Rapid Quantification of Sodium Dithionite by Ion Chromatography Travis James, Allen Apblett, and Nicholas F. Materer* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078-3071, United States ABSTRACT: Sodium dithionite is an oxidizable sulfur oxyanion often employed as a reducing agent in environmental and synthetic chemistry. This industrially important reagent slowly decomposes into a variety of sulfur oxyanions. Thus, a rapid method to assess purity of a given sample is crucial. Despite the importance of this material to the wood pulping and textile industries, a rapid, reliable method to quantify dithionite remains a challenge. Current methodologies require extensive bench-top chemistry. Ion chromatography can provide a simple one-step method that can easily be automated to rapidly and accurately determine the concentration of dithionite. The results are in excellent agreement with those obtained using a multistep iodometric titration. ■ INTRODUCTION Sodium dithionite (Na 2 S 2 O 4 ) is an easily oxidized sulfur oxyanion produced using one of the following production methods: zinc-mediated oxidation of sulfur dioxide, oxidation of sulfur dioxide with sodium formate, or reduction of sodium bisulfite with sodium amalgam. 1 This compound is often employed as a reducing agent in environmental and synthetic chemistry. 2-8 Industrially, its use is extensive in vat dyeing procedures in the wood pulping, paper, and textile industries, where it finds use as both a reducing agent and a bleaching agent. 1,9-16 Over the past 15 years, these two industries have consumed almost 80% of the 129 000 tons of sodium dithionite which the United States produces annually. 17-20 Unfortunately, dithionite is unstable and slowly decomposes into various oxyanions. Thus, for industrial processes, knowing the purity of the dithionite reagent is critical. This study presents a simple and rapid analysis of dithionite using ion chromatography. The use of a simple mobile phase, a commercially available column, and ion conductivity detection makes this method readily accessible to an industrial laboratory where simplicity, speed, and precision are critical. In solutions, dithionite disproportionates into bisulfite and thiosulfate. 21 The hydrated salts are also unstable, reacting in air to form bisul fite and sulfate salts. 21,22 Due to these decomposition pathways, a rapid and accurate method to determine dithionite concentration is required for its use in industry. Typical methods involve the addition of dithionite to a solution of formaldehyde to form hydroxymethanesulfinate (HOCH 2 SO 2 - or rongalite) and hydroxymethanesulfonate (HOCH 2 SO 3 - or HMS). With relatively pure samples of dithionite, direct quantification of rongalite can be accom- plished using iodometric titrations. 23 However, the decom- position of dithionite typically results in formation of significant quantities of thiosulfate, which prevents the accurate determi- nation of dithionite. Other impurities may also play a role. Thus, the American Association of Textile Chemists and Colorists advised that the Merriman method 23 should only be used for relatively pure samples of dithionite. 24 In 1930, Wollak described a method utilizing three iodometric titrations under different conditions to determine the concentrations of dithionite, thiosulfate, and bisulfite. 25 Szekeres has sug- gested 26,27 the addition of a bromate-bromide solution. However, the Wollak method was found to be unsatisfactory by Danehy and Zubritsky, 28 and Zocher and Saechtlin. 29 In a series of papers, Kilroy 30-32 addressed these concerns and published a revised three-step method. This final iodometric method yields precise and accurate determinations at the expense of complex bench-top chemistry. 32 Since accuracy of the final result depends on a combination of the individual titrations, great care is also required. In addition to iodometric titrations, the purity of dithionite has been quanti fied by both Methylene Blue 22 and hexacyanoferrate(III) 33 titrations. However, these procedures are limited to analyzing dithionite only and no other sulfur oxyanions. Cyclic voltammetry 34,35 has also been utilized to determine the concentrations of dithionite in the solutions. However, the use of a mercury electrode may be unsuitable for routine industrial analysis. Raman spectroscopy 36 can be used, although the complex spectra of potential mixtures of different sulfur oxyanions and overlapping bands add difficulty for routine analysis. Finally, chromatographic methods, including capillary electrophoresis, 35 isotachophoresis, 37 ion-pair chro- matography, 38 and ion chromatography, 39,40 have been reported. Of the chromatographic studies, the capillary electrophoresis work of Carvalho and Schwedt was the most quantitative, determining dithionite, thiosulfate, sulfite, and sulfate. 35 However, these papers did not directly compare their proposed methodologies with the results from the established multistep iodometric titration procedure. This study presents an ion chromatography analysis of dithionite and compares the results to the established multistep iodometric titration. ■ EXPERIMENTAL SECTION Dithionite Samples. A total of five samples were analyzed (see Table 1). Sample 1 was unopened, while samples 2 and 3 have been used as reagents present in our laboratories. The lot analysis for these three samples was performed less than 1 year Received: December 5, 2011 Revised: April 19, 2012 Accepted: April 27, 2012 Published: April 27, 2012 Article pubs.acs.org/IECR © 2012 American Chemical Society 7742 dx.doi.org/10.1021/ie202847t | Ind. Eng. Chem. Res. 2012, 51, 7742-7746