Hydration and Ion Pairing in Aqueous Sodium Oxalate Solutions

Dielectric spectra have been measured for aqueous sodium oxalate solutions up to the saturation concentration (0.04≤c mmol L^?1]≤0.25) at 25 °C over the approximate frequency range 0.2≤ν GHz]≤20. The spectra exhibit a process at about 1 GHz associated with the presence of ion pairs, in addition to the dominant solvent relaxation process at about 18 GHz. Detailed analysis of the solvent dispersion amplitude indicates that the oxalate ion is highly hydrated but that its solvation sheath is “fragile”, decreasing quickly with increasing solute concentration. The NaOx(aq)^? ion pair is shown to be of the double‐solvent‐separated (2SIP) type, with an infinite dilution association constant K_A=1.04±0.02. Analysis of the ion‐pair relaxation time as a function of solute concentration gave rate constants for the formation (k₁₂=(7.3±0.4)10⁹ L mol^?1 s^?1) and dissociation (k₂₁=(6.7±0.5)10⁸ s^?1) of the ion pair. These values are reasonably close to the diffusion‐controlled values predicted by the Eigen theory, consistent with a 2SIP structure for the ion pair.