Sulfur dioxide (SO2) structure, properties, uses, risks

The sulfur dioxide is a gaseous inorganic compound formed by sulfur (S) and oxygen (O), its chemical formula being SO_two. It is a colorless gas with an irritating and suffocating odor. In addition, it is soluble in water, forming acid solutions. Volcanoes expel it into the atmosphere during eruptions.

It is part of the biological and geochemical cycle of sulfur, but it is produced in large quantities by certain human activities such as oil refining and the burning of fossil fuels (coal or diesel for example).

The SO_two it is a reducing agent that allows the paper pulp to remain white after bleaching with other compounds. It also serves to remove traces of chlorine in water that has been treated with this chemical.

It is used to preserve some types of food, to disinfect containers where the fermentation of grape juice is produced to produce wine or barley to make beer..

It is also used as a fungicide in agriculture, to obtain sulfuric acid, as a solvent and as an intermediate in chemical reactions..

The SO_two present in the atmosphere is harmful to many plants, in water it affects fish and is also one of those responsible for "acid rain" which corrodes materials created by humans.

Article index

• 1 Structure
• 2 Nomenclature
• 3 Properties
  • 3.1 Physical state
  • 3.2 Molecular weight
  • 3.3 Melting point
  • 3.4 Boiling point
  • 3.5 Density
  • 3.6 Solubility
  • 3.7 pH
  • 3.8 Chemical properties
  • 3.9 Aqueous solutions of SO2
  • 3.10 Other properties
• 4 Obtaining
• 5 Presence in nature
• 6 Uses
  • 6.1 In the production of sulfuric acid
  • 6.2 In the processed food industry
  • 6.3 As solvent and reagent
  • 6.4 As a reducing agent
  • 6.5 In various applications
• 7 Effects of endogenous SO2
• 8 Risks
  • 8.1 Ecotoxicity
  • 8.2 Effects of ingesting it with food
• 9 References

Structure

The sulfur dioxide molecule is symmetrical and forms an angle. The angle is due to the SW_two It has a pair of solitary electrons, that is, electrons that do not form a bond with any atom but are free.

Nomenclature

- Sulfur dioxide

- Sulfur anhydride

- Sulfurous oxide.

Properties

Physical state

Colorless gas.

Molecular weight

64.07 g / mol

Melting point

-75.5 ºC

Boiling point

-10.05 ºC

Density

Gas: 2.26 at 0 ° C (relative to air, that is, air density = 1). This means that it is heavier than air..

Liquid: 1.4 to -10 ° C (relative to water, i.e. density of water = 1).

Solubility

Soluble in water: 17.7% at 0 ° C; 11.9% at 15 ° C; 8.5% at 25 ° C; 6.4% at 35 ° C.

Soluble in ethanol, diethyl ether, acetone, and chloroform. It is less soluble in non-polar solvents.

pH

Aqueous solutions of SO_two they are acidic.

Chemical properties

The SO_two it is a powerful reducing and oxidizing agent. In the presence of air and a catalyst it oxidizes to SO₃.

SW_two + OR_two → SO₃

The lone electron pairs make it sometimes behave like a Lewis base, in other words, it can react with compounds where there is an atom that is missing electrons.

If the OS_two it is in the form of a gas and dry does not attack iron, steel, copper-nickel, or nickel-chromium-iron alloys. However, if it is in a liquid or wet state, it causes corrosion to these metals..

The SO_two liquid with 0.2% water or more produces strong corrosion to iron, brass and copper. It is corrosive to aluminum.

When it is liquid it can also attack some plastics, rubbers and coatings.

Aqueous SO solutions_two

The SO_two it is very soluble in water. It was long considered that in water it forms sulfurous acid H_twoSW₃, but the existence of this acid has not been proven.

In OS solutions_two in water the following equilibria occur:

SW_two + H_twoO ⇔ SO_two.H_twoOR

SW_two.H_twoO ⇔ HSO₃^- + H₃OR⁺

HSO₃^- + H_twoO ⇔ SO₃^two- + H₃OR⁺

Where HSO₃^- is the bisulfite ion and SO₃^two- is the sulfite ion. The sulfite ion SO₃^two- occurs mainly when an alkali is added to the SO solution_two.

Aqueous solutions of SO_two possess reducing properties, especially if they are alkaline.

Other properties

- It is extremely heat stable, even up to 2000 ° C.

- It is not flammable.

Obtaining

The SO_two It is obtained by combustion of sulfur (S) in the air, although small amounts of SO are also formed₃.

S + O_two → SO_two

It can also be produced by heating various sulfides in the air, burning pyrite minerals and minerals containing sulfides, among others..

In the case of iron pyrite, when oxidized, iron oxide (iii) and SO are obtained._two:

4 FeS_two + 11 O_two → 2 Fe_twoOR₃ + 8 SO_two↑

Presence in nature

The SO_two It is released into the atmosphere by the activity of volcanoes (9%) but it is also caused by other natural activities (15%) and by human actions (76%).

Explosive volcanic eruptions cause significant annual fluctuations or variations in SO_two in the atmosphere. It is estimated that 25% of the SO_two emitted by volcanoes is washed away by rain before reaching the stratosphere.

Natural sources are the most abundant and are due to the biological cycle of sulfur.

In urban and industrial areas, human sources predominate. The main human activity that produces it is the burning of fossil fuels, such as coal, gasoline and diesel. Other human sources are oil refineries, chemical plants and gas production.

In mammals it is generated endogenously, that is, within the body of animals and humans due to the metabolism of sulfur-containing amino acids (S), especially L-cysteine.

Applications

In the production of sulfuric acid

One of the most important applications of the OS_two is in obtaining sulfuric acid H_twoSW₄.

2 SO_two + 2 h_twoO + O_two → 2 H_twoSW₄

In the processed food industry

Sulfur dioxide is used as a food preservative and stabilizer, as a moisture control agent, and as a flavor and texture modifier for certain edible products..

It is also used to disinfect equipment that comes into contact with foodstuffs, fermentation equipment, such as those in breweries and wineries, food containers, etc..

It allows to preserve fruits and vegetables, increases their shelf life, prevents loss of color and flavor and helps in the retention of vitamin C (ascorbic acid) and carotenes (precursors of vitamin A).

It is used to preserve wine, as it destroys bacteria, fungi and unwanted yeasts. It is also used to sterilize and prevent the formation of nitrosamines in beer..

It is also used to soak corn kernels, to whiten beet sugar, and as an antimicrobial in the manufacture of high fructose corn syrup..

As a solvent and reagent

It has been widely used as a nonaqueous solvent. Although it is not an ionizing solvent, it is useful as a proton-free solvent for certain analytical applications and chemical reactions..

It is used as a solvent and reagent in organic synthesis, an intermediate in the production of other compounds such as chlorine dioxide, acetyl chloride and in the sulfonation of oils..

As a reducing agent

It is used as a reducing agent despite not being so strong, and in alkaline solution the sulfite ion is formed, which is a more energetic reducing agent..

In various applications

The SO_two is also used:

- In agriculture as a fungicide and preservative for grapes after harvest.

- To make hydrosulfites.

- To bleach wood pulp and paper, as it allows to stabilize the pulp after bleaching with hydrogen peroxide H_twoOR_two; the SO_two acts by destroying the H_twoOR_two remainder and thus maintain the brightness of the pulp, since the H_twoOR_two can cause a reversal of brightness.

- For whitening textile fibers and wicker articles.

- To treat water as it eliminates the residual chlorine that remains after the chlorination of drinking water, wastewater or industrial water.

- In the refining of minerals and metals, as a reducing agent for iron during mineral processing.

- In petroleum refining to trap oxygen and retard corrosion, and as an extraction solvent.

- As an antioxidant.

- As an alkali neutralizer in glass manufacturing.

- In lithium batteries as an oxidizing agent.

Effects of the OS_two endogenous

Certain studies have revealed that SO_two endogenous or produced by the body itself has a beneficial effect on the cardiovascular system, including the regulation of heart function and the relaxation of blood vessels.

When SO is produced in the body_two is converted to its bisulfite derivatives HSO₃^- and sulfite SO₃^two-, which exert a vasorelaxing effect on the arteries.

The SO_two endogenously lowers hypertension, prevents the development of atherosclerosis, and protects the heart from myocardial damage. It also has an antioxidant action, inhibits inflammation and apoptosis (programmed cell death).

For these reasons, it is thought that it may be a possible new therapy for cardiovascular diseases..

Risks

- Exposure to SO_two gaseous can lead to burns to eyes, skin, throat and mucous membranes, damage to bronchial tubes and lungs.

- Some studies report that it has a potential risk of damage to the genetic material of mammalian and human cells.

- It is corrosive. It is not flammable.

Ecotoxicity

Sulfur dioxide is the most common pollutant gas in the atmosphere, especially in urban and industrial areas..

Its presence in the atmosphere contributes to the so-called “acid rain” that is harmful to aquatic organisms, fish, terrestrial vegetation and corrosion to human-made materials..

The SO_two it is toxic to fish. Green plants are extremely sensitive to SO_two atmospheric. Alfalfa, cotton, barley and wheat are damaged at low environmental levels, while potatoes, onions and corn are much more resistant.

Effects of ingesting it with food

Although it is harmless to healthy people, when used in the concentrations recommended by authorized health agencies, SO_two can induce asthma in sensitive people who ingest it with food.

The foods that usually contain it are dried fruits, artificial soft drinks and alcoholic beverages.

References

1. U.S. National Library of Medicine. (2019). Sulfur dioxide. Recovered from pubchem.ncbi.nlm.nih.gov.
2. Huang, Y. et al. (2016). Endogenous Sulfur Dioxide: A New Member of Gasotransmitter Family in the Cardiovascular System. Oxid Med Cell Longev. 2016; 2016: 8961951. Recovered from ncbi.nlm.nih.gov.
3. Cotton, F. Albert and Wilkinson, Geoffrey. (1980). Advanced Inorganic Chemistry. Fourth Edition. John Wiley & Sons.
4. Windholz, M. et al. (editors) (1983). The Merck Index. An Encyclopedia of Chemicals, Drugs, and Biologicals. Tenth Edition. Merck & CO., Inc.
5. Pan, X. (2011). Sulfur Oxides: Sources, Exposures and Health Effects. Health Effects of Sulfur Oxides. In Encyclopedia of Environmental Health. Recovered from sciencedirect.com.
6. Tricker, R. and Tricker, S. (1999). Pollutants and contaminants. Sulfur dioxide. In Environmental Requirements for Electromechanical and Electronic Equipment. Recovered from sciencedirect.com.
7. Bleam, W. (2017). Acid-Base Chemistry. Sulfur Oxides. In Soil and Environmental Chemistry (Second Edition). Recovered from sciencedirect.com.
8. Freedman, B.J. (1980). Sulfur dioxide in foods and beverages: its use as a preservative and its effect on asthma. Br J Dis Chest. 1980; 14 (2): 128-34. Recovered from ncbi.nlm.nih.gov.
9. Craig, K. (2018). A Review of the Chemistry, Pesticide Use, and Environmental Fate of Sulfur Dioxide, as Used in California. In Reviews of Environmental Contamination and Toxicology. Volume 246. Recovered from link.springer.com.