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Aluminium Sulfate - Formula, Preparation, N-factor, Uses, FAQs

Aluminium Sulfate - Formula, Preparation, N-factor, Uses, FAQs

Edited By Team Careers360 | Updated on Jul 02, 2025 04:56 PM IST

A white crystalline solid, which is highly soluble in water and chemically known as Al2(SO4)3 is Aluminium Sulfate, also known as alum. Aluminium Sulfate is widely used in commercial and industrial processes. You must be aware that water, before reaching our houses, is cleaned and purified by municipal water treatment facilities, and these treatment facilities widely use Aluminium sulfate as a coagulating agent that binds with dirt and other impurities, leading to the formation of large flocs which can be filtered easily. Apart from water treatment plants, this compound is used in the paper and textile dyeing industries. This compound is acidic, and due to this property, it is also used as a pH regulator in some processes. This compound is inexpensive and can be easily accessed.

This Story also Contains
  1. Formula and Structure of Aluminium Sulfate :
  2. Preparation:-
  3. Properties Of Aluminium Sulfate
  4. How to Derive the Formula of Aluminium Sulfate
  5. N factor of Al2(SO4)3 and equivalent weight-
  6. Aluminium Sulfate uses:
Aluminium Sulfate - Formula, Preparation, N-factor, Uses, FAQs
Aluminium Sulfate - Formula, Preparation, N-factor, Uses, FAQs

In this article, we cover the concept of Aluminium Sulfate, which falls under the chapter P-block in Group 13, which is important for boards and JEE Mains Exam and NEET entrance exam.

Aluminium Sulfate is odourless and has a sweet taste. It is a water-soluble chemical compound, but is insoluble in ethanol. Aluminium Sulfate emits lethal fumes of sulfur oxide after undergoing the decomposition process.

Aluminium Sulphate

Formula and Structure of Aluminium Sulfate :

The chemical formula for aluminium sulfate can be represented as - Al2(SO4)3

Dialuminium Trisulfate or Filter alum are some of the other terms used to address aluminium sulphate.

The molar mass of Aluminium Sulfate is 342.15 g/cm3.

Aluminium sulphate structure


Preparation:-

Aluminium Sulfate can be prepared in the laboratory by the following method-

  • Aluminium Sulfate is prepared by the addition of aluminium hydroxide-Al(OH)3 to sulphuric acid- H2SO4.

Background wave

2Al(OH)3+3H2SO4Al2(SO4)3+6H2O

It can also be prepared by heating aluminium in a solution of sulfuric acid.

2Al+3H2SO4Al2(SO4)3+3H2

  • Similarly,Al2(SO4)3 is also manufactured from alum schists, clays or bauxite and cryolite respectively.

Alum schists comprise iron pyrite, aluminium silicate, and several bituminous elements. They undergo the roasting process, where sulphuric acid is formed and made to act on the clay to form aluminium sulphate. A comparable requirement of conditions is produced during the weathering. process; being another process followed for the production of aluminium sulphate.

The mass retrieved is now systematically removed with water and aluminium sulfate solution is prepared of required specific gravity. This solution is made to stand for some duration of time to separate. Calcium and basic iron(III) sulfate are formed. It is then evaporated until iron (II) sulfate crystallises out on chilling. It is then drawn off and evaporated until it gains the required specific gravity and is allowed to stand for some time followed by decantation.

  • The material is calcinated lightly and then blended with sulphuric acid and water and heated progressively to boiling. If concentrated acid is utilized then no external warmth or heat is required as the formation of aluminium sulfate is an exothermic reaction. The clear solution is extracted after being allowed to stand for a specific extent of time. In this way, aluminium sulfate is extracted from clay or bauxite.

  • Calcium carbonate is blended with cryolite ore and heated resulting in the formation of sodium aluminate. It is then again heated and precipitated by using sodium bicarbonate or by passing a current of carbon dioxide via the solution. The precipitate is then dissolved in a sulphuric acid solution. In this way, aluminium sulfate is manufactured using cryolite ore.

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Properties Of Aluminium Sulfate

Molecular Weight/Molar Mass342.15g/mol
Density 2.672 g/cm3
Boiling Point214° F/ 101.11°C
Melting Point770 °C


How to Derive the Formula of Aluminium Sulfate

The cation and the anion in the aluminium sulfate molecule are (Al3+) and (SO2-4), respectively.

The valency of aluminium is 3, and hence it forms a tri cation: (Al3+)

The valency of sulfate ion is 2 and so it forms a

dianion: (SO2-4)

The ions combine in a way to form an electrically neutral salt.

By cross-multiplying their valencies we get –

Al2(SO4)3

The combination of ions takes place in such a way that they form an electrically neutral salt.

N factor of Al2(SO4)3 and equivalent weight-

N factor is the capability of accepting or releasing electrons. This ability is reflected by a change in the oxidation state of the species.

Hence, the n factor for salts can be simply defined as the multiplication of charges of ions present in the salt.

Equivalent weight is defined as the ratio of molecular mass of the charged ions and charges on ionic species (n factor) i.e. ratio of molecular weight of salt to the n factor.

How to find n factor of Al2(SO4)3:

Let us find the n factor of Al2(SO4)3

Al2(SO4)3 compound consists of aluminium and sulfate ions having charges +3 and -2 respectively.

These ions are symbolised as – Al3+ and SO2-4.

By multiplying the charges present in the ions, we get-

3 × 2 = 6

And so, the n factor for Al2(SO4)3 is 6.

Equivalent weight can be given as = M/n factor

= M/6

Related Topics:

Aluminium Sulfate uses:

  • Aluminum Sulfate functions as a mordant in the dyeing industry. It assists the dye in printing on paper or a piece of fabric.

  • Aluminium Sulphate's major use in the industrial sector is as a coagulant in water treatment plants. Aluminium Sulfate when mixed with water results in the formation of several forms having varied ionizing levels capable of attracting contaminants present in water and precipitating them out.

  • Aluminum Sulfate finds its application in paper manufacturing industries as well. It is used to eliminate unwanted foreign particles along with water. It also helps in adhering to materials and neutralizing the charges when used in the pulp itself.

  • Aluminium Sulfate helps regulate algal growth in water bodies. Aluminium Sulfate being an acidic compound is often used by gardeners to adjust the soil pH.

  • It is also used as an accelerator and waterproofing agent in the concrete business.

  • One can find its application as a firefighting foam and fireproofing agent as well.

Also, check-


Frequently Asked Questions (FAQs)

1. Discuss the chemical formula of Aluminium Sulfate.

Aluminium sulfate is represented chemically as Al2(SO4)3. This compound consists of two ions of Aluminium and three ions of (SO4). Aluminium sulfate is a fundamental chemical used widely in water purification, paper, and textile dying industries.  

2. Discuss the preparation of Aluminium Sulfate.

There are several methods used in the preparation of Aluminium Sulfate, but the most widely used one is the reaction of Aluminium hydroxide with Sulfuric acid.  

The reaction is represented as: 

2Al(OH)+ 3H2SO4 -àAl2(SO4) + 6H2O

3. What is N-factor, and also discuss its importance.

N-factor of Al2(SO4)is 6. N-factor plays a very important role in stoichiometric calculations like titration reactions, as it helps in determining the number of equivalents in a reaction.  

4. What are the uses of Aluminium Sulfate?

Aluminium Sulfate is used in various industries:

  • In the water treatment industry as a water purifier.
  • In the textile industry acts as a mordant to fix dyes on fabric
  • It is used in the paper industry as a sizing agent. 
5. What are the safety considerations taken into account while dealing with aluminium sulfate?

Aluminium sulfate is safe, but there are certain precautions that we can follow: 

  • Inhalation of aluminium sulfate dust and fumes should be avoided.
  • It can cause skin irritation upon contact

So to protect ourselves from these side effects, we can use a mask, gloves, and goggles when handling this compound. This compound should be stored in a dry and cool place. 

6. What is the chemical formula of aluminium sulfate?
The chemical formula of aluminium sulfate is Al2(SO4)3. This formula represents two aluminium atoms bonded to three sulfate ions.
7. Why does aluminium sulfate have a 2:3 ratio of aluminium to sulfate?
Aluminium sulfate has a 2:3 ratio of aluminium to sulfate because aluminium has a +3 charge (Al3+) and sulfate has a -2 charge (SO4^2-). To balance these charges, two Al3+ ions combine with three SO4^2- ions, resulting in the formula Al2(SO4)3.
8. What is the difference between hydrated and anhydrous aluminium sulfate?
Hydrated aluminium sulfate contains water molecules in its crystal structure, typically Al2(SO4)3·18H2O, while anhydrous aluminium sulfate (Al2(SO4)3) does not contain any water molecules. The hydrated form is more commonly used in industry.
9. How does the structure of aluminium sulfate contribute to its properties?
The structure of aluminium sulfate consists of Al3+ ions coordinated with water molecules and SO4^2- ions. This structure contributes to its high solubility in water and its ability to form complexes, which are important for its various applications.
10. What is the difference between aluminium sulfate and alum?
While the terms are often used interchangeably, alum typically refers to a double salt of aluminium sulfate with another sulfate, such as potassium aluminium sulfate (KAl(SO4)2·12H2O). Aluminium sulfate (Al2(SO4)3) is a single salt.
11. What are the main industrial uses of aluminium sulfate?
Aluminium sulfate is widely used in water treatment as a flocculating agent, in paper manufacturing to improve paper quality, in dyeing and printing textiles, as a food additive, and in fire extinguishers.
12. How does aluminium sulfate act as a flocculating agent in water treatment?
When added to water, aluminium sulfate hydrolyzes to form Al(OH)3, which is a gelatinous precipitate. This precipitate attracts and binds to suspended particles, causing them to clump together (flocculate) and settle out of the water, thereby purifying it.
13. Is aluminium sulfate acidic or basic in nature?
Aluminium sulfate is acidic in nature. When dissolved in water, it undergoes hydrolysis to produce H+ ions, resulting in an acidic solution.
14. What is the role of aluminium sulfate in papermaking?
In papermaking, aluminium sulfate acts as a sizing agent. It helps to make the paper less absorbent and more resistant to ink penetration by creating a barrier on the paper fibers.
15. Can aluminium sulfate be used as a mordant in dyeing?
Yes, aluminium sulfate can be used as a mordant in dyeing. It helps to fix dyes to fabrics by forming a complex with the dye molecules and the fabric fibers, improving color fastness.
16. What is the N-factor of aluminium sulfate?
The N-factor (normality factor) of aluminium sulfate is 6. This is because one mole of Al2(SO4)3 can provide 6 moles of H+ ions in an acidic solution, or 6 moles of OH- ions in a basic solution.
17. Why is the N-factor of aluminium sulfate important?
The N-factor is important because it helps in calculating the concentration of aluminium sulfate in terms of normality, which is useful in titrations and other chemical reactions where the number of reactive species matters more than the number of moles.
18. How does the pH of a solution affect the behavior of aluminium sulfate?
The behavior of aluminium sulfate is highly pH-dependent. In acidic conditions, it remains mostly in solution as Al3+ ions. As pH increases, it forms various hydroxide complexes, eventually precipitating as Al(OH)3 in neutral to slightly alkaline conditions.
19. How does aluminium sulfate interact with hard water?
When added to hard water, aluminium sulfate reacts with calcium and magnesium ions, forming insoluble compounds that precipitate out. This process can help soften water by removing these hardness-causing ions.
20. How does the presence of aluminium sulfate affect the freezing point of water?
Like most dissolved salts, aluminium sulfate lowers the freezing point of water. This is due to the colligative properties of solutions, where the presence of dissolved particles interferes with the formation of ice crystals.
21. How is aluminium sulfate prepared in the laboratory?
Aluminium sulfate is typically prepared by reacting aluminium hydroxide or aluminium oxide with sulfuric acid. The general reaction is: 2Al(OH)3 + 3H2SO4 → Al2(SO4)3 + 6H2O or Al2O3 + 3H2SO4 → Al2(SO4)3 + 3H2O.
22. What is the reaction between aluminium sulfate and sodium hydroxide?
When aluminium sulfate reacts with sodium hydroxide, it forms aluminium hydroxide and sodium sulfate: Al2(SO4)3 + 6NaOH → 2Al(OH)3 + 3Na2SO4. This reaction is the basis for using aluminium sulfate in water treatment.
23. How does the solubility of aluminium sulfate change with temperature?
The solubility of aluminium sulfate generally increases with temperature. This means that more aluminium sulfate can dissolve in water at higher temperatures, which is important for its various applications.
24. How does aluminium sulfate affect soil pH?
Aluminium sulfate can lower soil pH, making it more acidic. This property is often used by gardeners to adjust soil pH for plants that prefer acidic conditions, such as azaleas and rhododendrons.
25. Can aluminium sulfate be used in organic farming?
Aluminium sulfate is generally not allowed in organic farming due to concerns about aluminium accumulation in soil and potential toxicity to plants. Organic farmers typically use alternative methods for pH adjustment and pest control.
26. How does the presence of aluminium sulfate affect the oxidation-reduction potential of water?
Aluminium sulfate can affect the oxidation-reduction potential (ORP) of water. It generally lowers the ORP, which can impact various chemical and biological processes in the water. This effect is important to consider in applications like water treatment and aquaculture.
27. What is the environmental impact of using aluminium sulfate in water treatment?
While aluminium sulfate is effective in water treatment, there are concerns about residual aluminium in treated water. High levels of aluminium in drinking water have been linked to health issues, so its use must be carefully monitored and controlled.
28. What is the role of aluminium sulfate in fire extinguishers?
In fire extinguishers, aluminium sulfate is used as a foaming agent. When mixed with sodium bicarbonate, it produces a foam that can smother fires, particularly those involving flammable liquids.
29. How does aluminium sulfate compare to other flocculants in water treatment?
Aluminium sulfate is one of the most widely used flocculants due to its effectiveness and low cost. However, it may not be as effective in very cold water or in water with high organic content compared to some other flocculants like polyaluminium chloride.
30. What is the effect of aluminium sulfate on plant growth?
Aluminium sulfate can have both positive and negative effects on plant growth. While it can help acidify soil for acid-loving plants, excessive use can lead to aluminium toxicity, which can inhibit root growth and nutrient uptake in many plants.
31. Can aluminium sulfate be used to remove phosphates from water?
Yes, aluminium sulfate is effective at removing phosphates from water. It reacts with phosphate ions to form insoluble aluminium phosphate, which precipitates out of the solution, helping to control algae growth in water bodies.
32. What safety precautions should be taken when handling aluminium sulfate?
When handling aluminium sulfate, one should wear protective gloves, eyewear, and clothing to avoid skin and eye contact. It can cause irritation and should not be ingested. Proper ventilation should be ensured when working with the powdered form to avoid inhalation of dust.
33. How does aluminium sulfate affect the taste of water?
Aluminium sulfate can impart a slightly astringent taste to water if used in excess. However, when used properly in water treatment, it should not significantly affect the taste of the treated water.
34. What is the role of aluminium sulfate in leather tanning?
In leather tanning, aluminium sulfate can be used as a tanning agent. It helps to preserve the leather by cross-linking collagen fibers, making the leather more resistant to decomposition and more durable.
35. Can aluminium sulfate be used to treat swimming pool water?
While aluminium sulfate can be used to clarify swimming pool water by acting as a flocculant, it is not commonly used for regular pool maintenance. Other chemicals like chlorine and pH adjusters are more typically used for pool water treatment.
36. What is the crystal structure of hydrated aluminium sulfate?
Hydrated aluminium sulfate, Al2(SO4)3·18H2O, typically crystallizes in a monoclinic crystal system. The water molecules are coordinated around the aluminium ions and also form hydrogen bonds with the sulfate ions, creating a complex three-dimensional structure.
37. How does aluminium sulfate affect the conductivity of water?
Aluminium sulfate increases the electrical conductivity of water when dissolved. This is because it dissociates into ions (Al3+ and SO4^2-) which can carry electrical charge through the solution.
38. What is the role of aluminium sulfate in cosmetics?
In cosmetics, aluminium sulfate is sometimes used as an astringent and antiperspirant. It can help to reduce sweating and tighten pores, but its use in personal care products is controversial due to concerns about aluminium absorption.
39. How does aluminium sulfate compare to iron sulfate in water treatment?
Both aluminium sulfate and iron sulfate are used as coagulants in water treatment. Aluminium sulfate is generally more effective over a wider pH range and produces less colored residuals, but iron sulfate may be preferred in some situations, such as phosphate removal.
40. Can aluminium sulfate be used to adjust the pH of soil for all plants?
No, aluminium sulfate should not be used to adjust soil pH for all plants. It's primarily useful for acid-loving plants. For most plants, other pH adjusters like sulfur or lime are preferred, as excessive aluminium can be toxic to many plant species.
41. What is the effect of aluminium sulfate on aquatic life?
Excessive aluminium sulfate in water bodies can be harmful to aquatic life. It can cause gill damage in fish and affect the survival of aquatic invertebrates. Therefore, its use in water treatment must be carefully controlled to minimize environmental impact.
42. How does aluminium sulfate interact with organic matter in water?
Aluminium sulfate can form complexes with organic matter in water. This interaction can affect its effectiveness as a coagulant and may lead to the formation of disinfection by-products if the water is later chlorinated.
43. What is the difference between aluminium sulfate and aluminium chloride?
While both are aluminium salts, aluminium sulfate (Al2(SO4)3) contains sulfate ions, while aluminium chloride (AlCl3) contains chloride ions. They have different properties and uses; for example, aluminium chloride is more commonly used as a catalyst in organic synthesis.
44. How does temperature affect the hydrolysis of aluminium sulfate?
Higher temperatures generally increase the rate of hydrolysis of aluminium sulfate in water. This means that the formation of various aluminium hydroxide species occurs more rapidly at higher temperatures, which can affect its behavior in applications like water treatment.
45. Can aluminium sulfate be used to remove fluoride from water?
Yes, aluminium sulfate can be used to remove fluoride from water, although it's not as effective as some other methods. It works by forming aluminium fluoride complexes which can be removed through precipitation or adsorption.
46. What is the role of aluminium sulfate in the production of aluminium hydroxide?
Aluminium sulfate is a key starting material in the production of aluminium hydroxide. It reacts with a base (usually sodium hydroxide) to precipitate aluminium hydroxide: Al2(SO4)3 + 6NaOH → 2Al(OH)3 + 3Na2SO4.
47. How does aluminium sulfate affect the turbidity of water?
Aluminium sulfate reduces water turbidity by acting as a coagulant. It causes small particles suspended in water to clump together, forming larger particles that can be more easily filtered out or settled to the bottom.
48. What is the relationship between aluminium sulfate and bauxite processing?
Aluminium sulfate is not directly involved in bauxite processing (which produces aluminium metal), but it is often produced as a by-product or derivative product in aluminium refineries. The aluminium hydroxide produced in the Bayer process can be reacted with sulfuric acid to form aluminium sulfate.
49. How does aluminium sulfate compare to polyaluminium chloride in water treatment?
While both are used as coagulants, polyaluminium chloride (PAC) is often more effective than aluminium sulfate, especially in cold water or water with high organic content. PAC also typically produces less sludge and works over a wider pH range.
50. Can aluminium sulfate be used in food preservation?
Aluminium sulfate is approved as a food additive in some countries, where it can be used as a firming agent in some pickled vegetables. However, its use in food is limited due to concerns about aluminium consumption.
51. What is the effect of aluminium sulfate on the surface tension of water?
Aluminium sulfate slightly increases the surface tension of water when dissolved. This is because the ions it forms in solution interact more strongly with water molecules than water molecules interact with each other.
52. How does aluminium sulfate affect the growth of algae in water bodies?
Aluminium sulfate can help control algae growth in water bodies by removing phosphates, which are essential nutrients for algae. It also forms flocs that can entrap and remove algae cells from the water column.
53. What is the role of aluminium sulfate in the production of zeolites?
Aluminium sulfate can be used as a source of aluminium in the synthesis of some zeolites. The aluminium ions from aluminium sulfate become incorporated into the zeolite structure during the synthesis process.
54. How does aluminium sulfate interact with proteins?
Aluminium sulfate can interact with proteins through electrostatic interactions and by forming complexes. This property is utilized in some applications, such as in the tanning of leather, where it helps to crosslink collagen proteins.
55. What is the effect of aluminium sulfate on the boiling point of water?
Like other dissolved salts, aluminium sulfate raises the boiling point of water. This is a colligative property, meaning it depends on the concentration of dissolved particles rather than their specific nature.

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