Mohr’s salt, also known as ammonium ferrous sulphate or ammonium iron (II) sulphate, is a crystalline inorganic salt with a unique pale green colour.
Fe(SO4)(NH4)2SO4 is Mohr’s salt formula (anhydrous). Fe(SO4)(NH4)2SO4.6H2O is the formula for hydrated Mohr’s salt, the formula for Hexahydrate salt. As a result, it has two distinct cations: Fe2+ and NH4+. As a result, it’s a ferrous sulphate and ammonium sulphate double salt.
To make Mohr’s salt, dissolve a small amount of sulfuric acid in water and add an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate. The crystallisation process is now applied to the resultant solution mixture.
After that, we acquire tiny light green crystals of Mohr’s salt.
Experiment:
Titration of KMnO4 with Mohr’s Salt
Aim:
To prepare the 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:
In the presence of sulfuric acid, potassium permanganate is a powerful oxidant. The following concepts are known:
- Mohr salt is a double salt with the formula (NH4)2. FeSO4.
- 6H2O that forms a single crystalline structure.
- Ferrous ammonium sulphate is the chemical term for Mohr’s salt.
The reducing agent in this titration is Mohr salt, whereas the oxidising agent is potassium permanganate. As a result, Mohr’s salt and potassium permanganate undergo a redox reaction. The ferrous ion from Mohr’s salt is oxidised in this redox process.
The pink colour of manganese present in potassium permanganate, which is in the 7 oxidation state, is reduced to colourless Mn2.
The chemical reaction and the molecular chemical equation is given below.
The reaction involved is as follows –
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
The oxidation-reduction titrations are used in this titration. When sulfuric 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, which is known as permanganate titration.
Materials Required:
- Dilute sulfuric acid
- Potassium permanganate solution
- Chemical balance
- Pipette
- Conical flask
- Funnel
- Burette
- Burette stand
- Weighing bottle
- Measuring flask
- Wire gauze
- Burnet
Apparatus Setup:
- In burette – KMnO4 solution
- In Conical flask – 10ml of Ferrous Ammonium Sulfate (Mohr’s salt) Sulfuric acid
- Indicator – Self indicator (KMnO4)
- End Point – Colourless to permanent pale pink colour.
Chemicals Required – distilled water, dilute sulphuric acid, potassium permanganate solution.
(a) Preparation of 0.05M standard solution of ferrous ammonium sulfate:
Let’s check how we can calculate the same using 250ml of Mohr’s salt having the required normality of 0.05N.
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
- Using a chemical balance, weigh an empty watch glass.
- In a chemical balance, weigh 4.9 grammes of Mohr’s salt.
- Transfer the Mohr’s salt into the measuring flask using a funnel.
- Now, without removing the funnel from the flask, wash it with distilled water.
- Make the solution with distilled water up to the specified point and ensure the Mohr’s salt is completely dissolved.
- This is a Mohr’s salt standard solution with a concentration of 0.05N.
(b) Potassium permanganate titrated against a standard ferrous ammonium sulphate (Mohr’s salt) solution:
- Clean the burette and pipette with distilled water before rinsing them with the solution used to fill them.
- Fill the burette with potassium permanganate solution after rinsing it with the potassium permanganate solution.
- To correctly locate the endpoint, secure the burette in the burette stand and lay the white tile beneath the burette.
- Using standard ferrous sulphate solution, rinse the pipette and conical flask.
- Fill the conical flask with 10 mL of 0.05N standard Mohr’s salt solution.
- Add a test tube full of sulfuric acid to prevent manganese from oxidising and forming manganese dioxide.
- Before beginning the titration, record the initial reading in the burette.
- Begin titration by titrating against potassium permanganate solution while gently swirling the solution in the flask.
- The purple colour of KMnO4 is initially removed using ferrous ammonium sulphate. The terminus is revealed by the emergence of a persistent pink colour.
- Repeat the titration until the results are consistent.
- On the burette readings, make a note of the upper meniscus.
- Calculate the molarity of KMnO4 provided by recording the reading in the observation table below.
Observation Table:
Volume of Mohr’s salt solution taken – 10ml.
| S.No. | Burette Reading | Volume of KMnO4 used in ml | |
| Initial | Final | ||
| 1. | |||
| 2. | |||
| 3. | |||
Calculations:
(a) Normality of KMnO4 solution:
Consider y ml of given KMnO4 solution are equivalent to 20ml of N/10 Mohr’s salt solution.
According to the law of equivalents,
N1V1 = N2V2
- N1, N2 are the 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 and molarity of the given KMnO4 solution are found out as 2/y x 31.6 g/l and N/5 moles/litre, respectively.
Results and Discussion:
- Molarity of given KMnO4 solution is _________ moles/liter
- The strength of given potassium permanganate solution is _______ g/L
Precautions:
1. Because potassium permanganate is a darkish substance, read the top meniscus first.
2. Before using the pipette and burette, make sure they’re clean.
3. To acidify the potassium permanganate, use dilute sulfuric acid.
4. Before beginning the experiment, clean all of the apparatus with distilled water and then rinse with the solution used in them.
5. Once it approaches the terminus, take accurate readings rather than relying on average readings.
6. Avoid using a rubber cork burette since KMnO4 can harm it.
7. When taking burette readings, use the antiparallel or auto parallel card.
8. Only two decimal places should be used to calculate the strength of the unknown answer. Now you might wonder, why is diluted sulphuric acid is suitable for permanganate titration?
Conclusion
Well, you need to note that in acidic media, KMnO4 is an excellent oxidising agent. If no acid is employed, KMnO4 will oxidise to MnO2, resulting in a brown precipitate. Hence, we always use diluted acid for titration.
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