[jj089]

Hello,

We did an experiment in analytic chemistry where we dissolved an iron(II)-containing sample with concentrated sulfuric acid + water, titrated the resulting solution with K2Cr2O7 (aq), and measured the potential with a multimeter after each addition of the titrant. The reaction is:

6Fe2+(aq) + Cr2O72−(aq) + 14H+(aq) → 6Fe3+(aq) + 2Cr3+(aq) + 7H2O(l)

I did 3 trials. In the first trial, I mistakenly let the dissolved iron(II)-containing sample sit in a fume hood for at least 5 mins before titrating it. I had this mistake corrected on the other two trials. We know that Fe2+ oxidizes in air into Fe3+, which means that I had significant amount of Fe3+ in my first solution compared to the other two.

I observed that the equivalence point for the 1st trial is significantly greater than the other two

 11.49 mL > 11.36 mL > 11.34

I also noticed that the same trend in the initial potentials of the solutions

390 mV > 386 mV > 372 mV

It seems like there is a relationship between the initial Fe3+ concentration, the initial potential, and the equivalence point. However I can't find anything directly relating the three. Could someone tell me if these three are in fact connected or not?

Attempt at a solution:
Nernst Equation and Le Chatelier's Principle?

Any help would be greatly appreciated. 

[Borek]

Nernst equation is the way to go with the initial potential. 5 min is not that long.

Note that your first titration should use LESS titrant, not more, if the iron were oxidized to Fe(III) by the air oxygen.

[jj089]

Note that your first titration should use LESS titrant, not more, if the iron were oxidized to Fe(III) by the air oxygen.

Thank you.

That's what I thought at first too. It didn't make sense that it took more titrant to reach the equivalence point.