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.

Accurately take the mass of a sample of approximately 0.25-0.30 g of standard solid FAS in a weighing boat. Place the sample of FAS into a clean and dry 250 mL beaker. Label the beaker with the corresponding mass for identification. Do not place any solid back in the reagent bottle! Dissolve the FAS sample in about ~10 mL of boiled distilled water, add 2.0 mL of 9 M H2SO4, and add 2 drops of 6M H3PO4 to the beaker. Swirl the beaker until you are sure all of the FAS has dissolved and all other contents have mixed. H3PO4 is used to decolorize Fe in the reaction and keep its reduction potential low.

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

1. The indicator doesn't required for this titration. Because, the reagent KMnO4 has deep purple colour. When this is titrated with FAS solution in presence of acidic medium, it converts in to colorless, this is due to conversion of Mn+7 ( deep purple colour ) in to Mn+2( colourless). At end point, light pink colour will be observed. ( At the end point all FAS molecules will be consumed and excess amount of Kmno4 is added from the burette. This excess Kmno4 spread throughout the solution and gives pink colour to it. Hence by seeing the colour we can tell that the reaction has reached end point. For this colour change we need not add any indicator. Simply we can conclude Kmno4 acts as self indicator .

2. H2SO4 and H3PO4 provides protons to drag oxygens from KMno4 to form water. Means they facilitate Mn+7 to react with F A S. They also plays vital role in reaction to take out oxygen as much as they can from Kmno4 to yield water.

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