pH-dependent structural changes of arsenic oxoacids in solution and solid phase: Raman spectrometry and computational studies

Reliably knowledge of the arsenic oxoacids in solution and solid-state and any possible relationship between them at different pHs is of concern in various scientific fields. This work compares the experimental Raman spectra of the inorganic arsenic oxoacids in aqueous solutions under acidic, neutral and alkaline conditions with the dry precipitates obtained from the corresponding aqueous solutions. Further, we explore the ability of quantum chemical methods to model the Raman spectra of the arsenic oxoacids in solution by considering explicit water molecules and conformational sampling, fitting reasonably well with the experimental spectra in the whole pH range covered. Hydrogen bridges and the coexistence of arsenic oxoacids facilitate strong overlapping in the Raman spectra of the As-(III) aqueous samples compared with the As-(V) ones. Some Raman shifts of the dry arsenic precipitates correlate well with the corresponding Raman spectra of the same arsenic species in aqueous solutions, allowing a practical use of the Raman spectrometry for indirect screening purposes of arsenic speciation in both condensed phases.

Automated summary

This study compares the experimental Raman spectra of inorganic arsenic oxoacids in different aqueous solutions with the dry precipitates obtained from these solutions. The ability of quantum chemical methods to model the Raman spectra of the arsenic oxoacids in solution is also explored. It was found that hydrogen bridges and the coexistence of arsenic oxoacids contribute to strong overlapping in the Raman spectra of As-(III) aqueous samples compared to As-(V) samples. Furthermore, some Raman shifts of the dry arsenic precipitates correlate well with the corresponding Raman spectra of the same arsenic species in aqueous solutions, allowing for practical use of Raman spectroscopy in indirect screening of arsenic speciation in condensed phases.