Protonation and sodium ion-pairing of the sulfite ion in concentrated aqueous electrolyte solutions

A protocol has been developed for the reliable titration of aqueous sulfite solutions which minimizes photodecomposition effects. This procedure has been used to measure the protonation constants of the sulfite ion in aqueous solution by glass electrode potentiometry at 25.0 degrees C and ionic strengths (I) from 0.1 to 5.0 M in NaCl media and at I = 1.0 M in KCI and Me4NCl media. These measurements provided evidence of weak but significant ion-pairing between SO32- and Na+ with a formation constant of log K-Na = 0.431 in 1.0 M Me4NCl. This was in very good agreement with the value log K-Na = 0.410 measured directly by Na+ ion-selective electrode potentiometry. Evidence is also presented far an extremely weak association of K+ and SO32- with log K-K = 0.22 in 1.0 M Me4NCl.