Structural Characterization of the Forensic Sodium Rhodizonate Test for Lead

Abstract

The forensic sodium rhodizonate test is used to detect lead in gunshot debris. Areas of interest are sprayed with a sodium rhodizonate solution followed by a tartrate buffer solution of pH 2.8, producing a deep red compound indicative of lead and other metals. A confirmatory hydrochloric acid spray produces a violet color specific to lead. The associated chemical transformations of the insoluble lead rhodizonate complexes have not been specified. Lead rhodizonate, lead rhodizonate tartrate, and lead rhodizonate chloride coordination polymers were synthesized from dilute solutions of sodium rhodizonate and lead nitrate in water, tartrate buffer, and HCl buffer. Nanoparticles of the precipitates were analyzed by three-dimensional electron diffraction (3D ED). A crystal, Pb24+(C6O6)2–Cl22–·3H2O, was characterized in the space group R3̅m (a = 8.0708(3) Å, c = 16.576(3) Å), with rhodizonate layers bidentately coordinated to three lead ions, offset by ca. 0.05 Å from the rhodizonate planes. Solid-state nuclear magnetic resonance revealed the presence of a single axial lead site in the nitrate precipitate, a distribution of lead sites in the lead rhodizonate tartrate precipitate, and three distinct sites in the lead rhodizonate chloride precipitate, suggesting other Pb compounds in the bulk coexisting with the 3D ED product.