Characterization of precipitates formed from the tetraiodomercurate (II) anion and mercury(I) or silver(I) cations

Abstract

The nature of precipitates formed from Hg^I or Ag^I and the complex anion (HgI₄)^2– has been elucidated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray microanalysis (EDX). In the case of Hg^I, well crystallized tetragonal Hg₂I₂ is formed at the beginning of the precipitation from free I^– ions. Later, HgI₂ precipitates in the form of filamentous masses. On the other hand, Ag^I gives rise first to the formation of hexagonal crystals of AgI and, in a further stage, Ag₂(HgI₄) also precipitates. On adding an excess of tetraiodomercurate(II) solution, AgI is dissolved while filamentous HgI₂ precipitates. These results are rationalized, taking into account the various precipitation and complex formation equilibria involved.