In an upcoming lab we're measuring how pH effects cell potential E. An equal concentration of quinone and hydroquinone is added to a beaker and its cell potential is measured against an Ag/AgCl cell. Easy enough so far.
The hydroquinone is then titrated with Fe(CN)63+. We do this to find the E of the hydroquinone at whatever the current pH it is, which should be the measured E at 1/2 volume to equivalence point. What I don't understand is why this reaction occurs.
Here are the half reactions:
Fe(CN)63+ + e- --> Fe(CN)64- Eo=.356
Qu + 2H+ = 2e- --> H2Qu Eo=.700
Because we're adding the Fe(CN)63+, it must be the one reduced. However, the Ecell is then:
E=.365-.7 = -.344
And this should mean a non-spontaneous reaction by
ΔG=-nFE
But clearly it's not, or we wouldn't be using it. What am I doing wrong?