Principle of potentiometric titration 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?
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,
 $E_{cell} = E_{cathode}$