The difference between the initial amount of I−3I3−\text{I}_3^- and the amount in excess gives us the amount of I−3I3−\text{I}_3^- that reacts with the formaldehyde. This is an example of a back titration.
excess amount - amount consumed in 2nd rxn = amount consumed in 1st rxn
If the end point is a function of the titrant’s concentration, then adding the titrant too quickly leads to an early end point.
before it's had a chance to react with the titrand
titration error. If the end point and the equivalence point volumes coincide closely, then this error is insignificant and is safely ignored.
want end point to be as close as possible to equivalence point