We are trying to measure the concentration of Fe(III)EDTA complex which is converted to Fe(II)EDTA complex after a reaction. So, by this measurement, we will be able to calculate the extent of the reaction.
Chelatometric Determination of Ferrous Iron with 2-Pyridinealdoxime as an Indicator, journal of Pharmaceutical Science ,September 1963 Volume 52, Issue 9, Pages 858–860