary electrode chamber.
When I switched to a medium porosity glass frit, less analyte diffusion was noted but the curve was no longer a simple exponential decay. The current first decreased for about 15 minutes and then increased.
Finally, when I used a fine porosity frit, the current no longer decayed at all but actually increased over time.
It probably has to do with an increase in cell resistance, but why would the curve increase over time with the fine porosity frit? I would have expected no change in current over time.
Thank you for your help.
The fine porosity frit may have been such a high resistance that the compliance voltage of your potentiostat was reached! For most commercial designs this should have shown an OVERLOAD condition since the potentiostat was no longer holding the desired potential. The actual potential would have been the compliance of your potentiostat minus the iR drop across the frit minus the voltage required at the counter electrode to sustain the current on that electrode.It will change as the current changes through the electrolysis.
As you didn't give any of the particulars (potentiostat, compliance, or current) it's hard to back calculate, but the efffects should be similar to those you reported.
In the really old days we used to add some hydroxylamine to the counter electrode compartment to act as a depolarizer for the counter electrode and reduce the required voltage at that electrode. You also didn't say anything about the concentrations you were using or the supportting electrolyte. Try decreasing the concentration of your analyte and increasing the supporting electrolyte concentration.
Some words about compliance voltage are at
Drop me a note if you want to discuss this further.