ARSENOUS ACID (H3ASO3): PROPERTIES, RISKS AND USES - CHEMISTRY - 2024

The arsenious acid is an inorganic compound of formula H3AsO3. Its structure is analogous to trihydroxyamine and can be rewritten as As (OH) 3. It is in aqueous form and has not been isolated in a pure solid state. Its structure is presented in figure 1.

The manufacture of As (OH) 3 entails a slow hydrolysis of arsenic trioxide in water. The sum of the base converts the arsenic acid into the arsenite ions: -, 2- and 3-.

Figure 1: structure of arsenous acid.

It is a weak acid. The reactions attributed to aqueous arsenic trioxide are due to arsenous acid and its conjugate bases.

Physical and chemical properties of arsenious acid

Arsenous acid is a pyramidal molecule consisting of three hydroxyl groups attached to arsenic. The 1H NMR spectrum of arsenous acid solutions consists of a single signal consistent with the high symmetry of the molecule. It only exists in aqueous solution.

Said solution is colorless and has no characteristic aroma. It is stored in amber containers to avoid reaction with UV radiation (National Center for Biotechnology Information, 2017).

Figure 2: bottle containing arsenic acid.

Its molecular weight is 125.94 g / mol. Although the compound has not been isolated, its properties have been calculated using computational methods (© Royal Society of Chemistry, 2015) obtaining that it has a melting point of 271.52 ° C, a boiling point of 626.14 ° C and a solubility in water of 1 x 106 mg / l at 25 ° C.

The compound has a pKa of 9.2 for the reaction:

H3AsO3 ⇌ H2AsO3- + H +

The literature reports that the compound has an amphoteric character, although it dissociates as a base to a lesser degree than it dissociates as an acid, with the base having a pKb of 14 for the reaction:

As (OH) 3 ⇌ As (OH) 2+ + OH-

This leads to the formation of species such as acidic arsenic hydroxide sulfates (As (OH) (HSO4) +, As (OH) 2 (HSO4)) in concentrated solutions of sulfuric acid or to the formation of acid arsenic sulfate (As (HSO4) 3) in fuming sulfuric acid solutions.

Arsenic acid can act as an oxidizing agent or a reducing agent, remaining as elemental arsenic or arsenic acid respectively according to the half-reactions:

H3AsO3 + 3H + + 3e- → As + 3 H2O (ξ0 = +0.240 V)

H3AsO3 + H2O → H3AsO4 + 2H + + 2e- (ξ0 = -0.560 V)

Arsenious acid is a stronger oxidizing agent than its phosphonic counterpart, but a weaker reducing agent than it (Egon Wiberg, 2001).

Reactivity and hazards

Arsenous acid is a compound classified as stable, however it is a toxic and corrosive reagent.

The compound has a negligible fire and explosion hazard when exposed to heat or flame. In such a case, the container should be moved if possible, avoid breathing vapors or dust.

When heated, the compound releases toxic and corrosive arsenic oxide fumes. Self-contained breathing apparatus and protective clothing should be worn to avoid contact in case of fire.

It can be irritating to the eyes, skin and mucosa. May cause sensitization. Poisoning can affect the central nervous, gastrointestinal, and cardiovascular systems, liver, and kidneys. Compounds containing arsenic are highly toxic and carcinogenic.

In case of contact with the eyes

Wash with plenty of water for at least 15 minutes, occasionally lifting the eyelids until there is no evidence of chemical residue.

In case of skin contact

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

In case of ingestion

The conscious victim should be given large amounts of water to dilute the acid. Do not use gastric lavage or induce vomiting. Medical personnel must maintain airway and breathing.

In case of inhalation

Artificial respiration should be given if necessary. Move victim to cool place and keep warm and at rest.

In all cases, immediate medical attention should be sought (Material Safety Data Sheet Arsenious acid solution, 2007).

Applications

Arsenous acid is used to detect small amounts of iodine or iodine compounds. In the work of (Weegh, 1972) some aspects of the ceric reaction of arsenic acid were studied, paying attention to the optical properties of ceric solutions.

The kinetic effects of various compounds were also studied, especially the effects of chloride and bromide ions and the kinetic effects of chloride or bromide in combination with iodide.

Arsenous acid can also be used illicitly to cause abortions (Trend, 1858).

Arsenious acid reacts when heated to form arsenic trioxide (As2O3). This compound is also commonly known by the name of arsenous acid despite not being the same compound. The structure of arsenic trioxide is presented in Figure 3.

Figure 3: structure of arsenic trioxide.

Figure 1: Structure of arsenic trioxide.

This compound, also known as Trisenox, is used to treat leukemia patients who have not responded to other agents. This drug is approved for use by the US Food and Drug Administration (FDA) as a treatment for leukemia.

How the compound works is unclear. It can cause morphological changes and DNA fragmentation in promyelocytic leukemia cells, causing cell death and degradation or damage of PML / RAR alpha (a fusion protein) (Nurse's Drug Handbook 7th Ed, 2013).

Being poisonous to the human body, it is one of the highly debatable drugs in the field. Arsenic trioxide was first used as a traditional Chinese medicine called Pi Shuang. It is still used today to treat cancer patients and other health conditions. (Arsenious acid review, 2007-2016).

References

1. © Royal Society of Chemistry. (2015). Arsenous acid. Recovered from chemspider.
2. © Royal Society of Chemistry. (2015). Trihydroxyamine. Recovered from chemspider.
3. Arsenious acid review. (2007-2016). Recovered from medicalook.
4. Arsenous acid. (2014, July 28). Recovered from ebi.ac.uk.
5. Egon Wiberg, NW (2001). Inorganic Chemistry. Berlin: academic press.
6. Material Safety Data Sheet Arsenious acid solution. (2007, September 10). Recovered from t3db.ca.
7. National Center for Biotechnology Information. (2017, March 4). PubChem Compound Database; CID = 545,. Retrieved from PubChem.
8. Nurse's Drug Handbook 7th Ed. (2013). McGraw-Hill.
9. Trend, HG (1858). A Case in Which Arsenious Acid was Used to Procure Abortion. British medical journal original comunications, 725-726.
10. Weegh, WH (1972). The use of the ceric arsenious acid reaction for the determination of small amounts of iodine or iodine compounds. Clinica Chimica Acta Volume 39, Issue 2, 327-338.