Mohr’s Salt Titration with KMnO4

In the 19th century, German Chemist Karl Friedrich Mohr did great work in the development of titration technique. That’s why Mohr’s salt is named after German Chemist Karl Friedrich Mohr. Titration of KMnO4 with Mohr’s salt is a type of redox titration. 

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What is Mohr’s Salt? 

Ammonium ferrous sulfate or ammonium iron(II) sulfate is called Mohr’s salt. It is an inorganic light green coloured crystalline salt. Mohr’s salt formula is Fe(SO4)(NH4)2SO4 (anhydrous). Formula of hydrated Mohr’s salt is Fe(SO4)(NH4)2SO4.6H2O. It is hexahydrate salt. Thus, it has two different cations Fe2+ and NH4+ . So, it is a double salt of ferrous sulfate and ammonium sulfate.  

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Mohr’s salt is prepared by adding a little amount of sulfuric acid in water and then adding an equimolar mixture of hydrated ferrous sulfate and ammonium sulfate in it. Now this resulting solution mixture is subjected to crystallization. After this we get light green coloured small crystals of Mohr’s salt. 

Experiment: Mohr’s salt Titration with Potassium Dichromate

Aim – To prepare M/20 solution of Mohr’s salt and using this solution find out molarity and strength of the given potassium permanganate (KMnO4) solution. 

Theory – Redox titrations are those titrations in which a reducing agent is titrated against oxidizing agent or oxidizing agent is titrated against reducing agent. Mohr’s salt titration with potassium permanganate is also a redox titration. In this titration Mohr’s salt acts as reducing agent and potassium permanganate works as oxidizing agent. So, the reaction between Mohr’s salt and KMnO4 is a redox reaction in which oxidation and reduction takes place simultaneously. Potassium permanganate acts as oxidizing agent in all mediums such as neutral, acidic and basic but it acts as the strongest oxidizing agent in the acidic medium that’s why small quantity of diluted sulfuric acid is added in the conical flask before adding Mohr’s salt for titration. Reaction involved as follows – 

Molecular Equations – 

Reduction Half Reaction-

             2KMnO4 + 3H2SO4 🡪 K2SO4 + 2MnSO4 + 3H2O + 5[O]

Oxidation Half Reaction-

FeSO4(NH4)2SO4.6H2O + H2SO4 + 5[O]🡪Fe2(SO4)3 + 2(NH4)2SO4+ 13H2O5                      

Overall Reaction-

2KMnO4+10FeSO4(NH4)2SO4.6H2O+8H2O🡪K2SO4+2MnSO4+5Fe2(SO4)3+10(NH4)2SO4+68H2O

Ionic Reactions – 

Oxidation half – [Fe2+🡪Fe3+-e-]5

Reduction half- MnO4- + 8H++5e- 🡪 Mn2++4H2O

Overall equation- MnO4- + 8H++5Fe2+🡪 Mn2++ 5Fe3++ 4H2O

Potassium permanganate acts as self-indicator in this titration. Appearance of the end point can be detected by the colour change of KMnO4 from colourless to light pink.

Materials Required – 

Apparatus Required – weighing bottle, weight box, volumetric flask, conical flask, burette, pipette, clamp stand, chemical balance, beakers.

Chemicals Required – distilled water, dilute sulphuric acid, potassium permanganate solution. 

Procedure – Procedure for titration of Mohr’s salt by KMnO4 is as follows- 

    A.Preparation of 250ml of M/20 Solution of Mohr’s salt –

  1. Molar mass of Mohr’s salt is 392gmol-1. It is a primary standard. Since, 1000cm3 of 1M potassium permanganate require Mohr’s salt of =392g So, 250cm3 of M/20 potassium permanganate require Mohr’s salt of =
    \[\frac{\frac{392}{20}}{1000}\times 250\] = 4.9g

  1. Accurately weigh 4.9g of Mohr’s salt using a chemical balance and watch glass. 

  2. Now put weighed Mohr’s salt in a volumetric flask using a funnel. 

  3. Now add 5ml of dilute sulfuric acid and distilled water in the same flask and dissolve Mohr’s salt. 

  4. Now fill the volumetric flask by distilled water according to the required volume. 

  5. Thus, a standard solution is prepared for the titration.

 

  1. Procedure of Titration - 

  1. Fill the burette by potassium permanganate solution. 

  2. Take conical flask and add 5ml of dilute sulfuric acid in it. 

  3. Pipette out 10 ml of prepared standard Mohr’s salt solution in the same conical flask. 

  4. Place a white tile under the burette and place the conical flask containing Mohr’s salt solution and H2SO4 on it. 

  5. Note down the initial reading of the burette. 

  6. Start running potassium permanganate solution into the conical flask and keep shaking the conical flask slowly. 

  7. Stop titration when you obtain permanent pink colour in the conical flask as it indicates the end point. 

  8. Note down the final reading from the burette. 

  9. Repeat the procedure of titration until you get three concordant readings or values. 

Observation Table:

Volume of Mohr’s salt solution taken – 10ml.

S.No. 

Burette Reading

Volume of KMnO4 used in ml

Initial 

Final 

1. 




2. 




3. 




 

Calculation: Calculation for preparing M/20 Mohr’s salt solution, we have discussed in the procedure. 

N1M1V1 = N2M2V2_ _ _ _ _ _ _ _ (1)

Where N1 , M1, V1 are normality, molarity and volume of KMnO4 solution and N2 , M2 , V2 are normality, molarity and volume of Mohr’s salt solution. 

N1 = 5 (as MnO4- + 8H++5e- 🡪 Mn2++4H2O, gain of 5electrons)

V1 = Concordant value of KMnO4 obtained in the experiment. Suppose it’s ‘a’.

M1 =? 

N2 = 1(as Fe2+🡪Fe3+-e-)

V2 = 10ml 

M2 = 1/20M

Now putting the values of N1 , V1 , N2 , M2 ,V2 in equation (1)-

5M1 a = 1 1/20 10 

= M1= 1 1/20 10 5a  (you can calculate M1 by putting the value of a which you will get by experiment/titration)

Strength of KMnO4 solution = Molarity Molar mass 

                                                 =  1 1/20 10 5a [39+55+(16×4)]

                                                 = 1 1/20 10 5a 158

(by putting the value of a, you can calculate strength of KMnO4 solution)

Result – Molarity of given KMnO4 solution ______mole/l

               Strength of given KMnO4 solution______g/l

Precautions – Following precautions should be taken while performing the experiment – 

  1. Always rinse the burette and pipette before use. 

  2. Clean all the apparatus by distilled water before the experiment. 

  3. Always read upper meniscus in the burette as KMnO4 is dark in colour. 

  4. Use diluted sulfuric acid in the experiment. 

  5. Detect the end point when the solution gives permanent light pink colour. Don’t keep adding the KMnO4 solution after it. Immediately note the reading of burette. 

  6. Strength of KMnO4 solution should be taken up to three decimals. 

  7. Place white tile below the conical flask so that detection of the end point will be easier.

  8. Don’t use rubber cork burette as it can be attacked by KMnO4 solution. 

  9. Continue the titration at least until three concordant readings are obtained.