Tribasic barium arsenite, Ba3(AsO3)2, has been
prepared by the addition of potassium orthoarsenite to
an aqueous solution of barium chloride and drying the
precipitate at 100 C in a current of hydrogen gas. The
pH needs to at least 13.5. It is obtained as a white amorphous
powder, readily soluble in hot water and in dilute acids. When heated rapidly to about 730 °C in an inert
atmosphere, it decomposed into arsenic and the arsenate.
This is a double oxidation–reduction reaction in which
part of the tribasic arsenite is oxidized and part reduced:
4Ba3（AsO3）2 + heat ? 12BaO + 4As2O3 + 4As
4As2O302As2O5 + 4As + 2O2
4Ba3（AsO3）2 + heat ? 12BaO + 2As2O5 + 8As + 2O2
Slow heating below 300 °C causes loss of arsenious
oxide if heated in hydrogen or arsenic pentoxide if
heated in air:
Ba3(AsO3)2 + Heat (H2) ? 3BaO + As2O3
