Ruthenium Speciation during Acidification of Alkaline Solution

Ruthenium (Ru) has applications in nuclear medicine and is also a highly radioactive and volatile fission product. Due to its volatility in oxidizing environments, alkaline gas traps are used to capture its vapors. However, analyzing these solutions using inductively coupled plasma mass spectrometry (ICP-MS) or optical emission spectrometry (ICP-OES) requires acidification. This step causes much of the ruthenium in solution to precipitate, resulting in inaccurate measurements. Ruthenium speciation during the acidification with nitric acid of an alkaline ruthenium solution was monitored by using a closed reactor coupled with in situ high-speed UV-visible spectroscopy. This setup enabled real-time monitoring of the reaction while ensuring quantitative recovery of gaseous and solid byproducts, to close the analytical mass balance. The process followed a two-step mechanism, initially forming soluble ruthenium species, followed by ruthenium tetroxide (RuO₄) volatilization and the precipitation of an amorphous phase. Thermal and structural analyses (thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman, and XANES) confirmed its structural divergence from conventional RuO₂, K _y RuO₄ (y = 1 or 2), or Na _z RuO₄ (z = 1, 2, or 3). XANES measurements suggested the presence of +V oxidation states while also revealing its limited stability under the experimental conditions. Reaction pathways were proposed: first, ruthenate (RuO₄ ^2-) reacts with nitric acid to form perruthenate (RuO₄ ^-) and Ru₂O₅, and then RuO₄ ^- reacts with nitric acid to form RuO₄ and Ru₂O₅. This study provides a detailed methodology for the monitoring of ruthenium speciation during acidification with nitric acid, which can be used in other media. It supports the development of a routine analytical method for ruthenium determination in alkaline samples, addressing a critical gap in ICP-MS sample preparation.