Estimation of Fe (II) in Mohr’s salt solution using standard KMnO4 solution via Redox titration

Theory:

Titrations in which a reducing agent is titrated against an oxidizing agent or vice versa are called Redox titration. In a redox reaction if one substance is oxidized the other is reduced. Oxidation is defined as the process in which loss of electron from the element takes place and reduction is a process where electrons are accepted by the element or the ion. The reagent which undergoes reduction is an oxidizing agent and the reagent which undergoes oxidation is a reducing agent. The oxidation and reduction are complementary to one another and takes place simultaneously.

Here in the Experiment Fe2+ is oxidized to Fe3+  &  MnO4 is reduced to Mn2+.

Total reaction:

MnO4 + 8H+ + 5e = Mn2+ + 4H2O        ………….  Reduction

[ Fe2+ – e = Fe3+ ] x 5                           ……….   Oxidation

___________________________________________

MnO4 +5Fe2+ +8H+  = Mn2+ +5Fe3++ 4H2O

 

Chemicals: Mohr’s salt, KMnO4 solution, 1(N) H2SO4, Syrupy H3PO 4, Distilled water.

Apparatus:  Burette, Pipette, 250 ml conical flask, Measuring Cylinder, Dropper.

Procedure:

  1. Pipette out 10 ml of Mohr’s salt & taken In a 250 ml conical flask,
  2. Add 20 ml 1(N) H2 SO4 solution & 30 ml of distilled water followed by 1 ml of syrupy phosphoric acid (H3PO4).
  • The solution is allowed to titrate with standard KMnO4 solution drop wise taken in a burette, until the colour of the solution turns from colour less to faint pink.

Repeat the process thrice.

Observation table:

 No. of obs. Vol. of Mohr’s salt Burette reading Vol. of KMnO4 Mean Vol. of KMnO4

 

  Strength of       KMnO4

 

Initial final

Calculation:

1000 ml 1(N) of  KMnO4    55.85 gm of Fe(II)

X ml   Y (N) of KMnO4      55.85x XY / 5 gm of Fe(II)

= Z (say)

Hence, 10 ml of Mohr’s salt contains Z gm of Fe (II)

1000 ml of Mohr’s salt contains (Z x 100) gm of Fe (II).

Conclusion:    Hence, Given Mohr’s salt contains ……… gm/lit of Fe (II).

Precaution:  

i) Use of the balance should be done carefully.

ii) Use of syrupy phosphoric acid should be done carefully

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