Sodium-Mercury Amalgam Defined

The sodium-mercury amalgam formed at the mercury cathode in the chlor-alkali process is Na₁₁Hg₅₂, with the sodium and mercury atoms bound in a variety of polyhedra (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201108064). The century-old chlor-alkali process uses electrolysis cells to produce NaOH and Cl2 from brine. As the process runs, some sodium metal dissolves in the mercury, forming a solution of the solid amalgam in the liquid metal. The amalgam had defied characterization, but now Constantin Hoch of the University of Stuttgart and Arndt Simon of the Max Planck Institute for Solid State Research, both in Germany, have isolated single crystals of the amalgam and identified them as Na₁₁Hg₅₂. The crystal structure shows the Na atoms coordinated by Hg atoms in four classes of polyhedra, with Na–Hg distances ranging from 3.14 to 3.76 Å. The large difference in electronegativity between Na and Hg means that the bonds are more polar than those in classic metallic compounds, making the amalgam an interesting system for further study, Hoch and Simon report.