Potentiometric titration of sodium sulfate in sodium sulfite solutions

Abstract

A novel potentiometric titration procedure has been developed for the determination of sodium sulfate impurity in sodium sulfite solutions using 0.1 mol l^–1 lead nitrate solution as the titrant. Sodium sulfite was effectively removed by reaction with formaldehyde solution, leaving only sulfate to react quantitavely with the standard 0.1 mol l^–1 lead nitrate solution. Commercially available sodium sulfite solutions are manufactured to contain 18–20% m/m Na₂SO₃ and during method development the lowest level of sodium sulfate impurity measured was 0.3% m/m in a laboratory-prepared 18% m/m sodium sulfite solution. The potentiometric titration results agreed closely with those of a traditional gravimetric procedure, with no difference when the sodium sulfate level was 2.5% m/m and only 0.3% m/m maximum difference in absolute terms, when the sodium sulfate level was at 9% m/m. At low concentrations of sodium sulfite (i.e., about 0.2% m/m) the detection limit for the sodium sulfate impurity is 0.02% m/m.