Preparation of Ferrous Ammonium Sulphate (Mohr’s salt)

Preparation of Ferrous Ammonium Sulphate (Mohr’s salt) - Procedure, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 05:03 PM IST

Ferrous Ammonium Sulphate

Here we are talking about the ferrous ammonium sulphate that means it contains ions of ammonia, iron and sulphate.as it is known that it is supposed to contain the two ions which are equally strong therefore it is said that it is a double salt reaction. Talking about the form in which it is available is the crystalline form and the crystals of this ferrous ammonium sulphate are blue-green in colour. Do you know what is the name of sulphate meaning in Hindi? गन्धक शुल्वारि.The ferrous ammonium sulphate formula is (NH₄)₂Fe(SO₄)₂·6H₂O.

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There are various other physical properties of this ferrous ammonium sulphate. The first property to be discussed here is the melting point which is 100 degree celsius. It should be noted here that these salts in the compound dissolve in water to further give the aqua complex.

It is very important here to discuss the ferric ammonium sulphate formula or molecular formula of ferrous ammonium sulphate formula or chemical formula. It is given by: (NH₄)₂Fe(SO₄)₂.6H₂O with the chemical formula we will discuss the other chemical properties also which will start with its preparation.

Now we will discuss the preparation of the double salt of ferrous ammonium sulphate. preparation of ferrous sulphate.

We will now discuss the preparation of Mohr salt. To prepare a pure sample of ferrous ammonium sulphate it should be noted here that for preparing the Mohr’s salt the two equivalent quantities of salts are taken and those two salts are namely the hydrated ferrous sulphate and the ammonium sulphate.

The point which needs to be noted here that when the two of the salts are added and made the solution of it at that time a few drops of sulphuric acid needs to added and then the solution containing sulphuric acid will result in the formation of the crystals through the process called crystallisation.

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As we have discussed the preparation of the Mohr salt which is another of the ferrous ammonium sulphates. As we have said that the crystals are obtained at the end of the preparation and that too off the blue green colour then the shape of the crystals should also be known and the shape of the crystals are monoclinic.

As we have said that they are soluble in water then we should also know whether the crystals of ferrous ammonium sulphate are acid or base in nature? So the easiest way to determine the acidity or the basicity we need to know is that they turn the blue litmus paper or the solution red which means that they are acidic in nature.

Now we will discuss the preparation of Mohr salt in the complete experiment form

Aim: To prepare the crystals of Mohr's salt or the preparation of the double salt of ferrous ammonium sulphate.

Materials required: glass rod, beaker, funnel, wire gauze, burner, required salts, distilled water and a dilute sulphuric acid.

Theory: Here we are preparing the crystals of the Mohr salt or of the double salt of ferrous ammonium sulphate. It should be noted here that for preparing the Mohr’s salt the two equivalent quantity of salts are t when the two of the salts are added and made the solution of it at that time a few drops of sulphuric acid needs to added and then the solution containing sulphuric acid taken and those two salts are namely the hydrated ferrous sulphate and the ammonium sulphate which comprises what is called the double salt.

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Procedure

Firstly weight the equivalent weights of the two salts namely the ferrous sulphate and the ammonium sulphate
On washing the flask so taken with the distilled water add to it the weighed salts of ferrous sulphate and the ammonium sulphate.
When two of the salts are added and made the solution of it at that time a few drops of sulphuric acid needs to be added and then the solution contains sulphuric acid.
Heat the solution to get the clear solution.
Filter the solution using the filter paper.
The point of crystallisation should be noted by again heating the solution up to that point and while it is heating to reach the crystallisation point the stirring should be done in order to not allow the salt to settle at the bottom.
Once the crystallisation point is reached stop stirring the solution and allow it to cool at one place without much disturbance.
Once the crystals are cooled they are separated by adding the mother liquor to it.
Also check for the acidity and the basity using the litmus solution or the paper.

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Observations

It is observed that the crystals are formed.
The shape of the crystals obtained is monohedral.
As the acidity or basicity is checked it is observed that the blue litmus paper turns red which shows its acidic nature.

Result: the yield for the crystals so obtained is calculated.

Precautions

To get the clear solution it should not be heated at the high temperature rather it should be warm gently.
While heating the solution the face should not be kept close to the beaker or the flask.
The crystals obtained should not be disturbed while cooling otherwise proper shape would not be obtained.
Stop heating once the crystallisation point is reached otherwise overheating may lead to the conversion of ions.

Another similar name which comes is ferric ammonium sulphate which has the chemical formula:(NH₄)₂Fe(SO₄)₂.6H₂O. Ferric ammonium sulphate is used as an indicator by the medicinal companies in preparation of certain medicines and this indicator is basically the standardised solution of ferrous ammonium sulphate where the ferrous ions are mixed with the sulphuric acid solution in proper amount of water to get it as indicator.
So here we have discussed each and every detail of the ferrous ammonium sulphate and about the crystals.

Preparation of Double Salt of Ferrous Ammonium Sulphate: Preparation of Double Salt: Ferrous Ammonium Sulphate (i) Dissolve 3.5 g of ferrous sulphate and 1.7 g of ammonium sulphate (weighed separately), in 5 mL of distilled water contained in a 50 mL conical flask by heating.

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Frequently Asked Questions (FAQs)

1. Give the geometry and the shape of the crystals of Mohr’s salt?

The geometry of the Mohr salt crystals are monoclinic and while talking about the shape so the shape of the crystals is octahedral that means having eight corners.

2. Why the dilute sulphuric acid is added to the solution of the mohr's salt when the crystallisation process is about to start?

The dilute sulphuric acid is added to the solution of the Mohr’s salt when the crystallisation process is about to start because it prevents hydrolysis of the ferrous ions.

3. How to prepare ferric ammonium sulphate indicator?

Ferric ammonium sulphate is used as an indicator by the medicinal companies in preparation of certain medicines and this indicator is basically the standardised solution of ferrous ammonium sulphate where the ferrous ions are mixed with the sulphuric acid solution in proper amount of water to get it as indicator.

4. What are the bad effects of ferrous ammonium sulphate on human health?

We should note here that the ferrous ammonium sulphate is prepared in laboratories but it is very injurious to health as:

When inhaled causes respiratory diseases.
When remaining nearby for a longer time can lead to the feeling of nausea.
It further may lead to stomach pain.
It also causes drowsiness.

5. What does ammonium sulphate on heating gives?

When ammonium sulphate is heated firstly the partial burning or the process of decomposition takes place then on final decomposition in the second stage the ammonia, nitrogen and water are released.

6. What is Mohr's salt and why is it important in chemistry?

Mohr's salt, also known as ferrous ammonium sulfate, is a double salt with the formula (NH4)2Fe(SO4)2·6H2O. It's important in chemistry because it's a stable form of iron(II) that resists oxidation in air, making it useful for various analytical procedures and as a reducing agent.

7. Why is Mohr's salt preferred over ferrous sulfate in titrations?

Mohr's salt is preferred over ferrous sulfate because it's more stable and resistant to aerial oxidation. This stability ensures that the iron(II) content remains constant during storage and use, providing more accurate results in titrations.

8. How does the structure of Mohr's salt contribute to its stability?

The structure of Mohr's salt contributes to its stability through the presence of ammonium ions. These ions form hydrogen bonds with water molecules, creating a protective shell around the iron(II) ions and preventing their oxidation to iron(III).

9. What is the role of sulfuric acid in the preparation of Mohr's salt?

Sulfuric acid serves multiple purposes in the preparation of Mohr's salt. It provides the sulfate ions necessary for the salt formation, creates an acidic environment that prevents iron(II) oxidation, and helps dissolve the iron filings used as a starting material.

10. Why are iron filings used instead of iron powder in the preparation?

Iron filings are preferred over iron powder because they have a larger surface area, which increases the rate of reaction with sulfuric acid. This leads to faster and more efficient formation of ferrous sulfate, an intermediate in Mohr's salt preparation.

11. How does temperature affect the preparation of Mohr's salt?

Temperature plays a crucial role in Mohr's salt preparation. The reaction is typically carried out at around 50-60°C to increase the rate of reaction between iron and sulfuric acid. However, temperatures above 70°C should be avoided to prevent oxidation of iron(II) to iron(III).

12. Why is it important to filter the solution before crystallization?

Filtration before crystallization removes any unreacted iron filings and insoluble impurities. This ensures that the resulting Mohr's salt crystals are pure and free from contaminants that could affect its properties or analytical use.

13. What is the significance of the pale green color of Mohr's salt?

The pale green color of Mohr's salt is characteristic of iron(II) compounds. This color serves as a visual indicator of the salt's purity and the oxidation state of iron. Any change in color towards brown suggests oxidation to iron(III).

14. How does the addition of ethanol affect the crystallization process?

Ethanol is added to the concentrated solution to promote crystallization. It reduces the solubility of Mohr's salt in the mixture, causing the salt to precipitate out of solution and form crystals more readily.

15. Why is it crucial to dry Mohr's salt crystals at room temperature?

Drying Mohr's salt crystals at room temperature prevents the loss of water of crystallization. Higher temperatures could cause the salt to lose some of its six water molecules, altering its composition and properties.

16. How does the presence of ammonium ions in Mohr's salt affect its properties?

Ammonium ions in Mohr's salt contribute to its stability by forming hydrogen bonds with water molecules, creating a protective shell around iron(II) ions. They also make the salt more soluble in water compared to simple ferrous sulfate.

17. What is the significance of the "6H2O" in the formula of Mohr's salt?

The "6H2O" in the formula (NH4)2Fe(SO4)2·6H2O indicates that each molecule of Mohr's salt contains six water molecules of crystallization. These water molecules are an integral part of the crystal structure and contribute to the salt's properties.

18. How does the oxidation number of iron in Mohr's salt compare to that in rust?

In Mohr's salt, iron has an oxidation number of +2 (ferrous state), while in rust (iron oxide), iron typically has an oxidation number of +3 (ferric state). This difference is crucial for understanding the salt's reducing properties and stability.

19. Why is Mohr's salt considered a double salt?

Mohr's salt is considered a double salt because it contains two different cations: ammonium (NH4+) and iron(II) (Fe2+). These cations share the sulfate anions in a single crystalline structure, resulting in a compound with properties different from its component simple salts.

20. How does the solubility of Mohr's salt compare to that of ferrous sulfate?

Mohr's salt is generally more soluble in water than simple ferrous sulfate. This increased solubility is due to the presence of ammonium ions, which interact favorably with water molecules, enhancing the overall dissolution of the salt.

21. What precautions should be taken when handling concentrated sulfuric acid in the preparation process?

When handling concentrated sulfuric acid, always add acid to water slowly with stirring, never the reverse. Wear appropriate personal protective equipment including gloves, lab coat, and safety goggles. Work in a well-ventilated area or fume hood to avoid inhaling acid fumes.

22. How does the crystal structure of Mohr's salt differ from that of simple ferrous sulfate?

The crystal structure of Mohr's salt is more complex than simple ferrous sulfate due to the incorporation of ammonium ions and additional water molecules. This results in a monoclinic crystal system for Mohr's salt, as opposed to the orthorhombic system of ferrous sulfate heptahydrate.

23. Why is it important to use freshly prepared Mohr's salt solution in titrations?

Freshly prepared Mohr's salt solution is important in titrations because, despite its stability, some oxidation of iron(II) to iron(III) can occur over time when in solution. Using a fresh solution ensures the most accurate concentration of reducing agent for precise analytical results.

24. How does the presence of air affect the preparation and storage of Mohr's salt?

Air can cause gradual oxidation of iron(II) to iron(III) in Mohr's salt, especially in solution. During preparation, minimizing air exposure helps maintain the iron in its ferrous state. For storage, keeping the salt in airtight containers and using it promptly after preparation is advisable.

25. What is the role of cooling in the crystallization of Mohr's salt?

Cooling plays a crucial role in the crystallization of Mohr's salt by reducing its solubility in the solution. As the temperature decreases, the solution becomes supersaturated, promoting the formation and growth of Mohr's salt crystals.

26. How does the pH of the solution affect the stability of Mohr's salt during preparation?

The pH of the solution significantly affects Mohr's salt stability. An acidic environment (low pH) is maintained during preparation to prevent the oxidation of iron(II) to iron(III) and the formation of iron hydroxides. This acidity is crucial for the salt's stability and purity.

27. Why is it important to avoid using metal spatulas when handling Mohr's salt?

Metal spatulas should be avoided when handling Mohr's salt to prevent contamination. Some metals could react with the salt or introduce ions that might interfere with its properties or subsequent analytical use. Glass or plastic spatulas are preferred.

28. How does the molecular mass of Mohr's salt compare to that of ferrous sulfate heptahydrate?

The molecular mass of Mohr's salt [(NH4)2Fe(SO4)2·6H2O] is 392.14 g/mol, while that of ferrous sulfate heptahydrate [FeSO4·7H2O] is 278.01 g/mol. The higher mass of Mohr's salt is due to the additional ammonium sulfate component.

29. What is the significance of the pale violet color sometimes observed in Mohr's salt solutions?

A pale violet color in Mohr's salt solutions can indicate the presence of small amounts of manganese as an impurity. Pure Mohr's salt solutions should be pale green, so any violet tint suggests the need for further purification or the use of higher purity starting materials.

30. How does the reducing power of Mohr's salt compare to other common reducing agents?

Mohr's salt is a mild reducing agent compared to stronger reducers like sodium borohydride or zinc. Its reducing power comes from the iron(II) ion, which can be oxidized to iron(III). This moderate strength makes it suitable for controlled redox reactions in analytical chemistry.

31. Why is it important to use distilled or deionized water in the preparation of Mohr's salt?

Distilled or deionized water is used to avoid introducing impurities that could affect the purity of the Mohr's salt or interfere with its properties. Tap water may contain ions like chloride or calcium that could react with the components of Mohr's salt or alter its crystallization.

32. How does the presence of oxidizing agents affect Mohr's salt?

Oxidizing agents can convert the iron(II) in Mohr's salt to iron(III), changing its properties and reducing its effectiveness as a reducing agent. This is why Mohr's salt solutions should be protected from strong oxidizers and why its stability in air is an advantage over simple ferrous salts.

33. What is the effect of pressure on the crystallization of Mohr's salt?

Pressure typically has minimal direct effect on the crystallization of Mohr's salt from solution. However, reduced pressure (vacuum) can be used to accelerate the evaporation of solvent, which indirectly promotes crystallization by increasing the concentration of the salt in solution.

34. How does the stoichiometry of iron to ammonium sulfate affect the formation of Mohr's salt?

The correct stoichiometry for Mohr's salt formation is crucial. A 1:1 molar ratio of iron(II) sulfate to ammonium sulfate is required. Excess of either component can lead to impurities in the final product or affect the crystallization process.

35. Why is Mohr's salt sometimes preferred over standard solutions of potassium permanganate in redox titrations?

Mohr's salt is sometimes preferred over potassium permanganate because it's more stable and its solutions maintain a constant concentration for longer periods. Unlike permanganate, which can slowly decompose, especially in the presence of light or heat, Mohr's salt solutions remain reliable for extended periods.

36. How does the crystal habit of Mohr's salt differ from other sulfate salts?

Mohr's salt typically forms pale green, monoclinic crystals. This crystal habit is distinct from other sulfate salts like copper sulfate (triclinic) or sodium sulfate (orthorhombic). The unique crystal structure is due to the arrangement of the two different cations (NH4+ and Fe2+) with sulfate anions and water molecules.

37. What is the role of concentration in the crystallization of Mohr's salt?

Concentration plays a crucial role in Mohr's salt crystallization. As the solution becomes more concentrated (either through evaporation or cooling), it reaches supersaturation. At this point, crystal nucleation begins, and further concentration promotes crystal growth.

38. How does the presence of other transition metal ions affect the preparation of Mohr's salt?

The presence of other transition metal ions can interfere with Mohr's salt preparation. They may compete with iron(II) for sulfate ions or incorporate into the crystal structure, affecting the purity and properties of the final product. This is why using pure iron and sulfuric acid is important.

39. Why is Mohr's salt sometimes used as a primary standard in analytical chemistry?

Mohr's salt is used as a primary standard because it has a precise, known composition, high purity, and stability. It doesn't effloresce (lose water) or deliquesce (absorb water) under normal conditions, maintaining its composition. Its high molecular weight also minimizes weighing errors.

40. How does the electron configuration of iron in Mohr's salt contribute to its color?

The pale green color of Mohr's salt is due to the electron configuration of iron(II). The d-orbital splitting in the octahedral complex allows for d-d transitions that absorb light in the red region of the spectrum, resulting in the complementary green color being observed.

41. What is the significance of the term "hexahydrate" in relation to Mohr's salt?

The term "hexahydrate" in Mohr's salt refers to the six water molecules coordinated to each formula unit of the salt. These water molecules are integral to the crystal structure and contribute to the salt's stability and properties. Losing these waters of hydration would change the compound's characteristics.

42. How does the magnetic behavior of Mohr's salt compare to that of ferric compounds?

Mohr's salt, containing iron(II), is paramagnetic due to its unpaired electrons. However, it exhibits weaker paramagnetism compared to ferric (iron(III)) compounds. This difference in magnetic behavior can be used to distinguish between ferrous and ferric compounds in some analytical procedures.

43. Why is it important to avoid overheating during the preparation of Mohr's salt?

Overheating during Mohr's salt preparation can lead to several issues: oxidation of iron(II) to iron(III), loss of ammonia from ammonium ions, and potential loss of waters of hydration. These changes would alter the composition and properties of the final product, reducing its purity and effectiveness.

44. How does the solubility of Mohr's salt change with temperature?

The solubility of Mohr's salt generally increases with temperature, like most solid salts in water. This property is utilized in its preparation and purification. Hot solutions are used to dissolve more salt, which then crystallizes as the solution cools and becomes supersaturated.

45. What is the significance of the octahedral coordination in Mohr's salt?

The octahedral coordination in Mohr's salt refers to the arrangement of six water molecules around each iron(II) ion. This coordination geometry is crucial for the salt's properties, including its color, stability, and magnetic behavior. It also influences the salt's reactivity in solution.

46. How does the presence of Mohr's salt affect the conductivity of its aqueous solution?

Mohr's salt increases the conductivity of its aqueous solution significantly. As a strong electrolyte, it dissociates completely in water, providing ions (Fe2+, NH4+, and SO42-) that can carry electrical charge. The conductivity is higher than that of simple ferrous sulfate due to the additional ammonium ions.

47. Why is Mohr's salt sometimes used in photography?

Mohr's salt is used in some photographic processes as a reducing agent. Its ability to reduce silver ions to metallic silver in a controlled manner makes it useful in certain developing solutions. The stability of Mohr's salt ensures consistent results in these applications.

48. How does the crystal field splitting energy of iron in Mohr's salt compare to other iron complexes?

The crystal field splitting energy of iron in Mohr's salt is relatively low compared to some other iron complexes. This is due to the weak-field ligands (water molecules) coordinated to the iron(II). The low splitting energy contributes to its high-spin configuration and paramagnetic properties.

49. What role does entropy play in the formation of Mohr's salt crystals?

Entropy plays a significant role in Mohr's salt crystallization. As the crystals form, the overall entropy of the system decreases as dissolved ions become ordered in the crystal lattice. This entropy decrease is offset by the release of heat (enthalpy of crystallization) and the increased entropy of water molecules released from hydration shells.

50. How does the presence of Mohr's salt affect the freezing point of its aqueous solution?

Mohr's salt, like other solutes, lowers the freezing point of water. This freezing point depression is more pronounced than for simple ferrous sulfate due to the higher number of ions produced per formula unit (5 ions: Fe2+, 2NH4+, and 2SO42-), resulting in a greater colligative effect.

51. Why is Mohr's salt sometimes used in soil analysis?

Mohr's salt is used in soil analysis primarily for its reducing properties. It can be used to determine the oxidizing capacity of soils or to reduce certain soil components for further analysis. Its stability and known composition make it reliable for standardizing solutions used in soil testing.

52. How does the lattice energy of Mohr's salt compare to that of simple ferrous sulfate?

The lattice energy of Mohr's salt is generally lower than that of simple ferrous sulfate. This is due to the larger size of the