Oxalic-Acid vs KMnO4, Mohr’s Salt vs KMnO4

Redox titration is demonstrated by the titration of potassium permanganate (KMnO₄) against oxalic acid (C₂H₂O₄). The activity of the indicator close to the endpoint is akin to the other visual colour titrations used in oxidation-reduction (redox) titrations.

Aim: This experiment evaluated the concentration, molarity, and strength (KMnO₄) solution by titrating it against a 0.1M oxalic acid reference solution (COOH-COOH).

Theory: KMnO₄ is an oxidizing agent that operates more effectively in acidic than in alkaline environments. As a result, potassium permanganate is only utilized in an acidic medium for quantitative analysis. The following reaction in an acidic medium might be used to show its oxidizing activity –

Reaction – MnO₄⁻ + 8H⁺ + 5e⁻ 🡪 Mn2+ + 4H₂O. 

In this titration using KMnO₄, we utilise sulphuric acid. Purple is the colour of the solution containing MnO₄⁻ ion. The solution containing Mn+2 ions, on the other hand, is colourless. When potassium permanganate combines with a reducing agent, it also functions as a self-indicator.

Oxalic acid is used as a reducing agent in the experiment, and KMnO4 is placed in an acidic medium of H2SO4. As a result, no indication is required because potassium permanganate is a self-indicator. The following reactions can be used to titrate oxalic acid against potassium permanganate:

Reduction half-reaction: 2KMnO4 + 3H2SO4 🡪 K2SO4 + 2MnSO4 + 3H2O + O2

Oxidation half-reaction: H2C2O4  🡪 2CO2 + H2O + CO

Resultant: 2MnO4- + 5C2O42- +16H+ 🡪 2Mn2+ + 10CO2 + 8H2O

Sulphuric acid combines with potassium permanganate to generate manganous sulphate, which functions as a catalyst for reducing MnO4-. When potassium permanganate is added to a conical flask holding oxalic acid, it is initially discharged, leaving the solution colourless. A pink colour indicates the endpoint due to unreacted potassium permanganate after complete oxalic acid ion consumption (pink in colour). Sulphuric acid combines with potassium permanganate to generate manganous sulphate, which functions as a catalyst for reducing MnO4-. That is why, at first, the reaction pace is slow, but as time goes on, it becomes faster.

Material Required: oxalic acid, potassium permanganate solution, 1.0 M sulphuric acid, measuring flask, burette, burette stand, pipette, conical flask, funnel, weighing bottle, glazed tile(white), burner, wire gauze, and chemical balance.

Apparatus Setup: Set up the apparatus by putting potassium permanganate solution in the burette and oxalic acid solution in the conical flask. 

Procedure: (a) Prepare a 0.1N oxalic acid standard solution:

The amount of oxalic acid necessary for a 250ml solution with a normality of 0.1N can be estimated using the formula below.

A molecular number of electrons lost by one molecule Equals oxalic acid equivalent weight.

oxalic acid equivalent weight = 126/2 = 63

Normality and Equivalent weight = Strength

1/10 x 63 = 6.3 g/l of strength

The amount of oxalic acid required to make 1 litre of N/10 oxalic acid solution is 6.3 g.

1. Using a chemical balance available, weigh an empty watch glass.
2. In the watch glass, accurately weigh 6.3g of oxalic acid.
3. Transfer the oxalic acid into the measuring flask using a funnel.
4. Now, wash it with distilled water without removing the funnel from the flask, wash it with distilled water.
5. Make the solution with distilled water to the specified point, ensuring the oxalic acid is completely dissolved.
6. This solution is a 0.1N oxalic acid standard solution.

(b) Potassium permanganate solution titration against oxalic acid standard solution:

1. Fill the burette halfway with potassium permanganate solution after rinsing it with the potassium permanganate solution.
2. To accurately determine the endpoint, place the burette in the burette stand and the white tile underneath the burette.
3. In a conical flask, pipette out 10ml of 0.1N standard oxalic acid solution.
4. Add a test tube full of sulphuric acid to prevent manganese from oxidising and becoming manganese dioxide.
5. Before titrating with potassium permanganate, heat the mixture to 60°C.
6. Before beginning the titration, take a note of the initial reading in the burette.
7. Titrate the heated solution against a potassium permanganate solution while gently swirling the answer in the flask.
8. With oxalic acid, the purple colour of KMnO₄ is first removed. The finish point is revealed by the emergence of a permanent pink colour.
9. Repeat the titration until the results are consistent.
10. On the burette readings, make a note of the upper meniscus. To compute the molarity of KMnO4 supplied, record the task in the observation table below.

Calculations: The following formula is used to compute the strength of a given KMnO₄ in terms of molarity.

a1M1V1 = a2M2V2

Where a1 and a2 are the stoichiometric coefficient of oxalic acid and KMnO₄ in a balanced chemical equation.

a1 = number of electrons lost per formula unit of oxalic acid in a balanced equation of half cell reaction which is 2

a2 = number of electrons gained per formula unit of potassium permanganate in the balanced equation of half cell reaction which is 5. 

M1 = molarity of an oxalic acid solution 

M2 = molarity of potassium permanganate solution. 

V1 = volume of oxalic acid solution 

V2 = volume of potassium permanganate solution, 

Therefore,

KMnO4 = Oxalic acid

5M2V2 = 2M1V1

M2 = (2M1V1/5M2V2).

The strength of KMnO4 is calculated by using the molarity.

Strength = Molarity x Molar mass

 Precautions: While experimenting, adopt the following precautions:-

1. Always rinse the burette and remove any bubbles from the nozzle.
2. Before using the burette and other flasks, always rinse them with distilled water.
3. Because KMnO₄ is a dark-coloured solution, always read the top meniscus.
4. To acidify potassium permanganate, use sulphuric acid. Neither HCl nor nitric acid should be used.
5. Maintain a 50-60°C temperature for the oxalic acid solution.
6. Rubber cork should not be used.
7. The solution’s strength should be calculated to three decimals.
8. Burettes with broken nozzles should never be used.
9. While recording the reading at the endpoint, no drop should be hanging from the burette’s nozzle.
10. The endpoint should be identified with attention and precision.

 What is Mohr’s Salt?

Mohr’s salt comprises ferrous ammonium sulphate and ammonium iron (II) sulphate. It’s a crystalline salt that’s inorganic and light green. Fe(SO₄)(NH₄)₂SO₄ (anhydrous) is Mohr’s salt formula. Fe(SO₄)(NH₄)₂SO₄.6H₂O is the hydrated Mohr’s salt formula. It is a Hexahydrate salt. As a result, it has two different cations: Fe⁺ and NH⁴⁺. As a result, it’s a ferrous sulphate and ammonium sulphate double salt. Hexahydrate salt is what it is. 

Theory – Redox titrations are when a reducing substance is titrated against an oxidising agent or vice versa. The potassium permanganate titration by Mohr is likewise a redox titration. The reducing agent in this titration is Mohr’s salt, whereas the oxidising agent is potassium permanganate. As a result, the reaction between Mohr’s salt and KMnO₄ is a redox reaction in which both oxidation and reduction co-occur. In all mediums, such as neutral and essential, potassium permanganate functions as an oxidising agent; however, it operates as the most excellent oxidising agent in the acidic medium, which is why a tiny amount of diluted sulfuric acid is added to the conical flask before adding Mohr’s salt for titration. The following are the involved reactions:

The chemical reaction and the chemical equation are given below.

• Reduction half-reaction –

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

• Oxidation half-reaction – 

[2Fe(SO₄)(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 this process is given below.

• Oxidation half-reaction – [Fe2+ → Fe3+ + e–] x 5
• Reduction half-reaction – MnO₄– + 8H+ + 5e– → Mn2+ + 4H2O
• The overall ionic equation is – MnO4– + 8H+ + 5Fe2+ → Mn2+ + 5Fe3+ + 4H2O

The oxidation-reduction titrations are used in this titration. When sulphuric acid is used to titrate ferrous ammonium sulphate solution against potassium permanganate in the presence of an acidic media. To avoid manganese oxide precipitation, an acidic media is required. The self-indicator KMnO4 is used in this titration, known as permanganate titration. 

 Materials Required: Mohr’s salt (ferrous ammonium sulphate), Potassium permanganate solution, Dilute sulphuric acid, Chemical balance, Burette, Burette stand, Pipette, Measuring flask, Weighing bottle, White tile, Burnet, Wire gauze.

Apparatus Setup: In burette – KMnO₄ solution, Conical flask – 10ml of Ferrous Ammonium Sulfate (Mohr’s salt) + Sulphuric acid, Indicator – Self indicator (KMnO₄), End Point – Colourless to permanent pale pink colour.

Procedure: Potassium permanganate solution was titrated against a standard ferrous ammonium sulphate (Mohr’s salt) solution:

1. Wash the burette and pipette with distilled water before rinsing with the solution used to fill them.
2. Fill the burette halfway with potassium permanganate solution after rinsing it with the potassium permanganate solution.
3. To accurately discover the endpoint, place the burette in the burette stand and the white tile underneath the burette.
4. Using standard ferrous sulphate solution, rinse the pipette and conical flask.
5. Fill the conical flask with 10ml of 0.05N standard Mohr’s salt solution.
6. Add a test tube full of sulfuric acid to prevent manganese from oxidising and becoming manganese dioxide.
7. Before beginning the titration, take a note of the initial reading in the burette.
8. Begin the titration by titrating against a potassium permanganate solution while gently swirling the answer in the flask.
9. With ferrous ammonium sulphate, the purple colour of KMnO₄ is initially ejected. The terminus is revealed by the emergence of a persistent pink colour.
10. Repeat the titration until the results are consistent.
11. On the burette readings, make a note of the upper meniscus.
12. To compute the molarity of KMnO₄ supplied, record the reading in the observation table below.

 (c)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 electrons gained

N = M x 5

M = N/5 moles/ litre

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

 Precautions: While experimenting, adopt the following precautions:

1. Before using the burette and pipette, make sure they’re clean.
2. Before starting the experiment, clean all the equipment with distilled water.
3. Because KMnO₄ is black, always read the top meniscus in the burette.
4. In the investigation, use diluted sulfuric acid.
5. When the solution turns a permanent bright pink colour, you’ve reached the finish point. After that, stop adding the KMnO₄ solution. Take note of the burette reading right away. 

Conclusion

Volumetric analysis, molarity, molality normalcy, and redox titration are all words that students are familiar with. Students learn how to use the molarity equation to calculate the strength of KMnO₄and the reason for adding oil. H₂SO₄ and heating the oxalic acid before titration.