Limit test for iron

Limit test for iron

1. LIMIT TEST FOR
IRON

2. PRINCIPLE
Thelimittestforironisbasedonthereactionofironin
ammoniacal solutioninpresenceofcitricacid,with
thioglycollicacidwhenapalepinktodeepreddishpurple
colorisformedduetoferrouscompounds.
Thioglycollic(HS)CH
2COOH,isusefulanalogueofglycollic
acid,CH
2(OH)COOH whichispreparedbytheactionof
potassiumhydrogensulphideonmonochloroaceticacid.

3. PRINCIPLE
Thecolourproducedfromaspecifiedamountof
substancefromthetestiscompared byviewing
verticallywithastandard(ferricammoniumsulphate).
Ifthecolourfromtestsolutionislessdarkthanthe
standard,thenthesamplepassesthetest.
Citricaciddoesnotallowtheprecipitationofironby
ammoniabyformingacomplexwithit.Thecolourdue
toferrouscompoundgetsdestroyedbyoxidisingagents
andalkalis.Sointerferenceofothermetalcationsis
eliminatedbymakinguseof20%citricacid,whichforms
acomplexwithothermetalcations.

4. PREPARATIONOFSTANDARDSOLUTIONOFIRON:Itispreparedby
adding0.173gofferricammoniumsulphate[NH
4Fe(SO
4)
2.12H
2O]
to1.5mlofHClandaddingsufficientwatertoproduce1000ml.
METHOD:Asolutionisfirstofallobtainedfromaspecifiedamount
ofsubstance.ItisthentakeninaNesslercylinderandtoit2mlof
20%citricacidsolutionand2dropsofthioglycollicacidare
added.
Thesolutionaremixed,andmadealkalinewithasolutionofiron
freeammonia(Ferrousthioglycolateiscolourlessinacidor
neutralsolutions.InacidicmediaFe
2+
isoxidisedtoFe
3+
in
presenceofthioglycollicacidhencesolutionbecomescolourless.
Thepurplecolourisdevelopedonlyinthepresenceofalkali)and
dilutedto50mlmarkwithwater.
Thecolourobtainediscomparedwithstandardpreparedfrom
2mlofstandardsolutionsofiron,with50mlofwaterinNessler
cylinderfollowingthesameprocedureasforthetestsolution.

5. Ifthecolourproducedbytestsolutionhasbeenlessthanthat
ofstandard,thesamplepassesthelimittestforiron.