Stability of Sulfur(IV) Solutions in the Presence of Amines and the Tendency of Sulfite Ions to Disproportionate in Stock Solutions

Abstract Several sulfite solutions at the level of 0.050 mol.L−1 were kept at 25°C, in polyethylene flasks, in the presence and absence of air. The concentration of sulfite was periodically checked by the iodometric method. The presence of ethanolamine, ethylenediamine or tris(hydroximethyl)aminomethane, at 0.05 - 0.20 mol.L−1 concentration levels, showed a remarkable stabilizing effect at pH values higher than 10.5. The amine ionization was supressed by the common ion effect by using OH−, added as NaOH. The changes in the concentration of S(IV) solutions in the presence of amine were about 1 to 3 %, during the period of 85 days. The stabilizing effect depends only on the amine group and not on the mixed function alcohol/amine. It was shown that an adduct of R-NH2.SO2 type is not formed in aqueous medium. In the stock S(IV) solutions kept in highly alkaline medium a slow increase in pH was observed and disproportionation of sulfite ion was evidenced by formation of thiosulfate as revealed from ion chromatographic measurements. In the presence of oxygen dithionate ion is also formed.