Sulfur Dioxide in Fruit and Vegetable Products

Even when errors in true free sulfur dioxide content due to dilution or oxidation are avoided, the direct iodometric determination of sulfurous acid and sulfites is subject to error. Eolthoff and Menzel (1929) point out that an appreciable error occurs when sulfite solutions are titrated with iodine owing to oxidation by air and point out that correct results can be obtained only when the sulfurous acid or sulfite solution is allowed to flow into a solution of iodine. The titration of sulfurous acid or bisulfites is 

In addition to the iodometiic determination, direct precipitation as barium sulfate before and after treatment with bromine was suggested for both quantitative and qualitative test for sulfur dioxide in wine (see Monier-Williams, 1927). Precipitation of SO2 after oxidation with H2O1 as the benzidine sulfate was proposed by Rothen-fusser (1929) reduction of the molybdenum in phosphomolybdic acid by the sulfite ion present in an aqueous solution of the food was proposed by Sasaki (1928) as a colorimetric method formation of a blue color from a solution of 1% methylene blue and 5% iodine in potassium iodide was proposed by Svershkov (1939). Mathers (1949) proposed a turbidimetric method based on the distillation of wine into a dilute solution of lead acetate to form a colloidal suspension of lead sulfite whose spectral transmittance in the range of 400 to 700 mn could be used as a measure of sulfur dioxide. This is similar to turbidimetric methods based on turbidity produced by adding BaCfii to a 

The above substitutes for the iodometric determination with one or two exceptions have not been tested for specificity for free SO2. Mathers (1949) on the basis of the fact that free sulfur dioxide is removed early in the distillation of wine proposed that 10 ml. of the distillate from the alcohol determination be mixed with 0.5 ml. of 5% neutral lead acetate and the turbidity of the suspension formed be used to correct volatile acidity for sulfur dioxide. La Rosa (1950) assumed that the color formed on mixing the fuchsin-sulfuric acid-formaldehyde reagent with white wine was a measure of free SO2 but did not give any data to confirm this. The fuchsin procedure and the lead sulfite procedures are very sensitive, the former more so, and can be applied only to very small aliquots or to dilute solutions. 

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. 

The Ripper method in modified form was extended to the determination of total sulfur dioxide in frozen and dehydrated sulfited fruit and vegetable products. Jensen 

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III. Determination of Sulfur Dioxide in Fruit and Vegetable Products... 113... 

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