ALUMINUM SULFATE (AL2 (SO4) 3): STRUCTURE, USES, TOXICITY - CHEMISTRY - 2025

The aluminum sulfate is an inorganic aluminum salt of formula A the ₂ (SO ₄) _3, usually seen as a shiny white solid crystals. The color of the compound will be influenced by its iron concentration and any other impurities. There are two main types of aluminum sulfate: A and B.

The lower image shows white crystals of hydrated aluminum sulfate. The absence of brown colorations can be noted, indicative of iron ions within the crystal lattice.

Source: Martin Walker via Wikimedia Commons

Aluminum sulfate is a very soluble salt in water and in nature it is rare to find it in its anhydrous form. It is usually hydrated in the form of aluminum sulfate octadecahydrate or hexadecahydrate.

Likewise, it can form double salts with potassium, and with ammonium, compounds known as alums. In part this may be due to the affinity of water within hydrates to retain ions other than aluminum.

Aluminum sulfate can be broken down by the action of water into aluminum hydroxide and sulfuric acid. This property has allowed its use as a soil acidifier.

It is also a toxic compound, especially when in contact with its dust. However, it has a myriad of uses and applications ranging from water purification using the coagulation method, through its use in various industries, to its use for therapeutic purposes.

Structure

Source: Gabriel Bolívar

Aluminum sulfate has a ratio of two Al ³⁺ cations for every three SO ₄ ^2- anions (upper image), which can be seen directly in its chemical formula Al ₂ (SO ₄) ₃.

Note that Al ³⁺ are greyish in color, while SO ₄ ^2- are yellow (due to the sulfur atom) and red (due to the oxygen atoms).

The illustrated structure corresponds to the anhydrous form of Al ₂ (SO ₄) ₃, since no water molecule interacts with the ions. In its hydrates, however, water can be attracted by the positive centers of Al ³⁺, or by the negative tetrahedra of SO ₄ ^2- by means of hydrogen bonds (HOH- O-SO ₃ ^2-).

Al ₂ (SO ₄) ₃ ∙ 18H ₂ O, for example, contains 18 water molecules in its solid structure. Some of them may be in direct contact with Al ³⁺ or SO ₄ ^2-. In other words: aluminum sulfate can have internal or external coordinating water.

Likewise, its structure can host other cations such as Fe ³⁺, Na ⁺, K ⁺, etc. But for this, the presence of more SO ₄ ^2- anions is necessary. For what? To neutralize the increase in positive charges due to metallic impurities.

Aluminum sulfate can have many structures, although its hydrates tend to adopt the monoclinic crystal system.

Physical and chemical properties

Appearance

It occurs as a solid with lustrous white crystals, granules or powder.

Molecular weight

342.31 g / mol (anhydrous).

Odor

It is odorless

Taste

Moderately astringent sweet flavor.

Melting point

770º C anhydrous form (86.5º C octadecahydrate form)

Water solubility

31.2 g / 100 ml at 0 ° C; 36.4 g / 100 ml at 20 ° C and 89 g / 100 ml at 100 ° C.

Density

2.67 to 2.71 g / cm ³.

Solubility

Slightly soluble in ethyl alcohol.

Vapor pressure

Essentially zero.

Stability

Stable in the air.

Decomposition

When heated, around its melting point, it can decompose, emitting especially sulfur oxide, a toxic gas.

Corrosiveness

Aluminum sulfate solutions are corrosive to aluminum. Also the compound can corrode metals in the presence of moisture.

pKa

3.3 to 3.6. And its pH is 2.9 or more in a 5% aqueous solution.

What is it for (uses)

Coagulation or flocculation of water

-When mixed with water (drinking, served or waste), aluminum sulfate produces aluminum hydroxide, which forms complexes with the compounds and particles in solution, accelerating their sedimentation, which in the absence of treatment with aluminum sulfate it would take a long time.

-Aluminum sulfate is also used to clean swimming pool water, making it more attractive to use.

-Through the use of aluminum sulfate, a water devoid of turbidity and color is achieved, increasing its potability by facilitating the cleaning of the water. Unfortunately this purification method can lead to a slightly increased concentration of aluminum in the water.

-Aluminum is a very toxic metal that accumulates in the skin, brain and lungs, causing serious disorders. In addition, it is not known what function it fulfills in living beings.

-The European Union requires that the maximum allowed limit of aluminum in water does not exceed 0.2 mg / l. Meanwhile, the US Enviromental Protection Agency requires that the maximum limit of water contamination with aluminum does not exceed 0.05-0.2 mg / l.

-The treatment of used or sewage water with aluminum sulfate allows the elimination or reduction of phosphorus in them, economically.

Use as an ink mordant and in papermaking

-Aluminum sulfate has been used as a mordant of colorants or inks, helping to fixate on the material to be dyed. Its fixing action is due to Al (OH) ₃, whose gelatinous consistency collaborates with the adsorption of dyes on textiles.

-Although it has been used since approximately 2000 BC for the purpose described, only organic dyes require mordants. Synthetic colorants, on the other hand, do not require mordants to do their job.

-It has been displaced from the manufacture of paper, but it is still used in the production of paper pulp. It removes its impurities, as well as it has also been used to bind materials, neutralize charges and calibrate rosin.

Industrial uses

-Used in the construction industry in order to accelerate the setting of concrete. Additionally, it is used in the waterproofing of constructions.

-In the industry of soaps and fats it is used in the production of glycerin.

-It is used in the oil industry in the production of synthetic catalysts that they use during their operation.

-It is used in the pharmaceutical industry as an astringent in the preparation of drugs and cosmetics.

-It takes part in the elaboration of colorants such as carmine. It is also used as a colorant in the manufacture of styrene butadiene synthetic rubbers.

-In the sugar manufacturing industry it is used as a purifier of sugar cane molasses.

-It is used in the manufacture of deodorants. Why? Because it causes a narrowing of the ducts of the sweat glands, thus limiting the accumulation of sweat, a necessary medium for the growth of odor-causing bacteria.

-It is used in the tanning of leather, a process necessary for its use. Additionally, it is used in the manufacture of fertilizers.

-It is used as an additive in the preparation of paints, adhesives and sealants, as well as cleaning and furniture care products.

Medicinal and therapeutic uses

-Aluminum sulfate is an immune adjuvant. Therefore, it fulfills the function of processing antigens in such a way that when they are released in their site of action, they produce a greater production of antibodies to the antigens that were inoculated.

-Freund's adjuvant and BCG, as well as other adjuvants, including endogenous ones such as interleukins, are nonspecific for antigens, allowing an increase in the radius of immune action. This has allowed the development of vaccines against numerous diseases.

-The coagulant action of aluminum sulfate has allowed the elimination of numerous viruses in the treated water, among others: Q beta, MS2, T4 and P1. The results indicate that the treatment of water with aluminum sulfate produces an inactivation of such viruses.

-Aluminum sulfate is used in the form of a stick or as a powder in the treatment of small superficial wounds or abrasions produced when shaving.

-It is used in the manufacture of aluminum acetate, a compound used in the treatment of some ear conditions. It has also been used, without significant success, to alleviate the consequences of fire ant stings.

-Aluminum sulfate solutions are used, at a concentration of 5% to 10%, in the local treatment of ulcers that allow the control of their secretions.

-The astringent action of aluminum sulfate contracts the surface layers of the skin, coagulating proteins and healing wounds.

Other uses

-Aluminum sulfate helps in controlling the excessive growth of algae in ponds, lakes and streams. It is also used in the elimination of mollusks, among others the Spanish slug.

-Gardeners apply this compound to acidify alkaline soils. In contact with their water, aluminum sulfate decomposes into aluminum hydroxide and dilute sulfuric acid. Then, the aluminum hydroxide precipitates, leaving the sulfuric acid in solution.

-The acidification of the soils due to sulfuric acid is visualized in a very creative way, due to the existence of a plant called Hydrangea, whose flowers turn blue in the presence of an acidic soil; that is, they are sensitive and respond to changes in pH.

-Aluminum sulfate is involved in the production of a foam to fight and control fire. How? Reacts with sodium bicarbonate, releasing CO ₂. This gas blocks O ₂ access to the material's burning site; and consequently the ceasefire that was developing.

Aluminum sulfate production

Aluminum sulfate is synthesized by reacting an aluminum-rich compound, such as the mineral bauxite, with sulfuric acid at elevated temperatures. The following chemical equation represents the reaction:

Al ₂ O ₃ + H ₂ SO ₄ ---> Al ₂ (SO ₄) ₃ + 3 H ₂ O

Aluminum sulfate can also be formed by a neutralization reaction between aluminum hydroxide and sulfuric acid, according to the following reaction:

2 Al (OH) ₃ + 3 H ₂ SO ₄ + 12 H ₂ O ---> Al ₂ (SO ₄) ₃.18H ₂ O

Sulfuric acid reacts with aluminum to form ammonium sulfate and release hydrogen molecules as a gas. The reaction is outlined as follows:

2 Al + 3 H ₂ SO ₄ ---–> Al ₂ (SO ₄) ₃ + 3 H ₂

Types

Aluminum sulfate is classified into two types: type A and type B. In type A aluminum sulfate, the solids are white in color and have an iron concentration of less than 0.5%.

In type B aluminum sulfate, the solids are brown in color and have an iron concentration of less than 1.5%.

There are industries producing aluminum sulfate that have different criteria for its classification. Thus, an industry reports for its preparation of type A aluminum sulfate a maximum concentration of 0.1% of iron as ferric oxide. While for type B they indicate a maximum iron concentration of 0.35%.

Toxicity

-Aluminum sulfate is a compound that exerts its toxic action through contact with its dust, thus causing irritation to the skin, and in cases of frequent contact, a dermatitis.

-It causes strong irritation to the eyes, even causing permanent injuries.

-Its inhalation produces irritation of the nose and throat, which can cause coughing and hoarseness.

-Its ingestion causes gastric irritation, nausea and vomiting.

-There are toxic effects that, although they are not directly due to aluminum sulfate, they are indirectly due to its use. This is the case of certain toxic effects of aluminum, caused by the use of aluminum sulfate in the purification of water.

-Patients who are chronically dialyzed using salts prepared in purified water, through the use of aluminum sulfate, experience very severe health disorders. These disorders include anemia, dialysis dementia, and an increase in bone disease.

References

1. Wikipedia. (2018). Aluminum sulfate. Taken from: en.wikipedia.org
2. Aris Industrial. Aluminum sulfate solution type A and B. Taken from: aris.com.pe
3. Christopher Boyd. (June 9, 2014). Top Industrial Uses for Aluminum Sulfate. Taken from: chemservice.com
4. PubChem. (2018). Aluminum Sulfate Anhydrous. Taken from: pubchem.ncbi.nlm.nih.gov
5. Andesia Chemicals. (August 20, 2009). Aluminum Sulfate Safety Sheet.. Taken from: andesia.com
6. Chemical Book. (2017). Aluminum Sulfate. Taken from: chemicalbook.com