Quantitative Determination of Sulfur Oxide Species in White Liquor by FT-IR

Abstract

The Circle® cell has made possible the quantitative analysis of samples by infrared spectroscopy in aqueous solutions. With the use of this technique, which makes use of an FT-IR spectrometer, a novel analytical method is presented for the quantitative determination of sulfur oxide anions in white liquor. The method is appropriate only for oxidized sulfur species. However, it is shown that when Na2S concentrations are determined by another technique (e.g., ion chromatography or ion-selective electrode), the results can be used to carry out material/species balance on the unoxidized as well as oxidized samples of white liquor. The method is fast, straightforward, and convenient to use, and it does not require further treatments of white liquor solutions. Another advantage of this method is that it lends itself to in situ analysis. It is also general enough to be used for the quantitative analysis of other aqueous solutions. Linear regressions carried out on the SO=4, S20=3, and SO=3 species (in concentration ranges of 0–70, 0–45, and 0–42 g/L as salts, respectively) led to correlation coefficients of higher than 0.99; respective minimum detection limits of 0.01, 0.02, and 0.02 g/L were obtained. The latter can be further improved upon by employing longer-pathlength crystals. The method provides a way of carrying out material species balances, and studying reactions and their kinetics. The process does not use any toxic materials or environmentally unfriendly bleaching agents; it makes use of a combination of temperature, pressure, and molecular oxygen to oxidize white liquor to sulfate.