Ion-Pair and Solvent Relaxation Processes in Aqueous Na2SO4 Solutions

The complex dielectric permittivity of aqueous sodium sulfate solutions (0.025 ≤ c /mol dm -3 ≤ 1.6) has been determined in the frequency range 0.2 ≤ ν/GHz ≤ 20 with a commercial dielectric measurement system based on a vector network analyzer. The spectra were supplemented with interpolated literature data at 12 ≤ ν/GHz ≤ 89. To fit the complex permittivity spectra, a superposition of three Debye relaxation processes was necessary. The slow and intermediate dispersion steps are assigned to the tumbling motion of doubly solvent-separated (2SIP) and solvent-shared (SSIP) NaSO 4 - ion pairs, respectively. The fast process, of amplitude S 3, is due to the collective relaxation of the solvent. Effective solvation numbers were deduced from the effect of Na 2 SO 4 concentration on S 3 . From the ion-pair dispersion amplitudes, S 1 and S 2, the concentrations c 2SIP and c SSIP, and thus the overall stoichiometric stability constant, β NaSO 4 −, were determined.