Miscellaneous Compounds

Abstract

Arsenic occurs in two valence forms: trivalent (As₂O₃) reacting with water to give arsenious acid and forming arsenite salts, and pentavalent arsenic (As₂O₅) yielding arsenic acid and the corresponding arsenate salts. The arsenite compounds are more toxic to insects, mammals, and plants. Paris green (a complex between copper acetate and copper arsenite) was used in the USA since 1867. It is quite toxic to plants and animals; therefore, its use is limited to insect poison baits. Lead and calcium salts of arsenic have been the commonest arsenicals in use. Acid lead arsenate (PbHAsO₄) and basic lead arsenate [Pb₂(AsO₄)₂] are no longer used in the USA, but they still find some use in other countries for control of various beetles, weevils, leaf rollers, codling moths, and other insects damaging fruit trees. Several organoarsenicals are used as herbicides. In view of the high mammalian toxicity of lead, preparations of calcium arsenate are often preferred. Environmental pollution by lead presents a serious problem since lead is a cumulative poison.