Mohr’s Salt Titration with KMnO4

Ammonium ferrous sulphate or ammonium iron(II) sulphate is called Mohr’s salt. It is an inorganic light green coloured crystalline salt. Mohr’s salt formula is (NH₄)₂Fe(SO₄)₂SO₄ (anhydrous). The formula of hydrated Mohr’s salt is (NH₄)₂Fe(SO₄)₂SO₄.6H₂O. It is hexahydrate salt. Thus, it has two different cations, Fe²⁺ and NH^4+. So, it is a double salt of ferrous sulphate and ammonium sulphate.  

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

The titration of potassium permanganate with respect to the mohr salt is a major example of redox titration. The action of the indicator is analogous to the other types of visual color titrations in the close proximity of the end-point in oxidation-reduction titration. 

Experiment: Mohr’s salt Titration with Potassium Dichromate

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

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

Molecular Equations – 

Reduction Half Reaction -

             2KMnO₄ + 3H₂SO₄ → K₂SO₄ + 2MnSO₄ + 3H₂O + 5O

Oxidation Half Reaction -

FeSO₄(NH₄)₂SO₄.6H₂O + H₂SO₄ + O → Fe₂(SO₄)₃ + 2(NH₄)₂SO₄ + 13H₂O x 5           

Overall Reaction-

2KMnO₄+10FeSO₄(NH₄)₂SO₄.6H₂O+8H₂SO₄→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 a self-indicator in this titration. The appearance of the endpoint 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, burette stand, funnel, measuring flask, white tile, burnet and wire gauge. 

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

Apparatus setup - 

1. KMnO₄ is poured into the burette

2. 10ml of Ferrous Ammonium Sulfate (Mohr’s salt) + Sulfuric acid is poured into the conical flask.

3. KMnO₄ acts as a self indicator.

4. Colourless to permanent pale pink colour is the endpoint. 

(Image will be uploaded soon)

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

1. The 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 = (392 / 20) / 1000 × 250 = 4.9g

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

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

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

5. Now fill the volumetric flask with distilled water according to the required volume. 

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

B. The procedure of Titration - 

1. Fill the burette with potassium permanganate solution. 

2. Take a conical flask and add 5ml of dilute sulfuric acid to 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 endpoint. 

8. Note down the final reading from the burette. 

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

C. Titrating potassium permanganate solution against standard ferrous ammonium sulfate (Mohr’s salt)

1. Wash the pipette and the burette with distilled water and then rinse it with the corresponding solution which needs to be filled in them.

2. Rinse the burette with potassium permanganate solution and then fill the potassium permanganate solution in it.

3. Fix the burette in the burette stand and then place a white tile beneath it so that the endpoint can easily be traced.

4. Rinse the pipette as well as the conical flask with mohr salt which is also known as ferrous sulfate solution.

5. Take out 10 ml of standard more mohr salt solution in the conical flask.

6. Add 1 + 2 full of sulfuric acid so as to prevent the oxidation of manganese to form manganese dioxide.

7. Take the initial reading of the burette before starting that experiment.

8. Now against the potassium permanganate solution start the titration and slowly still the flask gently.

9. Initially, the purple colour of KMnO₄ will be discharged with ferrous ammonium sulfate. The appearance of the permanent pink colour indicates the endpoint. Repeat the experiment unless 3 Concordant value is obtained.

10. It is important to note down the upper meniscus reading of the burette.

Observation Table:

The volume of Mohr’s salt solution taken – 10ml.

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 with distilled water before the experiment.

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

4. Use diluted sulfuric acid in the experiment.

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

6. The strength of the KMnO₄ solution should be taken up to three decimals. 

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

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

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