

What is Mohr’s Salt?
Ammonium ferrous sulphate or ammonium iron(II) sulphate is called Mohr’s salt. It is an inorganic light green coloured crystalline salt. Mohr’s salt formula is (NH4)2Fe(SO4)2SO4 (anhydrous). The formula of hydrated Mohr’s salt is (NH4)2Fe(SO4)2SO4.6H2O. It is hexahydrate salt. Thus, it has two different cations, Fe2+ and NH4+. So, it is a double salt of ferrous sulphate and ammonium sulphate.
Mohr’s salt is prepared by adding a small amount of sulfuric acid to water and an equimolar mixture of hydrated ferrous sulphate and ammonium sulphate. Now, this resulting solution mixture is subjected to crystallisation. After this, we get light green coloured small crystals of Mohr’s salt.
The titration of potassium permanganate with respect to the mohr salt is a major example of redox titration. The action of the indicator is analogous to the other types of visual color titrations in the close proximity of the end-point in oxidation-reduction titration.
Experiment: Mohr’s salt Titration with Potassium Dichromate
Aim – To prepare 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 works 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 the small quantity of diluted sulfuric acid is added to the conical flask before adding Mohr’s salt for titration. The reaction involved is as follows –
Molecular Equations –
Reduction Half Reaction -
2KMnO4 + 3H2SO4 → K2SO4 + 2MnSO4 + 3H2O + 5O
Oxidation Half Reaction -
FeSO4(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
Ionic Reactions –
Oxidation half – [Fe2+→Fe3+-e-]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 -
KMnO4 is poured into the burette
10ml of Ferrous Ammonium Sulfate (Mohr’s salt) + Sulfuric acid is poured into the conical flask.
KMnO4 acts as a self indicator.
Colourless to permanent pale pink colour is the endpoint.
(Image will be uploaded soon)
A. Preparation of 250ml of M/20 solution of Mohr’s salt –
The molar mass of Mohr’s salt is 392gmol-1. It is a primary standard. Since 1000cm3 of 1M potassium permanganate require Mohr’s salt of =392g So, 250cm3 of M/20 potassium permanganate require Mohr’s salt of = (392 / 20) / 1000 × 250 = 4.9g
Accurately weigh 4.9g of Mohr’s salt using a chemical balance and watch glass.
Now put weighed Mohr’s salt in a volumetric flask using a funnel.
Now add 5ml of dilute sulfuric acid and distilled water in the same flask and dissolve Mohr’s salt.
Now fill the volumetric flask with distilled water according to the required volume.
Thus, a standard solution is prepared for the titration.
B. The procedure of Titration -
Fill the burette with potassium permanganate solution.
Take a conical flask and add 5ml of dilute sulfuric acid to it.
Pipette out 10 ml of prepared standard Mohr’s salt solution in the same conical flask.
Place a white tile under the burette and place the conical flask containing Mohr’s salt solution and H2SO4 on it.
Note down the initial reading of the burette.
Start running potassium permanganate solution into the conical flask and keep shaking the conical flask slowly.
Stop titration when you obtain permanent pink colour in the conical flask as it indicates the endpoint.
Note down the final reading from the burette.
Repeat the procedure of titration until you get three concordant readings or values.
C. Titrating potassium permanganate solution against standard ferrous ammonium sulfate (Mohr’s salt)
Wash the pipette and the burette with distilled water and then rinse it with the corresponding solution which needs to be filled in them.
Rinse the burette with potassium permanganate solution and then fill the potassium permanganate solution in it.
Fix the burette in the burette stand and then place a white tile beneath it so that the endpoint can easily be traced.
Rinse the pipette as well as the conical flask with mohr salt which is also known as ferrous sulfate solution.
Take out 10 ml of standard more mohr salt solution in the conical flask.
Add 1 + 2 full of sulfuric acid so as to prevent the oxidation of manganese to form manganese dioxide.
Take the initial reading of the burette before starting that experiment.
Now against the potassium permanganate solution start the titration and slowly still the flask gently.
Initially, the purple colour of KMnO4 will be discharged with ferrous ammonium sulfate. The appearance of the permanent pink colour indicates the endpoint. Repeat the experiment unless 3 Concordant value is obtained.
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: Calculation for preparing M/20 Mohr’s salt solution, we have discussed in the procedure.
N1M1V1 = N2M2V2_ _ _ _ _ _ _ _ (1)
Where N1, M1, V1 are normality, molarity and volume of KMnO4 solution and N2, M2, V2 are normality, molarity and volume of Mohr’s salt solution.
N1 = 5 (as MnO4- + 8H++5e- 🡪 Mn2+ +4H2O, gain of 5electrons)
V1 = Concordant value of KMnO4 obtained in the experiment. Suppose it’s ‘a’.
M1 =?
N2 = 1(as Fe2+ 🡪Fe3+ -e-)
V2 = 10ml
M2 = 1/20M
Now putting the values of N1, V1 , N2 , M2 ,V2 in equation (1)-
5M1 a = 1 1/20 10
= M1 = 1 1/20 10 5a (you can calculate M1 by putting the value of a which you will get by experiment/titration)
Strength of KMnO4 solution = Molarity Molar mass
= 1 1/20 10 5a 39+55+(16×4)
= 1 1/20 10 5a 158
(by putting the value of a, you can calculate strength of KMnO4 solution)
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.
Always rinse the burette and pipette before use.
Clean all the apparatus with distilled water before the experiment.
Always read the upper meniscus in the burette as KMnO4 is dark in colour.
Use diluted sulfuric acid in the experiment.
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.
The strength of the KMnO4 solution should be taken up to three decimals.
Place white tile below the conical flask so that detection of the endpoint will be easier.
Don’t use a rubber cork burette as it can be attacked by KMnO4 solution.
Continue the titration at least until three concordant readings are obtained.
FAQs on Mohr’s Salt Titration with KMnO4
1. What is Mohr's salt and why is it considered a good primary standard for titrations?
Mohr's salt is a stable double salt of ferrous sulphate and ammonium sulphate with the chemical formula (NH₄)₂Fe(SO₄)₂·6H₂O. It is considered an excellent primary standard because it is a non-hygroscopic crystalline solid, meaning it doesn't absorb moisture from the air, and it resists oxidation. This stability ensures that its weight remains constant, allowing for the preparation of a standard solution with a precisely known concentration.
2. What is the fundamental principle of the titration between Mohr's salt and KMnO₄?
This titration is a classic example of a redox reaction. In an acidic medium, Mohr's salt (containing Ferrous ions, Fe²⁺) acts as the reducing agent and is oxidised to Ferric ions (Fe³⁺). Potassium permanganate (KMnO₄, containing Permanganate ions, MnO₄⁻) acts as the oxidising agent and is reduced to colourless Manganous ions (Mn²⁺).
3. Why is dilute sulphuric acid added to the Mohr's salt solution before starting the titration?
Dilute sulphuric acid serves two essential purposes in this experiment. Firstly, it provides the acidic medium required for KMnO₄ to function effectively as a strong oxidising agent. Secondly, it prevents the hydrolysis of ferrous sulphate in the Mohr's salt, which would otherwise form an undesirable precipitate of ferric hydroxide.
4. How is the endpoint detected in the titration of Mohr's salt with KMnO₄?
Potassium permanganate (KMnO₄) uniquely functions as a self-indicator. The KMnO₄ solution is intensely purple, but as it reacts with the colourless Mohr's salt, it is reduced to the colourless Mn²⁺ ion. The endpoint is reached when all the Mohr's salt has reacted. The very next drop of KMnO₄ is no longer decolourised and imparts a persistent light pink colour to the solution, signalling the completion of the reaction.
5. What are the key precautions to take while performing the Mohr's salt titration experiment?
To ensure accurate results in this titration, students should observe the following precautions:
- Clean all glassware thoroughly with distilled water before use.
- Rinse the burette with the KMnO₄ solution and the pipette with the Mohr's salt solution.
- Always read the upper meniscus of the KMnO₄ solution in the burette due to its dark colour.
- Ensure the conical flask is swirled gently and continuously during the titration.
- Stop adding the titrant immediately upon the appearance of a permanent pale pink colour.
6. How is the molarity of the unknown KMnO₄ solution calculated from the titration results?
The molarity is determined using the molarity equation for redox reactions: a₁M₁V₁ = a₂M₂V₂. For this specific reaction, the equation is 1(M₁V₁) = 5(M₂V₂), where M₁ and V₁ are the molarity and volume of KMnO₄, and M₂ and V₂ are for Mohr's salt. The number 5 represents the number of electrons gained by Mn, and 1 represents the electron lost by Fe. By substituting the known values and the experimental burette reading (V₁), you can solve for the unknown molarity (M₁).
7. Why can't hydrochloric acid (HCl) or nitric acid (HNO₃) be used instead of sulphuric acid in this titration?
These acids are unsuitable for specific chemical reasons. Hydrochloric acid cannot be used because KMnO₄ is strong enough to oxidise its chloride ions (Cl⁻) into chlorine gas (Cl₂), which would consume extra KMnO₄ and lead to an inaccurate, higher result. Nitric acid is an oxidising agent itself and would interfere by reacting with the Mohr's salt, competing with the KMnO₄ and disrupting the intended redox reaction.
8. What do the different colour changes signify during the titration process?
The colour changes provide direct visual cues about the reaction's progress:
- Start of Titration: As purple KMnO₄ is added to the colourless Mohr's salt solution, it is instantly decolourised because the MnO₄⁻ ion is reduced to the colourless Mn²⁺ ion.
- During Titration: The solution in the flask remains colourless as long as there is unreacted Mohr's salt (Fe²⁺) available.
- At the Endpoint: The moment all the Fe²⁺ is consumed, the next drop of KMnO₄ has nothing to react with. This excess, unreacted MnO₄⁻ ion imparts its characteristic permanent light pink colour to the solution, indicating the reaction is complete.

















