二氧化硫检测方法

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1、氧化硫检测方法ASTA ANALYTICAL METHODS Method 30.0Determination 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) (Refe

2、rence 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 口g/g. Sulfites in non-allium fo

3、ods should beanalyzed by the optimized Monier-Williams method as described in AOAC 990.28 or EN 1988-1.A. Apparatus1. Distillation apparatus: as described in EN 1988-1 (1998) or AOAC 990.28 (2000) andwith a dual bubble-through system (see Figure).2. 1000 mL round-bottom flask with three 29/32 (or 24

4、/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. Cryostat set at - 5?C. 89. pH meter.B

5、. Reagents1. Deionized water.2. 0.01 N NaOH solution.3. Indicator: Dissolve 250 mg of methyl red in 100 mL of ethanol (analytical grade). Thisindicator is red for pH 6.2.4. 30% w/w or 3% w/w hydrogen peroxide HO solution, analytical grade.225. 37% concentrated hydrochloric acid HCl for analysis.6. 8

6、5% phosphoric acid HPO for analysis. 34Page 1 of 6 October 2010ASTA ANALYTICAL METHODS Method 30.07. 85% potassium hydroxide KOH in pellets for analysis.8. Potassium dihydrogen phosphate KHPO of purity 99% for analysis.249. Toluene, analytical grade.10. Cryostat circulating fluid: mixture of monopro

7、pylene glycol/water (2/1).11. Nitrogen flow with precision valve allowing a flow rate of about200 mL/minute.12. Buffer solutionSolution A (acid part): 0.59 M HPO and 0.96 M HCl solution.Introduce approximately 34200 mL of water into a 1 L volumetric flask, carefully add 40 mL of 85% HPO 34followed b

8、y 80 mL of 37% HCl with, respectively, a 50 and 100 mL graduated cylinderand, finally, dilute to volume with water.Solution B base part: 0.25 M KHPO and 0.258 M KOH solution.Introduce 34 g 24KHPO and 17 g KOH into a 1 L volumetric flask and dilute to volume with water. 24Check that the buffer compon

9、ents are properly made by mixing solutions A and solutionB in a 1:4 ratio (e.g. 15 mL of solution A and 60 mL of solution B). The pH must rangefrom 2.3 to 2.5. Deoxygenate these 2 solutions by bubbling nitrogen through them for 15min, less than 1 h before use.13. Hydrogen peroxide (3%, HO) solution.

10、 Into a 50-mL graduated cylinder, add 3 mL of 2230% HO solution with a graduated pipet and water to give a volume of 30 mL. 22Alternatively, one may purchase and use 30 mL of 3% HO solution directly for this 22purpose. Add 3 drops of methyl red indicator using a Pasteur pipet. With stirring, add 1to

11、 5 drops of 0.01 N NaOH and until the color of the indicator just turns yellow. Thissolution should be prepared fresh daily.C. Preparation of SampleDried allium powders can be tested for evidence of added sulfite without preparation but quantitative precision will be improved if powders are finely g

12、round to improve homogeneity (see Statistics section).D. ProcedurePart A. Collection of sulfite from the sample.1. Start circulating the coolant in the condenser, set the cryostat to -5?C and wait forapproximately 30 minutes until this temperature is reached. (Note 1)Page 2 of 6 October 2010ASTA ANA

13、LYTICAL METHODS Method 30.02. Weigh approximately 25 g of test sample, to the nearest 0.1 g, into a dry round-bottomflask. (Note 2)3. Place the round-bottom flask in the heating mantle. Connect the dropping funnel(stopcock closed), the condenser and the nitrogen inlet to the round-bottom flask and t

14、hechain of 2 bubblers to the upper condenser outlet as shown in the Figure.4. Add 25 mL toluene into the first bubbler and place the 50 mL graduated cylinder with 3%HO solution into position as the second bubbler (see Figure). 225. Add 120 mL of solution A via the dropping funnel with a 250 mL gradu

15、ated cylinder. Ifthe system is sealed properly, gas should begin to flow through the bubblers as liquidenters the round bottom flask.6. Lower the height of the second bubbler cylinder to improve the flow and to ensure that noHO solution goes up in the bubbler containing toluene. The tip of the bubbl

16、er tube 22should be submerged at least 1 cm below the surface of the HO solution. 227. Close the dropping funnel tap when a few milliliters of solution A remain in the funneland raise the second bubbler cylinder such that the tip of the second bubbler tube isapproximately 1 cm from the bottom of the

17、 cylinder.8. Open the nitrogen inlet valve slowly and check the various tapered joints for leaks.9. Allow the nitrogen to continue bubbling for 30 minutes (without heating) and then stopthe nitrogen flow. (Note 3)10. Immediately add 480 mL of solution B with a 500 mL graduated cylinder via thedroppi

18、ng funnel.11. Lower the height of the second bubbler cylinder as in step 6.12. Close the dropping funnel tap when a few milliliters of solution B remain in the funneland raise the second bubbler cylinder as in step 7.13. Immediately re-open the nitrogen flow.14. Turn on the heat to the round bottom

19、flask and after the mixture starts to boil, let it refluxfor 105 min.15. Stop the heating but leave the nitrogen flow on. Lower the HO cylinder and rinse the 22bubbler tip with water into the HO solution. 2216. Turn off the nitrogen flow and immediately remove the dropping funnel from the roundbotto

20、m flask to open the system and avoid suction of toluene from bubbler #1 back intothe system as it cools.Page 3 of 6 October 2010ASTA ANALYTICAL METHODS Method 30.0Part B. Titration of sulfite.1. If the HO solution remains yellow, the added sulfite content is below the quantification 22lim it of the

21、met hod ( 20 s. Record the volume of NaOH solution.1. Dropping funnel2. Round bottom flask3. Nitrogen inlet tube4. Allihn condenser5. Bubbler #1 (toluene)6. Bubbler #2 (HO solution) 22Figure. Assembled apparatus for the modified Monier-Williams method. Page 4 of 6 October 2010ASTA ANALYTICAL METHODS

22、 Method 30.0E. CalculationThe added sulfite content, expressed in 口g/g (ppm) equivalent SO, is equal to: 2,2,V0.01 meq NaOH32 mg S01000 口gV,320,SO2,2WmL1 meq SOmgW,Where V = the end point volume of 0.01 N NaOH expressed in mLW = the mass of dried garlic expressed in gramsExample :If V = 10 mL and W

23、= 25 g, t hen C = 320 x 10 / 25 = 128 ug/g of added sulfites. SO2F. StatisticsAn international collaborative trial between eight laboratories was completed in June 2010(Reference 2). The results of four samples (duplicate analyses per lab) are shown in the tablebelow.Sample 1 Sample 2 Sample 3 Sampl

24、e 4Average (ug/g) 37 561 44 0C) condensertemperature. Distinction of samples with detectable andnondetectable ( 10 口g/g) levels of thsulfite was achieved in 7 of 8labs (the 8 was an outlier). Samples 1 & 3 were coarselygranulated solids. The summary suggests that precision for these low level sample

25、s would beimproved by grinding them into a finer, more homogeneous powder prior to analysis.G. Notes1. The condenser temperature must not exceed -3.5?C during distillation or slight falsepositive interference might occur. The condenser should be covered with insulatingmaterial (not shown in Figure)

26、to maintain low temperatures. Avoid insulating withvulcanized rubber because this could introduce residual levels of sulfite to the testenvironment.2. All the glassware must be well dried before use.3. If a sample contains a high content of added sulfites, the HO solution may turn red 22rapidly befo

27、re heating.Page 5 of 6 October 2010ASTA ANALYTICAL METHODS Method 30.0H. References1. Lafeuille, J.-L.; Lefevre, S.; Achouri, D. “Determination of Added Sulfites in DriedGarlic with a Modified Version of the Optimized Monier-Williams Method,” J. AOACIntl. 2007, 90, 1090-1097.2. Lafeuille, J.-L. (McC

28、ormick France, EMEA Central Analysis Laboratory, Carpentras,France) “Final Statistical Results of the Collaborative Study: Added Sulfite in DriedGarlic,” Special report to the European Spice Association and American Spice TradeAssociation, 23 June 2010.I. Revision History10/01/10 First version.Page 6 of 6 October 2010