Arsenous acid
Encyclopedia
Arsenous acid, also known as arsenious acid, is the inorganic compound
with the formula H3AsO3. It is known to occur in aqueous solution
s, but it has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)3.
groups bonded to arsenic. The 1H NMR
spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry. In contrast, the nominally related phosphorus
species H3PO3 mainly adopts the structure HPO(OH)2; P(OH)3 is a very minor equilibrium component of such solutions. The differing behaviors of the As and P compounds reflect a trend whereby high oxidation states are more stable for lighter members of main group elements than their heavier congeners.
of arsenic trioxide
in water. Addition of base
converts arsenous acid to the arsenite ions
[AsO(OH)2]-, [AsO2(OH)]2-, and [AsO3]3-. The first pKa is 9.2. Reactions attributed to aqueous arsenic trioxide
are due to arsenous acid and its conjugate bases.
, is used as a herbicide
, pesticide
, and rodenticide.
Inorganic compound
Inorganic compounds have traditionally been considered to be of inanimate, non-biological origin. In contrast, organic compounds have an explicit biological origin. However, over the past century, the classification of inorganic vs organic compounds has become less important to scientists,...
with the formula H3AsO3. It is known to occur in aqueous solution
Solution
In chemistry, a solution is a homogeneous mixture composed of only one phase. In such a mixture, a solute is dissolved in another substance, known as a solvent. The solvent does the dissolving.- Types of solutions :...
s, but it has not been isolated as a pure material, although this fact does not detract from the significance of As(OH)3.
Properties
As(OH)3 is a pyramidal molecule consisting of three hydroxylHydroxide
Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and a hydrogen atom held together by a covalent bond, and carrying a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, as a ligand, a nucleophile, and a...
groups bonded to arsenic. The 1H NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
spectrum of arsenous acid solutions consists of a single signal consistent with the molecule's high symmetry. In contrast, the nominally related phosphorus
Phosphorus
Phosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
species H3PO3 mainly adopts the structure HPO(OH)2; P(OH)3 is a very minor equilibrium component of such solutions. The differing behaviors of the As and P compounds reflect a trend whereby high oxidation states are more stable for lighter members of main group elements than their heavier congeners.
Reactions
The preparation of As(OH)3 involves a slow hydrolysisHydrolysis
Hydrolysis is a chemical reaction during which molecules of water are split into hydrogen cations and hydroxide anions in the process of a chemical mechanism. It is the type of reaction that is used to break down certain polymers, especially those made by condensation polymerization...
of arsenic trioxide
Arsenic trioxide
Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...
in water. Addition of base
Base (chemistry)
For the term in genetics, see base A base in chemistry is a substance that can accept hydrogen ions or more generally, donate electron pairs. A soluble base is referred to as an alkali if it contains and releases hydroxide ions quantitatively...
converts arsenous acid to the arsenite ions
Arsenite
In chemistry an arsenite is a chemical compound containing an arsenic oxoanion where arsenic has oxidation state +3.The different forms of the anion are the next ones:* ortho-arsenite: AsO33-* meta-arsenite: AsO2-...
[AsO(OH)2]-, [AsO2(OH)]2-, and [AsO3]3-. The first pKa is 9.2. Reactions attributed to aqueous arsenic trioxide
Arsenic trioxide
Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...
are due to arsenous acid and its conjugate bases.
Toxicology
Arsenic containing compounds are highly toxic and carcinogenic. The anhydride form of arsenous acid, arsenic trioxideArsenic trioxide
Arsenic trioxide is the inorganic compound with the formula As2O3. This commercially important oxide of arsenic is the main precursor to other arsenic compounds, including organoarsenic compounds. Approximately 50,000 tonnes are produced annually...
, is used as a herbicide
Herbicide
Herbicides, also commonly known as weedkillers, are pesticides used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant...
, pesticide
Pesticide
Pesticides are substances or mixture of substances intended for preventing, destroying, repelling or mitigating any pest.A pesticide may be a chemical unicycle, biological agent , antimicrobial, disinfectant or device used against any pest...
, and rodenticide.