permanganate index use known excess KMnO4 with the sample of water first,
then the remaining KMnO4 is back titrated with excess standard C2O42-
the remaining C2O42- is finally titrated with standard KMnO4.
why not use standard C2O42- is back titrated remaining KMnO4, directly know the amount of remaining KMnO4 but use excess C2O42-?
C2O42- can be primary standard.