[MindNarrative]

As I understand it, the amperometric endpoint is the volume where the current vs. volume graph slope changes. But why would this be different from the volumetric endpoint? I have one chart each of the same titrant on two different concentrations of the same titrand, but one has the amperometric endpoint before the volumetric, and the other has it after. What gives?

[Borek]

What do you mean by "volumetric endpoint" - the one calculated from stoichiometry for known concentrations of titrant and titrand? Differences shouldn't be large, but they are not surprising.

Hint: take a look at the titration curve of acetic acid. At what pH would you expect the lowest conductivity of the solution? Is it the same pH titration ends at? Which comes first?

[MindNarrative]

By volumetric endpoint, yeah, it's the one calculated from stoichiometry. The lowest conductivity would be when the ion concentration is lowest, so....wait, would it be at 7.0? Now I'm all turned around.

These are my graphs, two concentrations of the same titrand:

http://i.imgur.com/0iDSfgc.png
http://i.imgur.com/VRHAhln.png

If I have it right, I'm supposed to extend the slopes on either side of the slope change, and the endpoint is where the lines intersect. The color changes happen at 25 mL and 10 mL, but the slope changes are on either end of the color change on both graphs.

Or is the amperometric endpoint where the conductivity = 0? My textbook doesn't have much on reading the graphs.

[Borek]

The lowest conductivity would be when the ion concentration is lowest, so....wait, would it be at 7.0?

Perhaps not exactly at 7.00, but conductivity is a sum of infiovidual ion conductivities. At pH 7.00 sum of concentrations of H⁺ and OH^- is the lowest possible.

Or is the amperometric endpoint where the conductivity = 0? My textbook doesn't have much on reading the graphs.

No way to have conductivity of 0. Think about what is present in the solution.

[MindNarrative]

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.

[Borek]

Oops, sorry, for some reason I was fixed on conductometry.

But in general, while fraction of a percent difference can be easily explained (equivalence points are almost always a bite shifted, as typically there is some equilibrium involved), if you observe huge differences there must be IMHO some basic problem with the data.