Despite the widespread use of the sodium rhodizonate (Na2C6O6) test in modern forensics for Pb detection in gunpowder residues, and despite the test’s proven reliability and reproducibility, the underlying chemistry behind this test is not well-understood. The limited solubility of many metal rhodizonate complexes and the limited stability of rhodizonate in water have been hampering many of the efforts to understand this colorimetric test. While alkali metal rhodizonate salts are water soluble, they decompose in aqueous solutions quite rapidly (~2 hours), limiting detailed metal complexation studies. We have now studied the complexation of rhodizonate with several divalent metals in mixed solvents, such as 50/50 water:ethanol and 50/50 water:acetone, which greatly improve the stability of rhodizonate. For example a 10-5 M sodium rhodizonate solution in 75/25 acetone:water is essentially stable for over 2 days. In such solvents we saw notable spectroscopic changes in the UV-Vis spectra upon titration of sodium rhodizonate with several metal cations, such as Pb(II), Cd(II) and Hg(II). FT-IR and 13C-NMR were used to characterize the formed complexes. The formation of ternary complexes with additional N-containing ligands such as 2,2’-bipyridine, 2,2’-bipyrimidine, 1,10-phenantholine, and imidazole was also studied by UV-Vis, FT-IR, 1H-NMR, and 13C-NMR. The rhodizonate-imidazole complexes were particularly noteworthy, as they produced different colors from the binary rhodizonate complexes and potentially be exploited for improved colorimetric testing for toxic metals.

Coordination chemistry of the rhodizonate anion with divalent toxic metals: Binary and ternary rhodizonate complexes for spectrophotometric M(II) detection

2019 Spring National Meeting

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