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Journal article
Ion Association and Hydration of Some Heavy-Metal Nitrate Salts in Aqueous Solution
The journal of physical chemistry. B, Vol.128(41), pp.10238-10246
2024
PMID: 39361423
Abstract
Aqueous solutions of four heavy-metal nitrate salts (AgNO3, TlNO3, Cd(NO3)2 and Pb(NO3)2) have been studied at 25 °C using broadband dielectric relaxation spectroscopy (DRS) at frequencies 0.27 ≤ ν/GHz ≤ 115 over the approximate concentration range 0.2 ≲ c/mol L–1 ≲ 2.0 (0.08 ≲ c/mol L–1 ≲ 0.4 for the less-soluble TlNO3). The spectra for AgNO3, TlNO3, and Pb(NO3)2 were best described by assuming the presence of three relaxation processes. These consisted of one solute-related Debye mode centered at ∼2 GHz and two higher-frequency solvent-related modes: one an intense Cole–Cole mode centered at ∼18 GHz and the other a small-amplitude Debye mode at ∼500 GHz. These modes can be assigned, respectively, to the rotational diffusion of contact ion pairs (CIPs), the cooperative relaxation of solvent water molecules, and its preceding fast H-bond flip. For Cd(NO3)2 solutions an additional solute-related Debye mode of small-amplitude, centered at ∼0.5 GHz, was required to adequately fit the spectra. This mode was consistent with the presence of small amounts of solvent-shared ion pairs. Detailed analysis of the solvent modes indicated that all the cations are strongly solvated with, at infinite dilution, effective total hydration numbers (Zt0 values) of irrotationally bound water molecules of ∼5 for both Ag+ and Tl+, ∼10 for Pb2+, and ∼20 for Cd2+. These results clearly indicate the presence of a partial second hydration shell for Pb2+(aq) and an almost complete second shell for Cd2+(aq). However, the hydration numbers decline considerably with increasing solute concentration due to ion–ion interactions. Association constants for the formation of contact ion pairs indicated weak complexation that varies in the order: Tl+ < Ag+ < Pb2+ < Cd2+, consistent with the charge/radius ratios of the cations and their Gibbs energies of hydration. Where comparisons were possible the present constants mostly agreed well with the rather uncertain literature values.
Details
- Title
- Ion Association and Hydration of Some Heavy-Metal Nitrate Salts in Aqueous Solution
- Authors/Creators
- Johannes Hunger - Max Planck Institute for Polymer ResearchRichard Buchner - University of RegensburgGlenn Hefter - Chemistry Department, Murdoch University, Murdoch, WA 6150, Australia
- Publication Details
- The journal of physical chemistry. B, Vol.128(41), pp.10238-10246
- Publisher
- ACS Publications
- Number of pages
- 9
- Grant note
The authors thank Ms Martina Knecht for recording the DRS data. J.H. acknowledges funding from the Max Planck Society.; G.H. and R.B. had no financial support.
- Identifiers
- 991005707260007891
- Copyright
- © 2024 The Authors.
- Murdoch Affiliation
- Murdoch University
- Language
- English
- Resource Type
- Journal article
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- 2.89 Ionic, Molecular & Complex Liquids
- 2.89.677 Liquid Water
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