二氧化硫检测方法.doc

Determination of Added Sulfites in Dried Allium (Modified Monier-Williams Method) Purpose: This modification of the Monier-Williams method is suitable specifically for detection of sulfites in dried allium (i.e. garlic, onion, shallot, leek, and chive) (Reference 1). Organosulfur components are removed in a toluene trap before sulfur dioxide is collected and oxidized to sulfuric acid with hydrogen peroxide in a second trap. Sulfuric acid is titrated with standard sodium hydroxide solution. The limit of quantitation is 10 μg/g. Sulfites in non-allium foods should be analyzed by the optimized Monier-Williams method as described in AOAC 990.28 or EN 1988-1. A. Apparatus 1. Distillation apparatus: as described in EN 1988-1 (1998) or AOAC 990.28 (2000) and with a dual bubble-through system (see Figure). 2. 1000 mL round-bottom flask with three 29/32 (or 24/40) tapered joints (vertical arms) and appropriate heating mantle. 3. 250 mL dropping funnel with stopcock and tapered joint to fit round bottom flask. 4. 25 mL buret. 5. Graduated cylinders of 25, 50, 100 and 500 mL. 6. 1 L volumetric flask. 7. 3 mL graduated pipet. 8. Cryostat set at - 5°C. 9. pH meter. B. Reagents 1. Deionized water. 2. 0.01 N NaOH solution. 3. Indicator: Dissolve 250 mg of methyl red in 100 mL of ethanol (analytical grade). This indicator is red for pH 4.4 and yellow for pH value 6.2. 4. 30% w/w or 3% w/w hydrogen peroxide H2O2 solution, analytical grade. 5. 37% concentrated hydrochloric acid HCl for analysis. 6. 85% phosphoric acid H3PO4 for analysis. 7. 85% potassium hydroxide KOH in pellets for analysis. 8. Potassium dihydrogen phosphate KH2PO4 of purity 99% for analysis. 9. Toluene, analytical grade. 10. Cryostat circulating fluid: mixture of monopropylene glycol/water (2/1). 11. Nitrogen flow with precision valve allowing a flow rate of about 200 mL/minute. 12. Buffer solutionSolution A (acid part): 0.59 M H3PO4 and 0.96 M HCl solution. Introduce approximately 200 mL of water into a