STABILITY RELATIONS OF ANTIMONY AND ARSENIC COMPOUNDS IN THE LIGHT OF REVISED AND EXTENDED EH-PH DIAGRAMS

Abstract

New stability fields for antimony and arsenic compounds have been calculated in terms of Eh-pH diagrams at 25oC and 1 atm total pressure.The Eh-pH diagram for antimony shows substantial differences with existing diagrams. This concerns mainly the pentavalent oxide Sb2O5, which is metastable and does not feature in the new diagram. Both Sb2O4 (cervantite) and Sb2O5 are replaced by the ionic species SbO-3(aq), although Sb2O4 may retain a small stability field under oxidizing and strongly acidic conditions. The polymorphs of Sb4O6, senarmontite and valentinite, occupy a very large stability field, under moderately reducing to moderately oxidizing conditions, either acidic or alkaline. In the oxidizing environment antimony appears to be much more mobile than was suggested by earlier diagrams.Eh-pH diagrams for the systems As-O-H and As-O-H-S are similar to existing diagrams, but may need some rethinking on the stability of the polymorphs of As2O3, arsenolite and claudetite. An extension is made by including the diagram for the system As-Fe-O-H-S, so that arsenopyrite (FeAsS) being the most common occurrence of arsenic, can be included as well. Secondly, in this diagram, a stability field for scorodite (FeAsO4 . 2H2O), in the oxidizing environment can also be tentatively outlined.