[xshadow]

Hi All!!!
I have some doubts about the redox titration and potentiometry...
In particular i don't understand  very well the phisical-chemistry point of view of what happens in this redox titration (taken from my slides):



The example is about " the redox titration of a solution containing Fe2+ in HClO4 1M in a Becker...using  as titrant Ce4+"

The semi reactions are:
Fe2+ ----> Fe3+ +1e-
Ce4+ +1e- ---->Ce3+   (Ce4+ is the titrant)

Now my doubts originate looking the picture above:

1) Studying redox reaction I always  see redox reactions that involve  2 half-cells ,each one separated  by the other through a "salt bridge"...SO In a semi-cell is performed the oxidation reaction and in the other one the reduction reaction...But here,in this figure( a redox titration) ,the two  Half-reaction  occur in the same cell/becker!?!!?!..there aren't two semi cell...the two semi reactions aren't separated! is this correct??


2) in the figure there are two electrodes:
a) a calomelan electrode,used as reference electrode
b) a inert platinum electrode

My second question is: what is the role of these two electrodes in my redox titration of Fe2+ with Ce4+???
The electrons that  Fe2+  releases WHERE do they go ?? in which electrode They are released on the surface of the Pt electrode or on the calomelan one?? And the  electrons released move (themself) towards the other electrode (the calomelan one) or they stay there untill the molecules of Ce4+ catch them??? In this case what is the role of the reference electrode??


3) the voltmeter that i can see in that picture between the two electrodes what ΔE_cell measures???


Please help me to understan chemistry!!
Thanks!!!

[Borek]

You need to separate half cells only if you are interested in charge flowing thorough an external circuit (as in a battery). There is absolutely nothing that stops both reactions from taking a place in the same solution.

Every redox mixture has a measurable potential, the one given by the Nernst equation. Your two electrodes are for measuring it.