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.