Ion Association and Hydration of Some Heavy-Metal Nitrate Salts in Aqueous Solution

Aqueous solutions of four heavy-metal nitrate salts (AgNO3, TlNO3, Cd(NO3)(2) and Pb(NO3)(2)) have been studied at 25 degrees C using broadband dielectric relaxation spectroscopy (DRS) at frequencies 0.27 <= nu/GHz <= 115 over the approximate concentration range 0.2 less than or similar to c/mol L-1 less than or similar to 2.0 (0.08 less than or similar to c/mol L-1 less than or similar to 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 similar to 2 GHz and two higher-frequency solvent-related modes: one an intense Cole-Cole mode centered at similar to 18 GHz and the other a small-amplitude Debye mode at similar to 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 similar to 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 (Z(t)(0) values) of irrotationally bound water molecules of similar to 5 for both Ag+ and Tl+, similar to 10 for Pb2+, and similar to 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.