Mohr Salt Titration with KMnO₄

Table of Contents

AimTheoryMaterials RequiredApparatus SetupProcedureObservationsCalculationsResults and DiscussionPrecautionsViva Questions

The titration of potassium permanganate (KMnO₄) against Mohr salt is an example of redox titration. In close proximity to the endpoint, the action of the indicator is analogous to the other types of visual colour titrations in oxidation-reduction (redox) titrations.

Aim:

To determine the strength of a given potassium permanganate solution against a standard ferrous ammonium sulfate (Mohr’s salt) solution.

Theory:

Potassium permanganate is a strong oxidant in the presence of sulfuric acid. Mohr salt is a double salt forming a single crystalline structure having the formula FeSO₄.(NH₄)₂SO₄.6H₂O. The chemical name for Mohr’s salt is ferrous ammonium sulfate.

In this titration Mohr salt acts as a reducing agent and potassium permanganate acts as an oxidising agent. So, the reaction between Mohr’s salt and potassium permanganate is a redox reaction. In this redox reaction, ferrous ion from Mohr’s salt gets oxidised and pink coloured of manganese present in potassium permanganate, which is in the +7 oxidation state gets reduced to colourless Mn²⁺ state.

The chemical reaction and the molecular chemical equation is given below.

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

The ionic equation involved in the process is given below.

Oxidation half reaction – [Fe²⁺ → Fe³⁺ + e^–] x 5

Reduction half reaction – MnO₄^– + 8H⁺ + 5e^– → Mn²⁺ + 4H₂O

Overall ionic equation – MnO₄^– + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O

This titration is based upon oxidation-reduction titrations. When ferrous ammonium sulfate solution is titrated against potassium permanganate in the presence of acidic medium by sulfuric acid. Acidic medium is necessary in order to prevent precipitation of manganese oxide. Here KMnO₄ acts as a self indicator and this titration is called permanganate titration.

Materials Required:

1. Mohr’s salt (ferrous ammonium sulfate)
2. Potassium permanganate solution
3. Dilute sulfuric acid
4. Chemical balance
5. Burette
6. Burette stand
7. Pipette
8. Conical flask
9. Funnel
10. Measuring flask
11. Weighing bottle
12. White tile
13. Burnet
14. Wire gauze

Apparatus Setup:

1. In burette – KMnO₄ solution
2. In Conical flask – 10ml of Ferrous Ammonium Sulfate (Mohr’s salt) + Sulfuric acid
3. Indicator – Self indicator (KMnO₄)
4. End Point – Colourless to permanent pale pink colour.

Procedure:

(a) Preparation of 0.05M standard solution of ferrous ammonium sulfate:

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/mol

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

1. Weigh an empty watch glass using a chemical balance.
2. Weigh accurately 4.9gm of Mohr’s salt in a chemical balance.
3. With the help of a funnel, transfer the Mohr’s salt into the measuring flask.
4. Now wash the funnel with distilled water without removing the funnel from the flask.
5. Make the solution up to the marked point with distilled water and make sure the Mohr’s salt is fully dissolved.
6. This solution is 0.05N standard solution of Mohr’s salt.

(b) Titration of potassium permanganate solution against standard ferrous ammonium sulfate (Mohr’s salt) solution:

1. Wash and rinse the burette and pipette with distilled water and then rinse with the corresponding solution to be filled in them.
2. Rinse the burette with the potassium permanganate solution and fill the burette with potassium permanganate solution.
3. Fix the burette in the burette stand and place the white tile below the burette in order to find the endpoint correctly.
4. Rinse the pipette and conical flask with standard ferrous sulfate solution.
5. Pipette out 10ml of 0.05N standard Mohr’s salt solution into the conical flask.
6. Add a test tube full of sulfuric acid in order to prevent oxidation of manganese to form manganese dioxide.
7. Note down the initial reading in the burette before starting the titration.
8. Now start the titration, titrate against potassium permanganate solution and simultaneously swirl the solution in the flask gently.
9. Initially, the purple colour of KMnO₄ is discharged with ferrous ammonium sulfate. The appearance of a permanent pink colour reveals the endpoint.
10. Repeat the titration until concordant values are obtained.
11. Note down the upper meniscus on the burette readings.
12. Record the reading in the observation table given below in order to calculate the molarity of KMnO₄ given.

Observations:

Calculations:

(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/liter, respectively.

Results and Discussion:

1. Molarity of given KMnO₄ solution is _________ moles/liter
2. The strength of given potassium permanganate solution is _______ g/L

Precautions:

1. Potassium permanganate is dark, so always read the upper meniscus.
2. Rinse the pipette and burette before use.
3. Use dilute sulfuric acid for acidifying the potassium permanganate.
4. Clean all the apparatus with distilled water before starting the experiment and then rinse with the solution to be taken in them.
5. Take accurate readings once it reaches the endpoint and doesn’t go with average readings.
6. Do not use a rubber cork burette as it can be attacked by KMnO₄.
7. Use the antiparallel card or auto parallax card while taking the burette readings.
8. The strength of the unknown solution should be taken up to two decimal places only.

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