The Practical Experiment of Mohr’s Salt Titration with KMnO4

Titration of Mohr’s Salt with KMnO4 is an example of a redox reaction Redox reactions are also known as reduction and oxidation reactions. A redox reaction is the transfer of electrons from one species to another simultaneously in an aqueous solution. During an oxidation reaction, electrons are transferred from the species. In a reduction reaction, electrons are gained by the species. Redox reactions may occur simultaneously.  Here, we will also learn Mohr’s Salt Titration Equation. 

The titrations used to study redox reactions are called Redox Titrations, similar to acid-base titration indicators. These indicators are sensitive to changes in oxidation potential. The best indicators need to have a potential oxidation value between the solution being titrated and the titrant. These indicators produce a sharp, detectable colour change at the endpoint of the redox reaction.

Aim

Titration with Mohr’s Salt Fe(SO4) (NH4)2 solution to determine the concentration/molarity of the KMnO4 solution

Theory

Mohr’s salt is double iron (ferrous II) sulphate and ammonium sulphate; Mohr’s salt is a green-coloured inorganic compound with a Fe2+ cation and an NH4+ cation. Mohr’s salt is made by adding a small volume of H2SO4 in water to this add equimolar mixture of FeSO4 and (NH4) 2 SO4 the resulting product is crystallised.

Potassium permanganate acts as a powerful oxidising agent in an alkaline medium. The oxidising action of KMnO4 in the acidic medium for quantitative analysis, mostly acidic medium, is used. Mohr’s salt should be taken in a conical flask before that less volume of dil. H2SO4 should be taken in a conical flask.

This chemical reaction involves one oxidation and another reduction reaction 

1. The half-oxidation equation is represented as follows:

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

When potassium permanganate reacts with Sulphuric acid, potassium sulphate and two molecules of Manganese sulphate and three water molecules form.

1. The half -reduction equation is represented as follows:

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

When two molecules of ferrous ammonium sulphate are reacted with Sulphuric acid, ferrous tri sulphate and two molecules of ammonium sulphate and 13 water molecules formed.

1. The complete reaction is represented as follows:

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

When potassium permanganate and ferrous ammonium sulphate react with sulphuric acid, potassium sulphate, two molecules of manganese sulphate, ferrous tri sulphate, and 68 molecules of water formed.

Material Required

• Measuring flask

• Burette

• Burette holder

• Dropper and pipette

• Conical flask

• Funnel

• Scale for weighing
• Chemical balance

Reagents  

• Mohr’s Salt

• Potassium permanganate solution

• dilute Sulphuric acid

Procedure 

1. Preparation of M/20 Mohr’s Salt standard solution

Weigh 4.9 g of Mohr’s salt using a balance, transfer it to a volumetric flask, and add 5 ml of sulphuric acid. Then add distilled water to dissolve the solute completely. Mark up to the 100mL mark by adding water. Shake the conical flask well.

1. Titration of Mohr’s salt solution versus Potassium Permanganate solution
2. Take a clean burette and rinse with a potassium permanganate solution. In the case of an air bubble present in the burette, open the nozzle to remove it. Use a glass stop cock burette rather than a rubber stop cock since it attracts permanganate ions.
3. In a clean and dry conical flask, take 10 mL of M/20 Mohr’s salt solution and add approximately 5 mL of dilute H2 SO4. Sulphuric acid is added to stop the formation of any manganese dioxide precipitates at the time of titration.
4. Slowly add Permanganate solution from burette to conical flask at regular intervals to the conical flask, which is swirling gently, continue addition till colour change noticed.
5. Note down the initial and final readings in the burette. Repeat the titration till three concordant readings are obtained. Consider the upper meniscus of burette readings because of the dark colour of KMnO4.
6. Write down the readings as shown in the recording table and calculate the strength of the KMnO4solution in moles/litre.

Recording Table for Titration of KMnO4 solution against standard Mohr’s salt solution

Calculations

The following equation can calculate the strength of the unknown solution molarity.

N1 M1 V1 = N2 M2 V2 (Equation 1)

Where

For Mohr’s salt solution vs potassium permanganate titration:

N1 = 5, (total No. of electrons given per formula unit of Mohr’s salt solution in a balanced equation of half-cell reaction)

N2 = 1, (total No. of electrons received per formula unit of Potassium Permanganate in the equation of half-cell reaction)

M1 and M2 are the Mohr’s salt solution and Potassium Permanganate solution molarities, respectively.

V1 and V2 are the Mohr’s salt solution and Potassium permanganate solutions, volumes respectively.

On putting the values of N1 and N2 into the equation, we get

Mohr’s salt solution KMnO4

Adding values to the equation we get,

 5M1V=1×10×1/20

5M1V=0.5

M1=1/10 V

M1=V/10

The molarity of Potassium Permanganate solution can be calculated by the following equation, which gives the strength of the solution:

Molarity x Molar mass = Strength

 KMNO4 strength = molar mass (M) molarity (M)

Molarity(M) molar mass = 158

V/10 = 158

V=18.8.

Result

• KMnO4 solution Molarity is found to be _______mole/litre.
• KMnO4 solution Strength is found to be ___grams/litre.

Precautions:

1. Always use proper personal protective equipment while doing experiments.

1. Always use clean, undamaged, and dry glassware.

1. The burette and the pipette should be rinsed thoroughly with the solutions to be taken in them.

1. Remove the funnel from the burette after the addition of KMNO4.

1. Carefully add the solvent from the burette, ensuring that no drop of the liquid is hanging at the tip of the burette when the endpoint has arrived.

1. Always make sure the tip of the pipette is dipped in the liquid while drawing the liquid.
2. Very carefully add the last drop of the solution from the pipette. Don’t blow it out.
3. Remember to add dilute H2SO4 solutions before titrating Mohr’s salt solution against Potassium Permanganate.
4. Take three trials.
5. The strength of the solution must be calculated up to the fourth decimal place.

Conclusion

From this redox titrimetric analysis, we analysed the strength/molarity of the KMnO4 solution by titrating it with a Mohr’s salt solution.