Not conductivity, I meant current. Again, turned around. The amperometric equivalence point is where the redox system is balanced, so net current = 0, so on the graph where y = 0. So that would mean the slope changes mark the region where the midlog of the titration begins and ends, like a typical titration curve, right?
The titrand is Fe2+ and the titrant is Ce 4+. As I understand it, before adding Ce, Fe2+ is being oxidized, which reduces the electrode, giving positive current. At the equivalence point, the oxidation of Fe 2+ and the reduction of Fe 3+ occur in equal proportions, so net current at the working electrode is 0. After the equivalence point, Fe is getting reduced, so the current is negative.
But I still don't understand why stoichiometric and amperometric equivalence points can be different, especially to the degree in my data. The current is 0 at 50% and 80% of the volume calculated by stoichiometry, which means there is something I'm missing, but I don't know what.