二氧化硫检测方法.docx

二氧化硫检测方法.docx

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二氧化硫检测方法

二氧化硫检测方法

ASTAANALYTICALMETHODSMethod30.0

DeterminationofAddedSulfitesinDriedAllium（ModifiedMonier-WilliamsMethod）

Purpose:

ThismodificationoftheMonier-Williamsmethodissuitablespecificallyfordetectionofsulfitesindriedallium（i.e.garlic,onion,shallot,leek,andchive）（Reference1）.Organosulfurcomponentsareremovedinatoluenetrapbeforesulfurdioxideiscollectedandoxidizedtosulfuricacidwithhydrogenperoxideinasecondtrap.Sulfuricacidistitratedwithstandardsodiumhydroxidesolution.Thelimitofquantitationis10μg/g.Sulfitesinnon-alliumfoodsshouldbe

analyzedbytheoptimizedMonier-WilliamsmethodasdescribedinAOAC990.28orEN1988-1.

A.Apparatus

1.Distillationapparatus:

asdescribedinEN1988-1（1998）orAOAC990.28（2000）and

withadualbubble-throughsystem（seeFigure）.

2.1000mLround-bottomflaskwiththree29/32（or24/40）taperedjoints（verticalarms）

andappropriateheatingmantle.

3.250mLdroppingfunnelwithstopcockandtaperedjointtofitroundbottomflask.

4.25mLburet.

5.Graduatedcylindersof25,50,100and500mL.

6.1Lvolumetricflask.

7.3mLgraduatedpipet.

.Cryostatsetat-5?

C.8

9.pHmeter.

B.Reagents

1.Deionizedwater.

2.0.01NNaOHsolution.

3.Indicator:

Dissolve250mgofmethylredin100mLofethanol（analyticalgrade）.This

indicatorisredforpH<4.4andyellowforpHvalue>6.2.

4.30%w/wor3%w/whydrogenperoxideHOsolution,analyticalgrade.22

5.37%concentratedhydrochloricacidHClforanalysis.

6.85%phosphoricacidHPOforanalysis.34

Page1of6October2010

ASTAANALYTICALMETHODSMethod30.0

7.85%potassiumhydroxideKOHinpelletsforanalysis.

8.PotassiumdihydrogenphosphateKHPOofpurity>99%foranalysis.24

9.Toluene,analyticalgrade.

10.Cryostatcirculatingfluid:

mixtureofmonopropyleneglycol/water（2/1）.

11.Nitrogenflowwithprecisionvalveallowingaflowrateofabout200mL/minute.

12.Buffersolution

SolutionA（acidpart）:

0.59MHPOand0.96MHClsolution.Introduceapproximately34

200mLofwaterintoa1Lvolumetricflask,carefullyadd40mLof85%HPO34

followedby80mLof37%HClwith,respectively,a50and100mLgraduatedcylinder

and,finally,dilutetovolumewithwater.

SolutionBbasepart:

0.25MKHPOand0.258MKOHsolution.Introduce34g24

KHPOand17gKOHintoa1Lvolumetricflaskanddilutetovolumewithwater.24

CheckthatthebuffercomponentsareproperlymadebymixingsolutionsAandsolution

Bina1:

4ratio（e.g.15mLofsolutionAand60mLofsolutionB）.ThepHmustrange

from2.3to2.5.Deoxygenatethese2solutionsbybubblingnitrogenthroughthemfor15

min,lessthan1hbeforeuse.

13.Hydrogenperoxide（3%,HO）solution.Intoa50-mLgraduatedcylinder,add3mLof22

30%HOsolutionwithagraduatedpipetandwatertogiveavolumeof30mL.22

Alternatively,onemaypurchaseanduse30mLof3%HOsolutiondirectlyforthis22

purpose.Add3dropsofmethylredindicatorusingaPasteurpipet.Withstirring,add1

to5dropsof0.01NNaOHanduntilthecoloroftheindicatorjustturnsyellow.This

solutionshouldbepreparedfreshdaily.

C.PreparationofSample

Driedalliumpowderscanbetestedforevidenceofaddedsulfitewithoutpreparationbutquantitativeprecisionwillbeimprovedifpowdersarefinelygroundtoimprovehomogeneity（seeStatisticssection）.

D.Procedure

PartA.Collectionofsulfitefromthesample.

1.Startcirculatingthecoolantinthecondenser,setthecryostatto-5?

Candwaitfor

approximately30minutesuntilthistemperatureisreached.（Note1）

Page2of6October2010

ASTAANALYTICALMETHODSMethod30.0

2.Weighapproximately25goftestsample,tothenearest0.1g,intoadryround-bottom

flask.（Note2）

3.Placetheround-bottomflaskintheheatingmantle.Connectthedroppingfunnel

（stopcockclosed）,thecondenserandthenitrogeninlettotheround-bottomflaskandthe

chainof2bubblerstotheuppercondenseroutletasshownintheFigure.

4.Add25mLtolueneintothefirstbubblerandplacethe50mLgraduatedcylinderwith3%

HOsolutionintopositionasthesecondbubbler（seeFigure）.22

5.Add120mLofsolutionAviathedroppingfunnelwitha250mLgraduatedcylinder.If

thesystemissealedproperly,gasshouldbegintoflowthroughthebubblersasliquid

enterstheroundbottomflask.

6.Lowertheheightofthesecondbubblercylindertoimprovetheflowandtoensurethatno

HOsolutiongoesupinthebubblercontainingtoluene.Thetipofthebubblertube22

shouldbesubmergedatleast1cmbelowthesurfaceoftheHOsolution.22

7.ClosethedroppingfunneltapwhenafewmillilitersofsolutionAremaininthefunnel

andraisethesecondbubblercylindersuchthatthetipofthesecondbubbler