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

Formula and Structure of Aluminium Sulfate

The chemical formula for aluminium sulfate can be represented as - $\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$

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.

Preparation Of Aluminium Sulfate

1. Reaction of Aluminium Hydroxide with Sulphuric Acid

In the laboratory, aluminium sulfate is prepared by adding aluminium hydroxide to sulphuric acid.

$2 \mathrm{Al}(\mathrm{OH})_3+3 \mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Al}_2\left(\mathrm{SO}_4\right)_3+6 \mathrm{H}_2 \mathrm{O}$

In this process, aluminium hydroxide reacts with sulphuric acid to produce aluminium sulfate and water. It is a neutralisation reaction commonly used for laboratory preparation.

2. Reaction of Aluminium Metal with Sulphuric Acid

Aluminium sulfate can also be prepared by heating aluminium metal with dilute sulphuric acid.

$2 \mathrm{Al}+3 \mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Al}_2\left(\mathrm{SO}_4\right)_3+3 \mathrm{H}_2$

Here, aluminium displaces hydrogen from sulphuric acid, forming aluminium sulfate and hydrogen gas.

3. Industrial Manufacture of Aluminium Sulfate

By Roasting process and Leaching Process

Alum schists contain iron pyrite, aluminium silicate and bituminous substances. These materials are roasted in air, producing sulphuric acid. The sulphuric acid then reacts with clay present in the mixture to form aluminium sulfate. The obtained mass is treated with water to dissolve aluminium sulfate. The solution is purified by removing impurities such as calcium compounds and iron sulfates through settling, crystallisation and decantation. Finally, the purified solution is evaporated to obtain aluminium sulfate.

4. Preparation from Cryolite Ore
Cryolite is heated with calcium carbonate to form sodium aluminate. The sodium aluminate solution is then treated with sodium bicarbonate or carbon dioxide, producing aluminium hydroxide as a precipitate.

The precipitated aluminium hydroxide is dissolved in sulphuric acid to produce aluminium sulfate.

$2 \mathrm{Al}(\mathrm{OH})_3+3 \mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{Al}_2\left(\mathrm{SO}_4\right)_3+6 \mathrm{H}_2 \mathrm{O}$

Also Read :

Properties Of Aluminium Sulfate

How to Derive the Formula of Aluminium Sulfate

The cation and the anion in the aluminium sulfate molecule are (Al³⁺) and ($\mathrm{SO}^{2-}{ }_4$), respectively.

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

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

Dianion: ($\mathrm{SO}^{2-}{ }_4$)

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

By cross-multiplying their valencies, we get –

$\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$

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

N factor of $\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$ 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 the 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., the ratio of molecular weight of salt to the n factor.

How to find n factor of $\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$:

Let us find the n factor of $\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$

$\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$ compound consists of aluminium and sulfate ions having charges +3 and -2, respectively.

These ions are symbolised as – $\mathrm{Al}^{3+}$ and $\mathrm{SO}^{2-}{ }_4$.

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

3 × 2 = 6

And so, the n factor for $\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$ is 6.

Equivalent weight can be given as = M/n factor

= M/6

Related Topics:

• Sulphuric Acid
• Diagonal Relationship Between Beryllium And Aluminium
• Alkaline Earth Metals

Uses of Aluminium Sulfate

• Aluminium 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 ionising levels capable of attracting contaminants present in water and precipitating them out.

• Aluminium 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 neutralising 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-

Some Solved Examples

Question 1: The aqueous solution of $\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$ is:

A. Neutral
B. Acidic
C. Basic
D. Amphoteric

Solution:

$\mathrm{Al}^{3+}\left(\right.$ from a weak base $\left.\mathrm{Al}(\mathrm{OH})_3\right)$ hydrolyzes and releases $\mathrm{H}^{+} \rightarrow$ solution becomes acidic.

Hence, the correct answer is option (A)

Question 2: Which ions are formed when $\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$ is dissolved in water?
A. $2 \mathrm{Al}^{3+}+3 \mathrm{SO}_4{ }^{2-}$
B. $2 \mathrm{Al}^{2+}+3 \mathrm{SO}_4^{-}$
C. $3 \mathrm{Al}^{+}+2 \mathrm{SO}_4{ }^{3-}$
D. $\mathrm{Al}^{4+}+\mathrm{SO}_4{ }^{4-}$

Solution:

The cation and the anion in the aluminium sulfate molecule are (Al³⁺) and ($\mathrm{SO}^{2-}{ }_4$), respectively.

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

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

Dianion: ($\mathrm{SO}^{2-}{ }_4$)

Hence , the correct answer is option (A)

Question 3: The first stage hydrolysis product of $\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3$ in water is:
A. $\mathrm{Al}(\mathrm{OH})_3$
B. $\mathrm{AlOHSO}_4$
C. $\mathrm{Al}_2 \mathrm{O}_3$
D. $\mathrm{Al}_2\left(\mathrm{SO}_3\right)_3$

Solution:

When aluminium sulfate is dissolved in water, the $\mathrm{Al}^{3+}$ ions undergo hydrolysis.
But complete hydrolysis forming $\mathrm{Al}(\mathrm{OH})_3$ does not occur immediately.
Instead, in the first stage of hydrolysis, a basic salt is formed:

$\mathrm{Al}_2\left(\mathrm{SO}_4\right)_3+\mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{AlOHSO}_4+\mathrm{H}_2 \mathrm{SO}_4$
This product is called basic aluminium sulfate.

Hence , the correct answer is option (B)