What Is Arsenic Acid?
Arsenic acid is an inorganic compound that is an oxoacid of arsenic.
It has the chemical formula H3AsO4 and is also known as orthoarsenic acid.
The formula is 141.943, the melting point is 35.5℃ (1/2 hydrate), the melting point is 160℃, and it is colorless crystal (1/2 hydrate) at room temperature. It is soluble in water with a density of 2.5 gcm-3 and a solubility in water of 96.2 g/100 g (at 20°C). Its acid dissociation constants, pKa, are 2.24, 6.96, and 11.50.
Arsenic acid and arsenate are designated as nonmedicinal toxicants.
Uses of Arsenic Acid
Arsenic acid is mainly used as an insecticide, rat poison, in the organic dye industry, and as a raw material for organic and inorganic arsenic preparations. In particular, arsenic acid has been used as an insecticide for termite control. However, due to its high toxicity, environmental pollution, and health hazards, it is currently not allowed to be used in Japan.
Arsenic acid has been isolated as a colorless crystal of 1/2 hydrate, which is hygroscopic and extremely soluble in water. Because of its high tidal solubility, it is often used in aqueous solution or hydrate form, or as arsenic acid.
Characteristics of Arsenic Acid
Arsenic acid anhydride and 0.5 hydrate are hygroscopic and easily deliquescent. The anhydride crystals are said to be slightly endothermically soluble in water. In arsenic acid, the oxidation number of the arsenic atom is +V (+5), the highest oxidation state, and the substance exhibits the properties of a trivalent acid.
In this respect, it is similar to phosphoric acid. Since it is tidally soluble and soluble in water, it is important to keep it tightly sealed during storage.
An aqueous solution of arsenic acid is a weak acid that undergoes three stages of electrolysis. First-step ionization produces dihydrogen arsenate (H2AsO4-), second-step dissociation produces hydrogen arsenate (HAsO42-), and third-step dissociation produces arsenic acid ion (AsO43-). The arsenic acid ion has a tetrahedral structure, similar to that of phosphate.
The first stage of electrolysis dissociates rather strongly, with an ionization degree of about 0.25 in a 0.1 mol dm-3 aqueous solution; the second and subsequent stages decrease sequentially, and the dissociation in acidic solutions is considered negligible.
Types of Arsenic Acid
Arsenic acid itself is highly deliquescent and difficult to handle, so it is basically not sold as a product in anhydrous form. It is sold as arsenic acid solution at a concentration of about 60%, and in solid form as salt. Basically, it is sold as a chemical for research and development and industrial use.
The most common arsenic acid salt is disodium hydrogen arsenate heptahydrate (Na2HAsO4-7H2O). This substance is a white crystal or crystalline powder at room temperature, soluble in water and virtually insoluble in ethanol. It is handled as a reagent product that can be stored at room temperature and has applications, such as a detection reagent for alkaloids.
Other Information on Arsenic Acid
1. Manufacturing Process of Arsenic Acid
Arsenic acid is produced by the oxidation of simple arsenic or diarsenic trioxide with concentrated nitric acid. When this solution is concentrated, fine plate-like crystals of 0.5 hydrate precipitate below 29.5°C, and above 29.5°C, arsenic trioxide (H5As3O10) precipitates.
Another method is obtained by dissolving diarsenic pentaoxide in water.
2. Chemical Reactions of Arsenic Acid
Aqueous solutions of arsenic acid exhibit weak oxidative activity, e.g., oxidizing iodide ions to iodine. Arsenic acid is also a substance that is desorbed by gentle heating. Triarsenic acid (H5As3O10) is formed, and further heating yields diarsenic pentaoxide from 250 °C, but complete dehydration requires temperatures above 500 °C.
Diarsenic acid (H4As2O7) or polyarsenic acid (Hn+2AsnO3n+1, (HAsO3)n) and its ions are unstable in aqueous solution and are quickly hydrolyzed to form arsenic acid or arsenate ion.