Thermodynamics of protonation and sodium binding of sulfate in concentrated NaCl and CsCl solutions studied by Raman spectroscopy

The protonation constant (pK(a)) of SO42-(aq) has been determined at ionic strengths 0.5 M less than or equal to I less than or equal to 4.0 M in NaCl and CsCl media at 25 degrees C by using Raman spectroscopy. These data wer e used to calculate the association constant of the NaSO4-(aq) ion pair in CsCl media. The results are in excellent agreement with previous values obt ained by other techniques. The pK(a) was also measured at I = 4 M in both m edia at temperatures up to 85 degrees C and the associated enthalpy and ent ropy changes were calculated. However, reliable thermodynamic data for the ion-pairing reaction could not be obtained at higher temperatures probably because of competition from CsSO4-(aq).