Mohr Salt Titration with KMnO4

Titration is a method used in analytical chemistry to determine the concentration of a solution by reacting it with a known volume and concentration of another solution. 

• In the titration of mohr salt with KMnO₄, a standard solution of KMnO₄ is slowly added to a solution of Mohr's salt until the reaction is complete. 
• The endpoint of the titration is determined by the appearance of a pink colour due to the reaction between KMnO₄ and Mohr's salt.

Key Terms: Mohr's salt, KMnO₄, titration, endpoint, analytical chemistry, cations, sulphuric acid

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What is Mohr’s Salt?

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In chemistry, Mohr’s salt is referred to as Ammonium Ferrous Sulphate or Ammonium Iron (II) Sulphate.

• Mohr’s salt is a light green-coloured, inorganic crystalline salt.
• Salt Formula: Fe(SO₄)(NH₄)₂SO₄ (anhydrous).
• Hydrated Salt Formula: Fe(SO₄)(NH₄)₂SO₄.6H₂O. This formula indicates that Mohr’s salt is a hexahydrate salt.
• The salt has two different cations, Fe₂ and NH₄.
• This is a double salt of ferrous sulphate and ammonium sulphate.
• Mohr’s salt is prepared by adding a small amount of sulphuric acid to water.

Next, an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate is added. This mixture is then subjected to crystallisation. The end product is light-green-coloured crystals of Mohr’s salt. The resulting crystals are fairly small in size.

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Experiment Mohr’s salt Titration with Potassium Dichromate

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Aim

To prepare an 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 work 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 a small quantity of diluted sulfuric acid is added to the conical flask before adding Mohr’s salt for titration.

Molecular Equations

Reduction Half Reaction –

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

Oxidation Half Reaction –

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

Overall Reaction-

2KMnO₄ + 10 FeSO₄(NH₄)₂SO₄.6H₂O + 8H₂SO₄ → K₂SO₄ + 2MnSO₄ + 5Fe₂(SO₄)₃ + 10(NH₄)₂SO₄ + 68H₂O

Ionic Reactions –

Oxidation half – [Fe²⁺→Fe³⁺-e^-] x 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 Sulphate (Mohr’s salt) + Sulphuric acid is poured into the conical flask.
3. KMnO₄ acts as a self-indicator.
4. Colourless to permanent pale pink colour is the endpoint.

Procedure

1. Preparation of 250ml of M/20 solution of Mohr’s salt –
2. The quantity of Mohr’s salt required for the 250ml of the solution having a normality of 0.05N can be calculated as follows.

The molar mass of Mohr’s salt = 392 g/mole

Strength = Normality x Equivalent weight

= (1/20) x 392 = 19.6 g/L

For preparing 250ml of N/20 Mohr’s salt solution, Mohr salt required

= (19.6/1000) x 250 = 4.9 gm

2. Accurately weigh 4.9gm of Mohr’s salt using a chemical balance and watch glass.
3. Put weighed Mohr’s salt in a volumetric flask using a funnel.
4. Add 5 ml of dilute sulfuric acid and distilled water in the same flask and dissolve Mohr’s salt.
5. Fill the volumetric flask with distilled water according to the required volume.
6. A standard solution is prepared for the titration.

1. Titration Method

1. Fill the burette with potassium permanganate solution.
2. Take a conical flask and add 5 ml 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 the 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.

2. Titrating potassium permanganate solution against standard ferrous ammonium sulphate (Mohr’s salt)

1. Wash the pipette and the burette with distilled water and then rinse them 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’s salt which is also known as ferrous sulphate solution.
5. Take out 10 ml of standard more Mohr’s 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 sulphate. 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

(a) Normality of KMnO₄ solution:

Consider y ml of given KMnO₄ solution is equivalent to 20ml of N/10 Mohr’s salt solution.

According to law of equivalents,

N₁V₁ = N₂V₂

• N₁, N₂ are normality of Mohr’s salt and KMnO₄ solution respectively.
• V₁, V₂ are volumes of Mohr’s salt and KMnO₄ respectively.

1/10 x 20 = N₂ x y

N₂ = 2/y

N = Normality of given KMnO₄ solution = 2/y

(b) Strength of KMnO₄ solution:

Strength = Normality x Equivalent mass

Equivalent mass of KMnO₄ = = 158/5

= 31.6

= 2/y x 31.6 g/liter

Molarity of KMnO₄ solution

N = M x Number of electron gained

N = M x 5

M = N/5 moles/ litre

The strength of and molarity of given KMnO₄ solution is found out as 2/y x 31.6 g/l and N/5 moles/litre, respectively.

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.

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Things to Remember

1. Mohr's salt is a double salt, a mixture of two different salts taken together as a single substance in a specific molecular proportion.
2. Potassium permanganate is a strong oxidant in the presence of sulfuric acid.
3. In this titration Mohr salt acts as a reducing agent and potassium permanganate act as an oxidising agent. So, the reaction is a redox reaction.
4. KMnO₄ acts as a self indicator and this titration is called permanganate titration.
5. If dilute sulphuric acid is not used, KMnO₄ may be oxidised to MnO₂ giving a brown precipitate.