Arsenic Acid
Arsenic Acid Formula- Arsenic acid is a chemical compound that has a formula H3AsO4. Descriptively, we write it as AsO(OH)3. Also, it is a colorless acid that is the arsenic analog of phosphoric acid. Besides, the phosphate and arsenate salts behave very correspondingly. Such arsenic acid has not been isolated and is only found in a solution where it is largely ionized. Furthermore, its hemihydrate form H3AsO4·1/2H2O does form stable crystals. On condensation, at 100 oC the crystalline samples dehydrate.
Arsenic Acid Formula and Structure
Its chemical formula is H3AsO4and the molecular formula is AsO(OH)3. Arsenic has 5 electrons, which it shares with 4 oxygen atoms. Besides, these 3 oxygen atoms complete their valance shell electron by sharing the electron with 3 hydrogen atoms. That gives arsenic, 4 oxygen, and 3 hydrogen atoms. The component is canonicalized (is a set of representations of the resonance structure of a molecule each of which contributes to the real structure).
Properties of arsenic acid
It has an idealized symmetry of tetrahedral species C3v with AS-O bond lengths ranging from 1.66 to 1.71 Å. While being a tricrotic acid, we can describe its acidity by three equilibria:
H3AsO4+ H2O ⇌H2AsO−
4 + H3O+ (K1= 10−2.19)
H2AsO−
4 + H2O ⇌HAsO2−
4 + H3O+ (K2= 10−6.94)
HAsO2−
4 + H2O ⇌AsO3−
4 + H3O+ (K3= 10−11.5)
Also, these Ka values are close to those for phosphoric acid. Furthermore, the highly basic arsenate ion (AsO3-4) is the product of the third ionization. Contrary to phosphoric acid, arsenic acid is an oxidizer, as scientists demonstrate it by its ability to convert iodide into iodine.
Preparation of arsenic acid
We can prepare arsenic acid by treating arsenic trioxide with concentrated nitric acid. As a byproduct, it produces dinitrogen trioxide.
As2O3+ 2 HNO3+ 2 H2O → 2 H3AsO4+ N2O3
After that, the resulting solution is cool down to give colorless crystal of the hemihydrate H3AsO4·1⁄2H2O. On the other hand, the dihydrate H3AsO4·2H2O produces when the crystallization occurs at lower temperatures.
Application of arsenic acid
Its commercial uses are limited due to its toxicity. Also, it is a precursor to a range of pesticides. Occasionally, we find its uses as a broad-spectrum biocide, as a wood preservative, a finishing agent for glass and metal, and a reagent in the synthesis of some dyestuffs and organic arsenic compounds.
Arsenic acid safety and health hazards
Like all arsenic compounds, it is tremendously carcinogenic and toxic and corrosive. Its contact with skin can cause irritation, itching, rash, and burning. If it comes in contact with the eyes then it causes irritation, redness, and watery eyes. Breathing it irritates the throat and nose. Furthermore, it can cause poor appetite, nausea, headache, diarrhea, stomach cramps, and vomiting.
Uses of arsenic acid
Historically, the agriculture industry uses arsenic acid in a variety of ways that include pesticides, insecticides, herbicides, cotton desiccants, soil sterilants, and defoliants, etc. In clothes, we use it as an anti-microbial. Also, the automobile industry uses them in manufacturing. We use it as a raw material in a variety of industries and products such as cosmetics, chemical manufacturing, production of metals, etc. Used as an agrochemical crop for growing. It is also an agent in glass manufacturing.
Solved example for you
Question: Mention a method to prepare arsenic acid?
Solution: We can directly prepare arsenic acid from its elemental arsenic form by moistening and treating it with ozone (O3) and water (3 H2O):
2 As + 3 H2O + 5 O3 → 2 H3AsO4 + 5 O2
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