Nickel Orthoarsenate, Ni₃(AsO₄)₂

Nickel Orthoarsenate, Ni₃(AsO₄)₂.8H₂O, occurs in nature as the mineral Annabergite, found at Annaberg in Saxony. It may be prepared by addition of ammonium arsenate to a nickel salt, the solution being kept neutral. If ammonia is present in excess the molecules of water are partially replaced by ammonia, yielding hydrated ammoniates, which are insoluble in water, but readily soluble in acids and in ammonia. The following ammoniates have been distinguished:

Ni₃(AsO₄)₂.NH₃.7H₂O, Ni₃(AsO₄)₂.2NH₃.6H₂O, and Ni₃(AsO₄)₂.3NH₃.5H₂O.