Speciation of Main-Group Metal Ions by Electrospray Mass Spectrometry. 1. Investigation of Aqueous Polyselenide Species and Effects of Cations and pH

Abstract

Polyselenide anion speciation in aqueous solution has been investigated by electrospray mass spectrometry (ESMS). Two methods of control over this speciation are reported: countercation substitutions and pH adjustments. The countercation has been found to affect the degree of disproportionation or comproportionation. Solutions of sodium tetraselenide displayed numerous Sen2- and HSen- species in neutral and basic soluions. In potassium solutions, the dominant species was Se42-·H2O. Solutions of cesium pentaselenide also showed preference for higher-order polyselenides. The pH dependence has been analyzed in terms of the percent Sen (n = 2−5) present. Di-, tri-, tetra-, and pentaselenide species were present over the entire pH range investigated, and trends in selenide species versus pH were developed. For example, at pH > 9, tetraselenide species were dominant.