All inorganic arsenic compounds are highly toxic and thus find only limited commercial applications.

[4] Pierre Macquer found a crystallizable salt which he called ‘sel neutre arsenical’.

This salt was the residue obtained after distilling nitric acid from a mixture of potassium nitrate and arsenic trioxide.

This reaction is reversible:[3] Strong oxidizing agents such as ozone, hydrogen peroxide, and nitric acid convert arsenic trioxide to the pentoxide.

Its reduced derivative arsenite, which is an As(III) compound, is even more toxic since it has a high affinity for thiol groups of cysteine residues in proteins.