Ion-pair and solvent relaxation processes in aqueous Na2SO4 solutions

The complex dielectric permittivity of aqueous sodium sulfate solutions (0.025 less than or equal to c/mol dm(-3) less than or equal to 1.6) has been determined in the frequency range 0.2 less than or equal to v/GHz less than or equal to 20 with a commercial dielectric measurement system based on a vector network analyzer. The spectra were supplemented with interpolated literature data at 12 less than or equal to v/GHz less than or equal to 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) NaSO4- 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 Na2SO4 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, beta(NaSO4-), were determined.