Sulfite/sulfur dioxide analysis

H. Mana and U. Spohn, Sensitive and Selective Flow Injection Analysis of Hydrogen Sulfite/Sulfur Dioxide by Fluorescence Detection with and without Membrane Separation by Gas Diffusion, Anal. Chem. 2001, 73, 3187. 

Mana, H. and U. Spohn. 2001. Sensitive and selective flow injection analysis of hydrogen sulfite/sulfur dioxide by fluorescence detection with and without membrane separation by gas diffusion. Anal. Chem. 73 3187-3192. 

The solid product from each of these sets of reactions is primarily calcium sulfite hemihydrate (CaSOs-5 H2O), which has been confirmed by x-ray diffraction analysis of scrubber sludgesJ l A similar set of reactions collects sulfur trioxide (SO3) from the flue gases, forming gypsum (CaS04 2H2O) as the solid product, but under normal boiler conditions sulfur trioxide makes up only about 0.5% of the total sulfur oxides, and so its removal is less important than the removal of sulfur dioxide.l " ... 

The method of Fan and Dasgupta (1994) relics on tlie reaction of formaldehyde with 1,3-cyclohexane-dione in acidified ammonium acetate to form the fluorescent dihydropyridine derivative in a flow injection analysis system. Formaldehyde trapped in water can be reacted with pararosaniline and sodium sulfite under mild conditions (neutral pH, room temperature equilibration) to produce a colored product that is measured at 570 nm (Petreas et al. 1986). The presence of bisulfite is an interference in this reaction so the method cannot be used to sample atmospheres that contain sulfur dioxide. In addition, the method is reported to suffer from interferences resulting from the presence of other aldehydes and phenol (Hoogenboom et al. 1987). The indirect method of Hoogenboom et al. (1987) relies on the reaction of excess bisulfite in an aqueous solution of formaldehyde with 5,5 -dithiobis(2-nitrobenzoic acid) to form a colored product, the absorbance of which is measured at 412 nm. The method reported by Naruse et al. (1995) relies on the formation of a colored product obtained by reacting the aqueous formaldehyde with acetylacetone and ammonium acetate in acetic acid. Absorbance is measured at 414 nm. 

Solid Calcium Sulfite. Recent thermodynamic studies of calcium sulfite by mass spectroscopy indicated that calcium sulfite dissociates into calcium oxide and sulfur dioxide (12). Under atmospheric pressure, this dissociation reaction is slow in the range below 250°C. We find under these conditions substantial decomposition of sulfite, yielding sulfate, elemental sulfur as well as thiosulfate. These observations are consistent with experiments by Brewer (13), and confirm old observatons made by wet-analysis of these complex solids (14). Our work confirms seventy year old literature reports which suggested evidence for thiosulfate, trithionate and dithionate in old pulping sulfite liquor which yellows when kept in air-free, sealed ampules (15-18). 

These were pyrolyzed directly in a 25-mL distillation flask fitted with a 3-in. Vigreux head. The sulfite was heated to 200°C under nitrogen at atmospheric pressure for 30 min. Vacuum (1 mmHg) was then applied, and the decomposition products were distilled slowly from the reaction mixture. The olefins were condensed in the air-cooled receiver, and the methanol and sulfur dioxide were allowed to pass into the dry ice trap. The yield of olefin was 86% from the cis isomer and 76% from the trans isomer.Analysis for the percentage of 1-substituted and 3-substituted cyclohexene was made by oxidation to the sulfones, and the position of the absorption maximum in the ultraviolet spectra was determined. The oxidized product from the cis isomer had an absorption maximum at 229 m u, corresponding to that of a mixture of 40% l-j -tolylsulfonyl-l-cyclohexene and 60% 3-p-tolylsulfonyl-i-cyclohexene. The oxidized product from the trans isomer had its absorption maximum at 232 m/z, corresponding to 80% of the 1-substituted isomer. 

The mechanism of sulfuric acid formation and scavenging in the Pasadena precipitation samples is also poorly understood. Aerosol sulfate measurements were too few for comparison with the excess sulfate concentrations, the sulfate in excess of that expected from sea salt. Sulfur dioxide concentrations during the storm were consistently 10 or 20 ppb with an uncertainty of 5 ppb. Gas scavenging of sulfur dioxide to form sulfite species and oxidation to sulfate before analysis of the sample could account for a large fraction of the excess sulfate. Equilibrium total sulfite was calculated from PSO2 i s sured at ground level and measured pH,by equations 17-20. 

Ion-selective electrodes using a sulfur dioxide gassensing membrane probe have been developed into commercial models for the determination of sul-fur(IV) oxo-species. A typical procedure for the determination of free sulfur dioxide requires the probe to be immersed in a stirred solution at pH 1, whereas total sulfur dioxide requires the sample to be treated with strong alkali prior to acidification and analysis at pH 1. Molecular sulfur dioxide passes through the membrane into a solution of hydrogen sulfite ion and the pH of the SO2 H20/HS03 buffer can be determined with a glass electrode. 

Two other methods have been approved by AOAC for determination of sulfites in food. One is a quantitative assay based on malachite green decolorization using flow injection analysis. Sulfite is released from a sample slurry with alkali, then the test stream is acidified to produce sulfur dioxide gas which diffuses across a Teflon membrane into a flowing stream of malachite green, and the extent of decolorization is measured at 615 nm. The other method is based on ion exclusion chromatography with sulfur dioxide being released by alkali extraction, and the diluted filtrate is injected onto an anion exclusion column linked to an electrochemical detector. 

The content of sulfur dioxide (SO2) in the atmosphere varies widely and may reach values as high as 1-5 mg m in heavily polluted air. Direct monitoring with an SO2 gas probe is often insufficiently sensitive and thus SO2 must be preconcentrated by absorption in a suitable solution. The most common procedure is based on absorption of SO2 in a tetra-chloromercurate(II) solution (TCM), in which Hg(S03)2 is formed. The pH of the absorption solution is maintained at a value of 6.9 that ensures virtually complete absorption of S02- Prior to analysis, amidosulfonic acid is added to remove nitrite and the pH is decreased to about unity, so that most of the sulfite present is converted into free sulfur dioxide, which is determined by an SO2 probe. With very low SO2 contents, a filter is soaked with a TCM solution and air is passed through the filter at a high flow rate and for a rather long time. These methods provide discontinuous values of the SO2 concentration in regular intervals. 

Total SulfuT Dioxide After Alkali Treatment The fact that neutral sodium sulfite does not combine with carbonyl compounds and that the hydroxysulfonic acid compounds are rapidly decomposed on treatment with alkali was used by Ripper (1892) as the basis for the determination of total sulfur dioxide in wine by direct iodine titration. In his method, 50 ml. of wine were pipetted into a 200-ml. flask containing 25 ml. of 1 iV KOH. The mixture was shaken and allowed to stand for 10 to 15 minutes. Then 10 ml. of dilute sulfuric acid (1 + 3) were added, and the solution titrated rapidly with 0.02 N iodine solution to a starch end point which persisted for some time. This method was used as the ofiicial direct titration method for wine in the first edition (1919) of the A.O.A.C. Methods of Analysis in the third (1930) edition it was extended to white grape juice, wine, and similar products (1N NaOH or KOH was used and the solution during standing for 15 minutes was occasionally agitated) hut it was dropped from the fourth (1935) and succeeding editions. Ripper compared his method with the Haas distillation method on ten wines whose SO2 content varied from 42 to 1488 mg. per liter and found the difference between the two to vary from 0 to 5 mg. 

Frenzel, W. and B. Hillmann. 1995. Gas-diffusion flow-injection analysis to the determination of sulfite and sulfur-dioxide in environmental-samples. Chem. Anal. Warsaw 40 619-630. 

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