Question

For the determination of the Fe in a Mohr's salt sample, do you expect to obtain more accurate results, with the data obtain after the direct titration using the indicator or with the data obtained with the potentiometric titration? Briefly justify your answer

Answer 1

The determination of Fe in Mohr's salt sample is titrated against a self indicator KMnO₄. Potassium permanganate changes colour of the solution from colourless to pink at the equivalence point. The Mohr salt here acts as the reducing agent and KMnO₄ is the oxidising agent. The oxidation state of Mn changes from +2 to +7. The potentiometric titration is a more accurate method of titration because it marks the equivalence point within a sudden rise in the potential. In this method, the potential is measured between the reference and indicator electrode against the volume of the titrant added. The equivalence point can be determined sharp at a single point of volume change of the titrant. In simple volumetric titration using a indicator ( self indicator in this case : titrant is indicator itself); generally equivalence point cannot be achieved and the end point can only be determined. But end point is just the point when the indicator changes it's colour. Redox indicator have two colours: one at the oxidised form and the other at the reduced form. Equivalence point is the point when the solution added (titrant) is chemically equal to the analyte solution ( here Mohr salt solution containing Fe). In direct titration, the end point and equivalence point might not be same as the indicator may change colour little after the equivalence point. Hence, one might miss the equivalence point in the process. Also since this method of determination is prone to human error (misreading the end point); given the fact that potentiometry uses an instrument which will give more accurate results, is another reason why potentiometric titration must be used.