Limit tests

Inorganic chemistry

2. LIMITTEST
•Thequantitativeorsemiquantitativetestdesignedtoidentifyandcontrolsmall
quantityofimpuritywhichislikelytobepresentinthesubstance.
•Purpose:Todeterminetheinorganicimpuritiespresentinthecompound.
•TYpes:Limittestforchlorides,sulphates,iron,leadandheavymetalarecarried
outinNesslercylinders.
•Apparatus:Nesselercylinders

3. Limittestforchloride
Principle:Basedonthereactionbetweensilvernitrateandsolublechloridestoobtain
silverchloridewhichisinsolubleindilutenitricacid.
Thesilverchlorideproducedinthepresenceofdilutenitricacidmakesthetestsolution
turbid.
Theextentofturbiditydependingupontheamountofchloridepresentinthesubstanceis
comparedwithastandardopalescenceproducedbyadditionofsilvernitratetoastandard
solutionhavingaknownamountofchlorideandthesamevolumeofdilutenitricacidas
usedinthetestsolution.
Iftheturbidityfromthesamplehasbeenlessthanthestandardturbidity,thesamplewill
passthelimittestandviceversa.
Cl
-
+AgNO
3 AgCl+NO
3
-

4. Preparation of test solution Preparation of standard solution
Specified weight of the substance
+
Water
(quantity as per directed in
pharmacopoeia)
+ 1 ml Nitric acid
Volume make up upto 50 mL with
water
+
1 ml Silver nitrate solution, Stirr with
glass rod and keep aside for 5 min.
1 ml of 0.05845% w/v solution of sodium
chloride ( in Nessler’s cylinder B)
+
1 ml Nitric acid
Volume make up upto 50 mL with
water
+
1 ml Silver nitrate solution, Stirr with
glass rod and keep aside for 5 min.
Method:
Transfer to Nessler’s cylinder A

5. Observationandresult:
Theopalescenceproducedbythesample(incylinderA)shouldnotbegreaterthanstandard
opalescence.Iftheopalescencefromthesamplehasbeenlessthanthestandardopalescence,the
samplewillpassthelimittest.

6. Limittestforsulphate
Principle:Thereactionbetweenbariumchlorideandsolublesulphatesinpresenceof
dilutehydrochloricacid.
ThebariumchloridetestsolutionintheI.P.hasbeenreplacedbyBariumSulphateReagent
whichishavingbariumchloride,sulphate-freealcohol,andasolutionofpotassium
sulphate.Potassiumsulphatehasaddedtoincreasethesensitivityofthetest.Theionic
concentrationsinthereagenthasbeensoadjustedthatthesolubilityproductofbarium
sulphategetsexceeded,andtheverysmallamountofsulphateispresentinthereagentacts
asaseedingagentforprecipitationofbariumsulphate,ifsulphateispresentinthe
substanceundertest.
Alcoholhelpstopreventsuper-saturation.Thesubstancepassesthelimittestifitproduces
turbiditythatislessthanstandard.
SO
4
2-
+BaCl
2 BaSO
4+2Cl
-

7. Preparation of test solution Preparation of standard solution
Specified weight of the substance
+
Water
(quantity as per directed in
pharmacopoeia)
+ 2 ml Hydrochloric acid
Volume make up upto 45 mL with
water
+
5 ml Barium sulphate reagent, stirr
with glass rod and keep aside for 5
min.
1 ml of 0.1089 per cent w/v solution of
potassium sulphate ( in Nessler’s
cylinder B)
+
2 ml Hydrochloric acid
Volume make up upto 45 mL with
water
+
5 ml Barium sulphate reagent, stirr
with glass rod and keep aside for 5
min.
Method:
Transfer to Nessler’s cylinder A

8. Observationandresult:
Theopalescenceproducedbythesample(incylinderA)shouldnotbegreaterthanstandard
opalescence.Iftheopalescencefromthesamplehasbeenlessthanthestandardopalescence,the
samplewillpassthelimittest.

9. LimittestforIron
Principle:Basedonthereactionofironinarnmonicalsolutionwiththioglycollicacidinthe
presenceofcitricacid.
AformationofpalepinktodeepreddishpurplecolourduetoformationofFerrous
thioglycollatecompoundsindicatingthespecifiedamountofsubstancefromthetestis
comparedbyviewingverticallywithastandard(ferricammoniumsulphate).
Ferrousthioglycollateiscolourlessinacidorneutralsolution.InacidicmediaFe2+is
oxidizedtoFe3+inpresenceofthiglycollicacid.Hencethesolutionbecomescolourless.The
purplecolourisdevelopedonlyinthepresenceofalkalie.Ifthecolourfromtestsolutionis
lessdarkthanthestandard,thenthesamplepassesthetest.Fe
2+
+2
COOH
SHCH
2
OOC
CH
2
SH
CH
2
SH
COO
Fe +
2H
+

10. Citricaciddoesnotallowtheprecipitationofironbyammoniabyformingacomplexwithit.
Thecolourduetoferrouscompoundgetsdestroyedbyoxidisingagentsandalkalies.The
thioglycollicacidreducesiron(III)toiron(II).
Thioglycollicacid,isaanalogueofglycollicacid,whichispreparedbytheactionofpotassium
hydrogensulphideonmonochloroaceticacid.
Itisacolourlessliquidhavingunpleasantodour.Thistestisverysensitive.Interferenceof
othermetalcationsiseliminatedbymakinguseof20%citricacid,whichformsacomplexwith
othermetalcations.

11. Preparation of test solution Preparation of standard solution
Specified weight of the substance
+
40 mL of water
(quantity as per directed in
pharmacopoeia)
+ 2 ml of 20% solution of iron free citric
acid and 0.1 mL thioglycollic acid
Make alkaline with iron free ammonia
solution
+
Volume make up upto 50 mL with water,
stirr with glass rod and keep aside for 5
min.
Method:
Transfer to Nessler’s cylinder A
2 ml of standard iron solution +
40 mL of water
(quantity as per directed in
pharmacopoeia)
+ 2 ml of 20% solution of iron free citric
acid and 0.1 mL thioglycollic acid
Make alkaline with iron free ammonia
solution
+
Volume make up upto 50 mL with water,
stirr with glass rod and keep aside for 5
min.
Transfer to Nessler’s cylinder B

12. Standardironsolution:Itispreparedbyadding0.173gofferricammoniumsulphate
[NH
4Fe(SO
4)
2.12H
2O]to1.5mlofHClandaddingsufficientwatertoproduce1000ml.Each
mlofsolutioncontains0.02mgofiron.
Observationandresult:
Thecolourproducedbythesample(incylinderA)shouldnotbegreaterthanstandard
colour.Ifthecolourfromthesamplehasbeenlessthanthestandardcolour,thesamplewill
passthelimittest.

13. LimittestforArsenic
Apparatus:Itishavingawidemouthedbottle
of120mlcapacityhavingmouthofabout2.5
cmindiameter.
Thebottleisfittedwithrubberbungthrough
whichglasstubepasses.
Theglasstubeis2cmlonghavinganexternal
diameterofabout0.8cmandinternaldiameter
ofabout0.65cm.
Thetubeisconstrictedatitslowerendtoabout
1mmdiameterwithaholeinthesideofthe
tubeneartheconstrictedpartnotlessthan2
mmdiameter.
Theupperendoftheglasstubehasbeenfitted
withtworubberbungs,eachhavingahole
boredcentrallyandexactly6.5minin
diameter.
Gutzeit apparatus

14. Principle:Thechemicalreactionsinvolvedinthemethodareasfollows:whenthesampleis
dissolvedinacid,thearsenicpresentinsamplegetsconvertedtoarsenicacidwhichgetsreduced
byreducingagentslikepotassiumiodide,stannousacidetc.toarseniousacid.Thenascent
hydrogenformedduringthereactionfurtherreducesarseniousacidtoarsine(gas),whichreacts
withmercuricchloridepapertogiveyellowstain.
H
3AsO
3 + 3H
2 AsH
3+ 3H
2O
H
3AsO
4 + H
2SnO
2 H
3AsO
3 + H
2SnO
3
H
3AsO
3+ 3H
2 AsH
3+ 3H
2O

15. Preparation of test solution Preparation of standard solution
5 mL of test substance + 1 g KI As T + 5 mL
stannous chloride + 10 g of zinc As T (glass
tube is kept in position)
Yellow stain produced on HgCl
2paper
Method:
Action is allowed for 40 min
1 ml of standard arsenic solution + 1 g KI As T + 5
mL stannous chloride + 10 g of zinc As T
(glass tube is kept in position)
Yellow stain produced on HgCl
2paper
Action is allowed for 40 min
Note***
Standardarsenicsolution:Arsenicsolutionishaving0.132gofAs
2O
3per100mlof
solution.Onemlofstandardsolutionisdilutedwithwatertomake100mlofdilute
arsenicsolution.

16. Observationandresult:
Thestainproducedbythesampleshouldnotbegreaterthanstandardstain.Ifthestain
fromthesamplehasbeenlessthanthestandardstain,thesamplewillpassthelimittest.

17. LimittestforLead
Principle:Limittestofleadisbasedonthereactionofleadanddiphenylthiocarbazone(dithizone)
inalkalinesolutiontoformleaddithizonatecomplex.
•ThetestdescribedinB.P.isdependentupontheformationofbrownishcolourationwhensodium
sulphideisaddedtodilutesolutionofleadsalts,theintensityofthecolourationvaryingwiththe
quantityofleadpresent.
•Iftheleadispresentinmorethantraces,acolloidalbrownishblackprecipitateofleadsulphide
isformedinsteadofthecolour.Thecoloursoobtainedismatchedagainststandardcolour
producedfromaknownamountoflead.Pb
2+
+
N=NC
6
H
5
S
NH.NHC
6
H
5
N=NC
6
H
5
S
NH.NHC
6
H
5
Pb
H
5
CN
6
=N
S
NH.NHC
6
H
5
Lead ion
Dithiazone
Lead dithiazonate

18. Preparation of primary solution Preparation of auxillary solution
4 g of substance is dissolved in water having
10 ml of acetic acid PbT
Make alkaline with PbT ammonia solution
+
1 ml of solution of potassium cyanide PbT
Filter the solution if they get turbid, Dilute
the solution to 50 ml with water
+
2 drops of solution of sodium sulfide PbT
and stir.
Method:
2 g of substance is dissolved in water having
7 ml of acetic acid PbT
+
2 ml of dilute solution of lead PbT
Make alkaline with PbT ammonia solution
+
1 ml of solution of potassium cyanide PbT
Filter the solution if they get turbid, Dilute
the solution to 50 ml with water
+
2 drops of solution of sodium sulfide PbT
and stir.

19. Observationandresult:
Comparethecoloursofprimaryandauxiliarysolution.Ifthecolourproducedinprimary
solutionislessthanthatofauxiliary,thesamplepassthelimittestoflead

20. LimittestforHeavymetals
Principle:Thequantityisstatedastheheavymetalslimitandisexpressedaspartsoflead
(byweight)permillionpartsofthetestsubstance.
Thelimittestforheavymetalshasbeenbaseduponthereactionofthemetalionwith
hydrogensulphide,undertheprescribedconditionsofthetestcausingtheformationof
metalsulphides.
Themetallicimpuritiesinsubstancesareexpressedaspartsofleadpermillionpartsofthe
substance.TheusuallimitasperI.P.hasbeen20ppm.
TheIndianPharmacopoeiahasadoptedthreemethodsforthelimittestsforheavymetals.
The'MethodI'isusedforthesubstancewhichgivesaclearcolourlesssolutionunder
specifiedconditions.'MethodII'isusedforthosesubstanceswhichdonotgiveclear
colourlesssolutionunderthetestconditionsspecifiedforMethodI.'MethodIII'is
usedforsubstancesthatgiveclearcolourlesssolutioninsodiumhydroxidemedium.

21. Preparation of test solution Preparation of standard solution
25 ml of the solution which is prepared as
per the procedure given under respective
monograph from I.P
Adjust the pH between 3 to 4 by adding
dilute acetic acid or dilute ammonia
solution1 ml of solution of potassium
cyanide PbT
Dilute with water to about 35 ml mix.
+
10 ml of freshly prepared hydrogen sulphide
solution, Mix well. Dilute with water to 50 ml.
Allow to stand for five minutes
MethodI:Forclearcolorlesssolutions
2 ml of standard lead solution, dilute it with
water top roduce 25 ml
Adjust the pH between 3 to 4 by adding dilute
acetic acid or dilute ammonia solution1 ml
of solution of potassium cyanide PbT
Dilute with water to about 35 ml mix.
+
10 ml of freshly prepared hydrogen
sulphide solution, Mix well. Dilute with
water to 50 ml. Allow to stand for five
minutes

22. MethodII:ThestandardsolutioncanbepreparedasdirectedundermethodI.
Testsolutionispreparedbyweighingaspecifiedquantityofsubstanceinacrucible.Moistenthe
substancewithsulphuricacid,igniteonalowflametillcompletelycharred.
Addfewdropsofnitricacidandheatto500°C.Allowtocool,add4mlofhydrochloricacidand
evaporatetodryness.
Moistentheresiduewith10mlofhydrochloricacidanddigestfortwominutes.Neutralizewith
ammoniasolutionandmakejustacidicwithaceticacid.
AdjustthepH1between3.0and4.0,filterifnecessary.Adjustthevolumeoffiltrateto35mlin
Nessler'scylinder,add10mlofhydrogensulphidesolution,diluteto50mlwithwaterand
comparethecolourwiththestandardsolution.

23. MethodIII:Thestandardsolutionispreparedbyusing2mlofstandardleadsolution,adding5ml
ofdilutesodiumhydroxidesolutionandmakingthevolumeto50mlwithwater.
•Forthetestsolutiontakeeither25mlsolutionpreparedasdirectedinmonographortake
specifiedquantityofsubstance,dissolvein20mlofwater,add5mlofdilutesodiumhydroxide
solutionandmakeupthevolumeto50ml.
•Toeachoftheabovesolutioninnesselrscylinderadd5dropsofsodiumsulphidesolution,mix
andsetasidefor5minutes.Thecolourproducedbytestsolutionisnotdarkerthanthestandard
solution..

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