He Arsenous acid Is an inorganic compound of formula H3AsO3. Its structure is analogous to trihydroxyamine and can be rewritten as As (OH) 3.
The compound is in aqueous form and has not been isolated in the pure solid state. Its structure is presented in figure 1 (arsenous acid, 2014).
Figure 1: structure of arsenous acid.
The preparation of As (OH) 3 involves slow hydrolysis of arsenic trioxide in water. The sum of the base converts the arsenious acid into the arsenite ions: [AsO (OH) 2] -, [AsO2 (OH)] 2- and [AsO3] 3-.
As (OH) 3 is a weak acid. The reactions attributed to aqueous arsenic trioxide are due to arsenic acid and its conjugated bases.
Physical and chemical properties of arsenous acid
Arsenious acid is a pyramidal molecule consisting of three hydroxyl groups attached to arsenic. The 1H NMR spectrum of arsenious 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 arsenious 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 a melting point of 271.52 ° C, a boiling point of 626.14 ° C and a solubility in water of 1 x 106 mg / at 25 ° C.
The compound has a pKa of 9.2 for the reaction:
H 3 AsO 3 ⇌ H 2 AsO 3 - + 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 acid, 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 arsenic hydroxide sulfates (As (OH) (HSO4) +, As (OH) 2 (HSO4)) in concentrated solutions of sulfuric acid or the formation of arsenic sulfate HSO4) 3) in solutions of fuming sulfuric acid.
Arsenious acid can act as oxidizing agent or reducing agent leaving as elemental arsenic or arsenic acid respectively according to the half-reactions:
H 3 AsO 3 + 3H + + 3e- → As + 3 H 2 O (ξ 0 = +0.240 V)
H 3 AsO 3 + H 2 O → H 3 AsO 4 + 2H + + 2e- (ξ0 = -0.560 V)
Arsenious acid is a stronger oxidizing agent than its phosphonic counterpart, but a weaker reducing agent than this (Egon Wiberg, 2001).
Reactivity and hazards
Arsenious acid is a compound classified as stable, however it is a toxic and corrosive reagent.
The compound has an insignificant risk of fire and explosion when exposed to heat or flame. In such case, the container should be moved if possible, avoid breathing vapors or dust.
When heated, the compound releases toxic and corrosive vapors of arsenic oxide. Self-contained breathing apparatus and protective clothing should be worn to prevent contact in case of fire.
May be irritating to eyes, skin and mucous membranes. May cause sensitization. Poisoning can affect the central nervous system, gastrointestinal and cardiovascular system, liver and kidneys. Arsenic-containing compounds are highly toxic and carcinogenic.
In case of contact with eyes
Wash with plenty of water for at least 15 minutes, occasionally lifting the eyelids until there is no evidence of chemical debris.
In case of contact with skin
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 dressing (secure, not tight).
In case of ingestion
The conscious victim must be given large amounts of water to dilute the acid. Do not use gastric lavage or induce vomiting. Medical personnel should maintain the airway and breathing.
In case of inhalation:
Give artificial respiration if necessary. Move the victim to a cool place and keep him warm and at rest.
In all cases seek immediate medical attention (Material Safety Data Sheet Arsenious acid solution, 2007).
Applications
Arsenious acid is used to detect small amounts of iodine or iodine compounds. In the work of (Weegh, 1972) some aspects of the arsenious acid reaction were studied, paying attention to the optical properties of the ceric solutions.
The kinetic effects of various compounds, especially the effects of chloride and bromide ions and the kinetic effects of chloride or bromide in combination with iodide, were also studied.
Arsenious acid can also be used illicitly to produce abortions (Trend, 1858).
Arsenious acid reacts on heating to form arsenic trioxide (As2O3). This compound is also commonly known by the name arsenious acid although it is not the same compound. The structure of arsenic trioxide is shown 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 patients with leukemia who have not responded to other agents. This drug was approved for use by the US Food and Drug Administration (FDA) as a treatment for leukemia.
The way the compound acts 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
- © Royal Society of Chemistry. (2015). Arsenous acid. Recovered from chemspider.
- © Royal Society of Chemistry. (2015). Trihydroxyamine. Recovered from chemspider.
- Arsenious acid review. (2007-2016). Retrieved from medicalook.
- Arsenous acid. (2014, July 28). Retrieved from ebi.ac.uk.
- Egon Wiberg, N.W. (2001). Inorganic Chemistry. Berlin: academic press.
- Material Safety Data Sheet Arsenious acid solution. (2007, September 10). Retrieved from t3db.ca.
- National Center for Biotechnology Information. (2017, March 4). PubChem Compound Database; CID = 545. Recovered from PubChem.
- Nurse's Drug Handbook 7th Ed. (2013). McGraw-Hill.
- Trend, H. G. (1858). The Case in Which Arsenious Acid was Used to Search Abortion. British medical journal original comunications, 725-726.
- Weegh, W. H. (1972). The use of the ceramic arsenious acid reaction for the determination of small amounts of iodine or iodine compounds. Clinica Chimica Acta Volume 39, Issue 2, 327-338.