A solution of potassium permanganate (KMnO4) will be standardized using solid sa

Question

A solution of potassium permanganate (KMnO4) will be standardized using solid samples of iron(II) ammonium sulfate hexahydrate (Fe(NH4)2(SO4)2•6H2O) called FAS for simplicity. Note that six hydrated water molecules are Part of the total molecule. The net ionic equation for the standardization of the permanganate solution is: ??????4?(????) + 5 ???? 2+(????) + 8 ?? +(????) ? ????2+(????) + 5 ???? 3+(????) + 4??2??(??) In this equation MnO4 - ions are reduced (charge decreases) to Mn2+ while Fe2+ ions are oxidized (charge increases) to Fe3+ ions. Note that the hydronium ions are supplied by making the solution acidic by adding sulfuric acid in excess to the beaker. This equation also shows that for every mole of permanganate, five moles of iron (II) are required. Thus, from the mass of Fe(NH4)2(SO4)2•6H2O used and from the volume of KMnO4 required for the titration you can calculate the molarity of the KMnO4 solution using stoichiometry. Standard solid samples FAS. KMnO4 is hygroscopic meaning that water will adsorb to the surface of the solid KMnO4 that was used to create the reagent solution you have before you. Therefore, when the mass of KMnO4 used is recorded, the mass of an unknown quantity of water is also included, meaning any molarity calculations derived from this mass are inaccurate! Therefore, a primary standard that is not hygroscopic

Why was an indicator not needed? Why was H2SO4 added? Why was H3PO4 added to the standardization reaction but not needed for the peroxide reaction?

Explanation / Answer

Ans. #1. The endpoint is marked by disappearance of pink color (MnO4-) due to reduction of Mn7+ (in KMnO4, pink colored) to Mn2+ (colorless).

Because reduction of MnO4- itself acts as an indicator for the titration endpoint, there is no need of adding external indicator.

#2. As shown in the balanced reaction, the reduction MnO4- by Fe2+occurs in acidic condition (acid in excess). So, for the reaction to occur, H2SO4 is added to the KMnO4 solution being titrated.

#3. Note that there all Fe2+ is oxidized to Fe3+ at the equivalence point. It is necessary to retain the oxidation state of Fe3+ for a persistent endpoint. To do so, small quantity of phosphoric acid is added to the solution being titrated. The phosphoric acid complexes Fe3+ ions and stabilizes them.

# Peroxide titration involves reaction between standardized KMnO4 and H2O2 only. Since there is no Fe used as reducing agent, there is no need of adding phosphoric acid to stabilize it.

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