Confocal Fluorescence Microscopy and Kinetics of the Cr ³⁺-Chromate Ion Oxidation Equilibria at the Solid Liquid Interface

Silica-borax pearl samples impregnated with 0.17 and 0.64% Cr³⁺ were characterized by specific surface area measurements, UV-Vis spectroscopy, energy-dispersive X-ray fluorescence and laser-scanning confocal microscopy. Pearl stability against oxidizing conditions was tested by adding samples to an aqueous hydrogen peroxide solution. The reaction was examined by UV-Vis spectroscopic measurements of the supernatant and laser-scanning confocal microscopy images of the substrate. Overall, hydrogen peroxide-induced Cr³⁺ to Cr⁶⁺ oxidation across the solid-liquid interface promoted solid matrix cleavage pearl degradation and concomitant formation of multiple scattering centers was observed. A dual-detection scheme was employed in the confocal microscopy measurements allowing us to separate scattering and absorptive contributions to the observed signals. The confocal microscopy images indicate that Cr³⁺ oxidation induced by hydrogen peroxide solutions occurs throughout the entire pearl sample and indicate that oxidation reactions induce leakage of chromate ion into aqueous solutions.

Keywords:
confocal fluorescence microscopy; chromium mobility; H₂O₂ oxidation equilibria; solid liquid interfaces; mass transfer phenomena; energy dispersive X-ray fluorescence