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In potentiometric titration between ascorbic acid and iodine, when we plot the graph of EMF with volume of iodine consumed, we get a sudden decrease in EMF at equivalance point--why? It is also negative--why?

Using Nernst equation at equivalence point we get $$E_{cell}=E_{STP}~~~(voltage ~at ~STP)$$

What is its physical meaning?

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In potentiometric analysis, there are two electrodes, an indicator electrode (platinum here) where reduction takes place and reference electrode (calomel electrode) where oxidation takes place.

So here the reference is not $E_{STP}$ cell but the $E_{cathode}$ that is the calomel electrode, so there is drop to $-241mV$ after the equivalance point. Hence, $E_{cell} = E_{cathode}$

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