Dielectric relaxation of aqueous Na2CO3 solutions

The complex dielectric permittivity of aqueous sodium carbonate solutions (0.05 less than or equal to c/mol L-1 less than or equal to 2.5) has been determined in the frequency range 0.2 less than or equal to nu/GHz less than or equal to 20 at 25.0 degrees C with a commercial dielectric measurement system based on a vector network analyser. A superposition of three Debye relaxation processes was necessary to fit the complex permittivity spectra. The slow and intermediate dispersion steps are assigned to double solvent separated (2SIP) and solvent-shared (SSIP) NaCO3-(aq) ion-pairs respectively. The absence of contact ion-pairs was confirmed by Raman spectroscopy. The fast process is assigned to the collective relaxation of the solvent. Equilibrium constants for the formation of NaCO3-(aq) were determined from the ion-pair dispersion amplitudes. The effect of Na2CO3 concentration on the water dispersion amplitude allowed effective solvation numbers to be deduced.