Determination of sulfite in rain water by ion chromatography

A method for the determination of sulfite in the rain water by using ion chromatography was investigated. Sulfite in the rain water is rapidly oxidized to sulfate due to the catalytic effects of Fe(III) and Mn(II) contained in the rain water. Therefore, it is difficult to determine sulfite in the rain water. The prevention of the oxidation of sulfite was tested by adding of triethanolamine (TEA) or ethylenediaminetetraacetic acid (EDTA) as the masking reagent of Fe(III) and Mn(II). As the result, it was found that sulfite in the rain water could be kept stable for more than one week in the presence of 2.5 raM TEA at pH 9–10 while the oxidation of sulfite could not be prevented at lower pH levels. Therefore, it is possible to determine sulfite in the rain water without the loss of oxidation by the addition of TEA during rain sampling. EDTA was also effective for the prevention of the oxidation of sulfite. However, the use of EDTA was not suitable for ion chromatographic analysis due to the appearance of EDTA peaks. A 2 raM Na₂CO₃/4 mM NaHCO₃ eluent was used for rain water analysis due to good separation of sulfite from nitrate, so that sulfite, chloride, nitrate, and sulfite in the rain water could be determined within 15 min by ion chromatography. The detection limit of sulfite by this method was 0.03 ppm at the injection of 100 µl sample solution. The concentration of sulfite in the rain water measured by this method was 0.07–1.8 ppm in Yokohama, May and June, 1985.