[zeoblade]

I have 25mL aliquot from 500mL Fe(II) digest using Fe(III) --> Fe(II) = 0.68V

I used somewhere near 12.65mL of 0.0996M Ce(IV) titrant to achieve end point using Ce(IV) --> Ce(III) = 1.44V

Equivalence is 0.819V, 12.60mL = 0.604V and 12.70mL = 1.038V

I'm really inarticulate to how to determine the exact volume at 0.819V

I tried working backwards in the combined Nerst equation:
0.819V = 1.44 - 0.05916log([Ce(III)]/[Ce(IV)])
0.819V - 1.44V = -0.05916log([Ce(III)]/[Ce(IV)])
0.621V/-0.05916 = log([Ce(III)]/[Ce(IV)])
10.50 = log([Ce(III)]/[Ce(IV)])
10^10.50 = [Ce(III)]/[Ce(IV)]

But I don't know where to go from here to work out the volume of Ce(IV), what step have I missed?

[zeoblade]

If I were to compare equivalence and end points between theoretical with experimental, I'm assuming I would obtain the same volumes in both theoretical and experimental. However, the theoretical voltage at equivalence is 0.819V, would the experimental also be 0.819V?

I found the experimental voltage just before equivalence at 12.60mL to be 0.604V but theoretical was 0.581V. Then just after equivalence at 12.70mL, experimental was 1.038V but theoretical was 1.057V. So experimental as a smaller range than theoretical but would the end points be equal or different? Would experimental be higher than theoretical?

[Borek]

I'm really inarticulate to how to determine the exact volume at 0.819V

I would just simply assume potential has been changing linearly between two points you have. I suppose it can be calculated in some more elaborated way, using Nernst equation, but it is a waste of time - difference will be very small, and my guess is that you are already trying to get more precision from your results that you really can (how have you measured volumes? What is expected tolerance of the glass used? Compare http://www.titrations.info/pipette-burette).

However, the theoretical voltage at equivalence is 0.819V, would the experimental also be 0.819V?

How was your theoretical voltage calculated? Have you taken into account ionic strength of the solution, activity coefficients, possible side reactions? Or have you just calculated it from standard potentials and Nernst equation?

[zeoblade]

Yes it is a waste of time in practice, this is just to compare differences between theoretical calculations and experimentally derived measurements. In other words, exploring the limitations of burettes because I can only read to 0.02mL confidently myself. The question asked if there was any difference between the equivalence and end point between theoretical and experimental.

Theoretical voltages were calculated using Nerst equation, experimental voltages were just read from the instrument