ARSENIC ACID (H3ASO4): PROPERTIES, RISKS AND USES - CHEMISTRY - 2024

The arsenic acid, hydrogen arsenate or ortoarsénico acid, is a chemical compound whose formula is H3AsO4. An arsenic oxacid comprises one oxo group and three hydroxyl groups attached to a central arsenic atom. Its structure is presented in figure 1 (CHEBI: 18231 - arsenic acid, SF).

Its structure is analogous to phosphoric acid (Royal Society of Chemistry, 2015) and can be rewritten as AsO (OH) 3. This compound is prepared by treating arsenic trioxide with nitric oxide according to the reaction: As2O3 + 2HNO3 + 2H2O → 2H3AsO4 + N2O3.

Figure 1: structure of arsenic acid.

The resulting solution is cooled to give colorless crystals of the H3AsO4 · ½H2O hemihydrate, although the H3AsO4 · 2H2O dihydrate occurs when crystallization takes place at lower temperatures (Budavari, 1996).

Arsenic acid is an extremely toxic compound. Many safety data sheets advise avoiding contact if possible.

Physical and chemical properties of arsenic acid

Arsenic acid is a white hygroscopic solid. Its appearance is shown in figure 2.

Figure 2: appearance of arsenic acid.

In aqueous solution, it is a viscous and transparent hygroscopic liquid (National Center for Biotechnology Information., 2017). Its molecular weight is 141.94 g / mol and its density is 2.5 g / ml. Its melting point is 35.5 ° C and its boiling point is 120 ° C where it decomposes.

Arsenic acid is very soluble in water, being able to dissolve 16.7 g per 100 ml, it is also soluble in alcohol. The compound has a pKa of 2.19 for the first deprotonation and 6.94 and 11.5 for the second and third deprotonation (Royal Society of Chemistry, 2015).

Arsenic acid is an oxidizing agent. It can corrode steel and reacts with galvanized metals and brass.

Arsenic acid solutions can develop very toxic gaseous arsine (AsH3) on contact with active metals, such as zinc and aluminum. When heated to decomposition, it produces toxic metallic arsenic vapors.

The solution is slightly acidic and a weak oxidizing agent. It reacts with alkalis to generate some heat and precipitate arsenates (ARSENIC ACID, LIQUID, 2016).

Reactivity and hazards

Arsenic acid is a stable, non-combustible compound that can be corrosive to metals. The compound is highly toxic and a confirmed human carcinogen.

Inhalation, ingestion, or skin contact with material can cause serious injury or death. Contact with the molten substance can cause severe burns to the skin and eyes.

Avoid contact with skin. The effects of contact or inhalation may be delayed. Fire can produce irritating, corrosive and / or toxic gases. Fire control or dilution product wastewater can be corrosive and / or toxic and cause contamination.

Symptoms of arsenic acid poisoning are coughing and shortness of breath if inhaled. There may also be redness on the skin, pain and a burning sensation if it comes into contact with it. Lastly, the symptoms in case of ingestion are redness and pain in the eyes, sore throat, nausea, vomiting, diarrhea and seizures.

In case of contact with the eyes

They should be washed with plenty of water for at least 15 minutes, occasionally lifting the upper and lower eyelids until there is no evidence of chemical remains.

In case of skin contact

Wash immediately with plenty of soap and water for at least 15 minutes while removing contaminated clothing and shoes. Cover burns with a dry sterile bandage (secure, not tight).

In case of ingestion

Rinse mouth and provide conscious victim with large amounts of water to dilute acid. In this case, gastric lavage should be used and vomiting should not be induced.

In case of inhalation

Artificial respiration should be given if necessary. The mouth-to-mouth method should not be used if the victim has ingested or inhaled the substance.

Artificial respiration should be performed with the aid of a pocket mask fitted with a one-way valve or other suitable respiratory medical device. The victim should be moved to a cool place and kept warm and at rest.

In all cases, immediate medical attention should be sought (National Institute for Occupational Safety and Health, 2015).

Arsenic acid is harmful to the environment. The substance is very toxic for aquatic organisms. Steps must be taken to limit the release of this chemical compound.

Applications

Arsenic acid, given its high toxicity, has limited uses. However, this compound was used as a pesticide and soil sterilant, although it is currently obsolete (University of Hertfordshire, 2016).

It has also been used in wood processing and as a desiccant in cotton production since 1995. Spraying the plants makes the leaves dry quickly without falling off. The plant needs to be dry enough so that the cotton bolls can come out easily.

Arsenic acid is used in the production of glass. Although the records consider the substance as an intermediate, this use of arsenic acid seems more like a "processing agent", similar to the use of diarsenic trioxide (As2O3) as a finishing agent.

This compound breaks the oxygen bonds between other elements through the redox reaction and produces gaseous oxygen that helps eliminate bubbles in the glass (Position Paper of the European Glass Industries on the, 2012).

Arsanilic acid or 4-aminophenylarsonic acid is a derivative of orthoarsenic acid. It is used as an arsenical antibacterial veterinary medicine used in the prevention and treatment of swine dysentery (ARSENIC ACID, SF).

Arsenate is the salt or ester of arsenic acid that has a negative ion of AsO43-. Arsenate resembles phosphate in many ways, since arsenic and phosphorus occur in the same group (column) of the periodic table.

Arsenate can replace inorganic phosphate in the glycolysis stage that produces 1,3-bisphosphoglycerate, producing 1-arsenic-3-phosphoglycerate instead. This molecule is unstable and rapidly hydrolyzes, forming the next intermediate down the road, 3-phosphoglycerate.

Thus, glycolysis continues, but the ATP molecule that would be generated from 1,3-bisphosphoglycerate is lost. Arsenate is a glycolysis uncoupler, which explains its toxicity.

Some species of bacteria obtain their energy by oxidizing various fuels while reducing arsenate to form arsenites. The enzymes involved are known as arsenate reductases.

In 2008, bacteria were discovered that employ a version of photosynthesis with arsenites as electron donors, producing arsenate (just as ordinary photosynthesis uses water as an electron donor, producing molecular oxygen).

The researchers conjectured that historically these photosynthetic organisms produced the arsenate that allowed the arsenate-reducing bacteria to thrive (Human Metabolome Database, 2017).

References

1. ARSENIC ACID. (SF). Recovered from chemicalland21.com.
2. ARSENIC ACID, LIQUID. (2016). Recovered from cameochemicals.noaa.gov.
3. Budavari, S. (. (1996). Merck Index - An Encyclopedia of Chemicals, Drugs, and Biologicals. Whitehouse Station, NJ: Merck and Co.
4. CHEBI: 18231 - arsenic acid. (SF). Recovered from ebi.ac.uk.
5. Human Metabolome Database. (2017, March 2). Showing metabocard for Arsenate. Recovered from hmdb.ca.
6. National Center for Biotechnology Information.. (2017, March 4). PubChem Compound Database; CID = 234,. Retrieved from PubChem.
7. National Institute for Occupational Safety and Health. (2015, July 22). ARSENIC ACID. Recovered from cdc.gov.
8. Position Paper of the European Glass Industries on the. (2012, September 18). Recovered from glassallianceeurope.
9. Royal Society of Chemistry. (2015). Arsenic acid. Recovered from chemspider.
10. Royal Society of Chemistry. (2015). Phosphoric acid. Recovered from chemspider.
11. University of Hertfordshire. (2016, January 13). arsenic acid. Recovered from PPDB.