Chemistry of Arsenic

To comprehend the activity, toxicology and metabolism of arsenic, it is essential to understand its chemistry. Arsenic (atomic number 33, atomic weight 75) is a metalloid of the nitrogen group of the periodic table. It can exist in three different valency states, including elemental (zero oxidation), trivalent (Asra) and penta-valent (AsV). Arsenic forms alloys with metals and also readily reacts with carbon, oxygen and hydrogen, forming covalent bonds.1 Trivalent arsenicals are more toxic than the pentavalent ones. The toxicity of an arsenical also varies with the physical state of the compound, and the rate of absorption and elimination. The pentavalent arsenic compounds include derivatives of the arsenic acid, H3AsO4 (the arsenates). The trivalent inorganic arsenic compounds are derivatives of the arsenious acid, H2As2O3 (the arsenites) and arsenic trioxide (As2O3). The organic arsenicals that contain (a) covalent bond(s) between arsenic and carbon are often formed from inorganic arsenic compounds through metabolism. They are generally excreted more rapidly from the body than the inorganic compounds.1 The chemical structures of the trivalent and pentavalent inorganic arsenicals and the methylated metabolites are shown in Table 16.1.

Table 16.1 Inorganic Asm and AsV compounds and the methylated metabolites

Chemicals with symbolic name Structure pKa iAs111 (arsenite)

13.5

iAsV (arsenate)

13.5

DMA (dimethylarsinic acid)

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