Hydroxide promotes ion pairing in the NaNO-NaOH-HO system

• Published in: Physical chemistry chemical physics : PCCP Vol. 23; no. 1; pp. 112 - 122
• Main Authors: Graham, Trent R, Dembowski, Mateusz, Wang, Hsiu-Wen, Mergelsberg, Sebastian T, Nienhuis, Emily T, Reynolds, Jacob G, Delegard, Calvin H, Wei, Yihui, Snyder, Michelle, Leavy, Ian I, Baum, Steven R, Fountain, Matthew S, Clark, Sue B, Rosso, Kevin M, Pearce, Carolyn I
• Format: Journal Article
• Published: 06.01.2021
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/d0cp04799f

Nitrite (NO 2 − ) is a prevalent nitrogen oxyanion in environmental and industrial processes, but its behavior in solution, including ion pair formation, is complex. This solution phase complexity impacts industries such as nuclear waste treatment, where NO 2 − significantly affects the solubility of other constituents present in sodium hydroxide (NaOH)-rich nuclear waste. This work provides molecular scale information into sodium nitrite (NaNO 2 ) and NaOH ion-pairing processes to provide a physical basis for later development of thermodynamic models. Solubility isotherms of NaNO 2 in aqueous mixtures with NaOH and total alkalinity were also measured. Spectroscopic characterization of these solutions utilized high-field nuclear magnetic resonance spectroscopy (NMR) and Raman spectroscopy, with additional solution structure detailed by X-ray total scattering pairwise distribution function analysis (X-ray PDF). Despite the NO 2 − deformation Raman band's insensitivity to added NaOH in saturated NaNO 2 solutions, 23 Na and 15 N NMR studies indicated the Na + and NO 2 − chemical environments change likely due to ion pairing. The ion pairing correlates with a decrease in diffusion coefficient of solution species as measured by pulsed field gradient 23 Na and 1 H NMR. Two-dimensional correlation analyses of the 2800-4000 cm −1 Raman region and X-ray PDF indicated that saturated NaNO 2 and NaOH mixtures disrupt the hydrogen network of water into a new structure where the length of the O O correlations is contracted relative to the typical H 2 O structure. Beyond describing the solubility of NaNO 2 in a multicomponent electrolyte mixture, these results also indicate that nitrite exhibits greater ion pairing in mixtures of concentrated NaNO 2 and NaOH than in comparable solutions with only NaNO 2 . The solubility and spectroscopy of sodium nitrite in sodium hydroxide solutions are detailed to provide insight into processes underlying solubility.