1) Poisonous compounds 
<chemistry> Inorganic or organic salts and esters of arsenic acid. ... (12 Dec 1998) ... 
Group of minerals that are compounds of one or more metallic elements associated with the arsenate radical (AsO4). The arsenates, together with the related vanadates, are classified in the phosphate group. Most arsenates are heavy, and none are hard. They are usually brittle and occur in small crystals or compact aggregates.
A group of inorganic insecticides that are fairly stable, but hydrolyse in water releasing arsenic acid.