Mercury and cadmium have adverse environmental and health effects and are regulated and controlled globally. As part of a project to design optical sensors for these toxic metals, the complexation of rhodizonate, (C6O62-), a six-member ring oxocarbon compound, with Hg(II) and Cd(II) has been investigated. UV-Visible spectral titrations of sodium rhodizonate and Hg(II) or Cd(II), at constant rhodizonate concentration in 1:1 water/acetone mixtures showed small but consistent spectral changes consistent with metal-rhodizonate interactions in solution. FT-IR analysis was performed by solvent drop grinding of Hg(II) or Cd(II) salts together with Na2C6O6. Through major changes in the intensity and location of the C-O rhodizonate stretches, it was determined that rhodizonate does complex with both Hg(II) and Cd(II). Through the same methodology, the complexation of rhodizonate with Hg(II) or Cd(II) was also investigated in the presence of the following nitrogen-donor co-ligands: imidazole, 1,10-phenanthroline, or 2,2’-bipyridine. Upon addition of each of the co-ligands to 1:1 mixtures of HgCl2 or CdCl2 with sodium rhodizonate in water/acetone 1:1, it was found that rhodizonate complexes in a different fashion with the metals in the presence of the co-ligand, especially for the imidazole titrations, which showed clear response for HgCl2 addition, with loss of absorbance at 490 nm, a new absorbance at 540 nm and an isosbestic point at 505 nm. FT-IR spectra after solvent drop grinding of sodium rhodizonate, Hg(II) or Cd(II) chloride salts, and organic co-ligand also showed distinct spectral changes consistent with complex formation. Thus it was concluded that in the presence of an N-type co-ligand, rhodizonate forms ternary complexes that include both the metal and the co-ligand, in addition to rhodizonate.

Complexation of Hg(II) and Cd(II) with rhodizonate and nitrogen donor ligands

255th National Meeting (2018)

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