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Analytical Chemistry
Unit 53 Qualitative analysis — detecting the presence of inorganic chemical species
Unit 54 Tests for functional groups, separation and purification of compounds
Unit 55 Quantitative methods of analysis
Unit 56 Instrumental analytical methods
Unit 57 Contribution of analytical chemistry to our society
Topic16
KeyC o ncepts
Quantitative methods of analysis• Gravimetric analysis• Precipitation titrations• Redox titrations involvingKMnO4
• Redox titrations involving I2
Qualitative analysis — detecting the presence of inorganic
chemical species• Cations• Common gases andvapours• Anions• Devising a separation scheme• Risks associatedwith tests
Contribution of analytical chemistry to our society
• Food anddrug analyses• Air quality control• Forensic chemistry• Clinical laboratory tests
Tests for functional groups, separation and purification
of compounds• Tests for organic functional groups• Distillation and fractionaldistillation• Liquid-liquid extraction• Re-crystallization• Chromatography• Checking thepurityof aproduct
Instrumental analytical methods• Colorimetry• Infrared spectroscopy• Mass spectrometry
Analytical Chemistry
Topic 16� Analytical Chemistry �Unit 53 Qualitative analysis — detecting the presence of inorganic chemical species
53.1 – 53.21
Summary
1 The following table summarizes results of some tests for cations.
Ion Test ActionofNaOH(aq) ActionofNH3(aq) Other test(s)
Aluminiumion,Al3+
whiteprecipitate, soluble inexcessNaOH(aq) to give acolourless solution
Al3+(aq) + 3OH–(aq)Al(OH)3(s)
Al(OH)3(s) +OH–(aq) [Al(OH)4]
–(aq)
whiteprecipitate
Al3+(aq) + 3OH–(aq)Al(OH)3(s)
—
Ammoniumion,NH4
+colourless gas (NH3) givenoff onwarming
NH4+(aq) +OH–(aq)
NH3(g) +H2O(l)
— —
Calciumion,Ca2+
whiteprecipitate
Ca2+(aq) + 2OH–(aq)Ca(OH)2(s)
— flame test—brick-red flame
actionofH2SO4(aq)—whiteprecipitate
Ca2+(aq) + SO42–(aq)
CaSO4(s)
Copper(II)ion,Cu2+
blueprecipitate
Cu2+(aq) + 2OH–(aq)Cu(OH)2(s)
blueprecipitate, soluble inexcessNH3(aq) to give adeepblue solution
Cu2+(aq) + 2OH–(aq)Cu(OH)2(s)
Cu(OH)2(s) + 4NH3(aq) [Cu(NH3)4]
2+(aq) +2OH–(aq)
flame test—bluishgreen flame
Iron(II)ion, Fe2+
greenprecipitate, turningbrownon standing
Fe2+(aq) + 2OH–(aq) Fe(OH)2(s)
greenprecipitate, turningbrownon standing
Fe2+(aq) + 2OH–(aq) Fe(OH)2(s)
—
Iron(III)ion, Fe3+
reddishbrownprecipitate
Fe3+(aq) + 3OH–(aq) Fe(OH)3(s)
reddishbrownprecipitate
Fe3+(aq) + 3OH–(aq) Fe(OH)3(s)
—
Lead(II)ion, Pb2+
whiteprecipitate, soluble inexcessNaOH(aq) to give acolourless solution
Pb2+(aq) + 2OH–(aq) Pb(OH)2(s)
Pb(OH)2(s) + 2OH–(aq) [Pb(OH)4]
2–(aq)
whiteprecipitate
Pb2+(aq) + 2OH–(aq) Pb(OH)2(s)
actionofHCl(aq)—whiteprecipitate,soluble inhotwater
Pb2+(aq) + 2Cl–(aq) PbCl2(s)
actionofH2SO4(aq)—whiteprecipitate
Pb2+(aq) + SO42–(aq)
PbSO4(s)
Magnesiumion,Mg2+
whiteprecipitate
Mg2+(aq) + 2OH–(aq)Mg(OH)2(s)
whiteprecipitate
Mg2+(aq) + 2OH–(aq)Mg(OH)2(s)
—
Potassiumion,K+
— — flame test— lilacflame
Sodiumion,Na+
— — flame test—goldenyellow flame
53.1 Introduction to analytical chemistry
53.2 Identifying the cation in a sample
53.3 Appearance
53.4 Solubility inwater
53.5 Flame test
53.6 Action ofheat
53.7 Action ofdilutehydrochloric acid
53.8 Action ofdilute sulphuric acid
53.9 Action ofdilute aqueous solutionof sodiumhydroxide
53.10 Action ofdilute aqueous ammonia
53.11 Identifying the cation in an unknown sample in thelaboratory
53.12 Devising a scheme to separate the cations in an aqueoussolution
53.13 Specific tests for identifying commongases andvapours
53.14 Identifying the anion in a sample
53.15 Action ofheat
53.16 Action ofdilutehydrochloric acid
53.17 Action of concentrated sulphuric acid
53.18 Action of aqueous solutionof silvernitrate
53.19 Devising a scheme to separate anions in an aqueous solution
53.20 Identifying the cation and anion in an unknown sample inthe laboratory
53.21 Risks associatedwith chemical tests
Qualitative analysis — detecting the presence of inorganic chemical speciesUnit 53
Topic 16� Analytical Chemistry �Unit 53 Qualitative analysis — detecting the presence of inorganic chemical species
Ion
Test ActionofdiluteHCl(aq) orH2SO4(aq)
ActionofBaCl2(aq)Actionof
concentratedH2SO4(aq)
Actionofdilute
HNO3(aq),followedbyAgNO3(aq)
Otherreagent(s)
Iodideion, I–
— — purple vapour(I2, SO2 andH2S)givenoff
NaI(s) +H2SO4(l)NaHSO4(s)
+HI(g)
8HI(g) +H2SO4(l) 4I2(s) +
H2S(g) + 4H2O(l)
yellowprecipitate
Ag+(aq) +I–(aq)
AgI(s)
actionofCl2(aq) +organicsolvent—apurpleorganiclayerforms*
*Cl2(aq)oxidizesBr–(aq) / I–(aq) toBr2(aq) / I2(aq).
3 The following table summarizes specific tests for some common gases andvapours.
Gasor vapour Colour Test Observation
Ammonia Colourless
Insert a piece of moist red litmuspaper into the gas.
It turns moist red litmuspaperblue.
Dipaglassrodindilutehydrochloricacid and insert it into the gas.
Dense white fumes areformed.
Carbondioxide Colourless Bubble the gas through limewater. The limewater turnsmilky.
Chlorine Greenishyellow
Insert a piece of moist blue litmuspaper into the gas.
Chlorine turns the moistblue litmus paper red andthenbleaches it.
Hydrogen Colourless Insert a burning splint into the gas. A ‘pop’ sound isheard.
Oxygen Colourless Insert a glowing splint into the gas. Theglowing splint relights.
Sulphurdioxide Colourless
Insert a piece of filter paper soakedwithacidifiedpotassiumdichromatesolution into the gas.
Thepaperturnsfromorangeto green.
Water vapour Colourless Insert a piece of blue cobalt(II)chloridepaper into thevapour.
The blue cobalt(II) chlorideturnspink.
Ion Test Action ofNaOH(aq) ActionofNH3(aq) Other test(s)
Zinc ion,Zn2+
whiteprecipitate, soluble inexcessNaOH(aq) to give acolourless solution
Zn2+(aq) + 2OH–(aq)Zn(OH)2(s)
Zn(OH)2(s) + 2OH–(aq) [Zn(OH)4]
2–(aq)
whiteprecipitate, solublein excessNH3(aq) to give acolourless solution
Zn2+(aq) + 2OH–(aq)Zn(OH)2(s)
Zn(OH)2(s) + 4NH3(aq) [Zn(NH3)4]
2+(aq) +2OH–(aq)
—
2 The following table summarizes results of some tests for anions.
Ion
Test ActionofdiluteHCl(aq)orH2SO4(aq)
Action of BaCl2(aq)Action of
concentratedH2SO4(aq)
Actionofdilute
HNO3(aq),followedbyAgNO3(aq)
Otherreagent(s)
Carbonateion,CO3
2–colourless gas(CO2) givenoff
CO32–(aq) +
2H+(aq)CO2(g) +
H2O(l)
whiteprecipitatesoluble indiluteHCl(aq)withcolourless gas (CO2)givenoff
Ba2+(aq) +CO32–(aq)
BaCO3(s)
vigorous reaction,colourless gas(CO2) givenoff
— —
Hypochloriteion,OCl–
greenishyellowgas (Cl2) givenoff
OCl–(aq) +2H+(aq) +Cl–(aq)
Cl2(g) +H2O(l)
— — — —
Sulphiteion, SO3
2–colourless gas(SO2) givenoff
SO32–(aq) +
2H+(aq) SO2(g) +
H2O(l)
whiteprecipitatesoluble indiluteHCl(aq)withcolourless gas (SO2)givenoff
Ba2+(aq) + SO32–(aq)
BaSO3(s)
vigorous reaction,colourless gas(SO2) givenoff
— —
Sulphateion, SO4
2–— whiteprecipitate
insoluble indiluteHCl(aq)
Ba2+(aq) + SO42–(aq)
BaSO4(s)
— — —
Chlorideion,Cl–
— — steamy fumes(HCl) givenoff
NaCl(s) +H2SO4(l)NaHSO4(s) +
HCl(g)
whiteprecipitatesoluble inNH3(aq)
Ag+(aq) +Cl–(aq)
AgCl(s)
—
Bromideion,Br–
— — reddishbrownvapour (HBr,Br2and SO2) givenoff
NaBr(s) +H2SO4(l)NaHSO4(s) +
HBr(g)
2HBr(g) +H2SO4(l) Br2(g) +
SO2(g) + 2H2O(l)
creamyprecipitatesoluble inconcentratedNH3(aq)
Ag+(aq) +Br–(aq)
AgBr(s)
actionofCl2(aq) +organicsolvent—anorangeorganiclayerforms*
ExamtipsExamtipsExamtipsExamtips ♦ Barium ions give a green flame in flame test.
♦ Ag2O(s),NOTAgOH(s), forms whenNaOH(aq) is added toAgNO3(aq). ✔ ✘
2Ag+(aq)+2OH–(aq) Ag2O(s) +H2O(l) brown
♦ Questions often ask about identifying a solution of sulphate ions byusing an aqueous barium chloride.
♦ Questions often ask about identifying a solution of chloride ions byusing an aqueous solution of silver nitrate.
Topic 16� Analytical Chemistry �Unit 53 Qualitative analysis — detecting the presence of inorganic chemical species
♦ Chlorineandsulphurdioxideformanacidicsolutionwhendissolvedinwater.
♦ Thechemicaltestscanbeusedtodistinguishbetweendifferentinorganicchemical species.
e.g.
Ba2+(aq) and Pb2+(aq) can be distinguished by one of the followingmethods:
– adding HCl(aq); only Pb2+(aq) gives awhite precipitate.
– adding NaOH(aq); only Pb2+(aq) gives a white precipitate which issoluble in excessNaOH(aq).
– adding KI(aq); only Pb2+(aq) gives a yellowprecipitate (PbI2(s)).
Cl–(aq) and Br–(aq) can be distinguished by one of the followingmethods:
– adding Cl2(aq); only Br–(aq) gives a brown solution.
– adding acidified AgNO3(aq); Cl–(aq) gives a white precipitate whileBr–(aq) gives a creamyprecipitate.
Mg(NO3)2(aq)andAgNO3(aq)canbedistinguishedbyoneofthefollowingmethods:
– adding zinc strip; only AgNO3(aq) forms a deposit on zinc.
– adding KCl(aq); only AgNO3(aq) gives awhite precipitate.
Na2CO3(aq), KCl(aq) and HNO3(aq) can be distinguished by addingacidified AgNO3(aq). Only Na2CO3(aq) shows effervescence while onlyKCl(aq) gives awhite precipitate.
♦ Questionsoftenaskaboutidentifyingsolutionsofbromideions/iodideions by adding aqueous chlorine.
– Chlorine oxidizes bromide ions to bromine.
Cl2(aq)+2Br–(aq) 2Cl–(aq)+Br2(aq)
Thebrominedissolvesinanorganicsolventtoformanorangeorganiclayer.
– Chlorine oxidizes iodide ions to iodine.
Cl2(aq)+2I–(aq) 2Cl–(aq)+ I2(aq)
The iodine dissolves in an organic solvent to form a purple organiclayer.
♦ Aqueoussodiumcarbonatesolutioncanprecipitateinsolublecarbonatesfrom solutions containing some cations.
♦ Questions often ask about the action of aqueous solution of lead(II)nitrate onother aqueous solutions.
unknownsamplesolution
Al3+(aq),Ca2+(aq), Mg2+(aq),
Pb2+(aq) or Zn2+(aq)
add a few drops of Na2CO3(aq)
whiteprecipitate
forms
precipitateturns brown upon
heating
blueprecipitate
colouredprecipitate
forms
Cu2+(aq)Ag+(aq)*
greenprecipitate
Fe2+(aq) or Ni2+(aq)
reddish brown precipitate
Fe3+(aq)
* Ag2CO3 precipitate decomposes to Ag2O upon heating.
Adding Pb(NO3)2(aq) to
solution containingObservation Ionic equation(s)
Bromide ions, Br–
Chloride ions,Cl–white precipitate forms,soluble in hotwater togive a colourless solution
Pb2+(aq)+2Br–(aq) PbBr2(s)Pb2+(aq)+2Cl–(aq) PbCl2(s)
Iodide ions, I–yellowprecipitate forms,soluble in hotwater togive a colourless solution
Pb2+(aq)+2I–(aq) PbI2(s)
Sulphate ions, SO42– white precipitate forms Pb2+(aq)+ SO4
2–(aq) PbSO4(s)
Carbonate ions,CO3
2–
Sulphite ions, SO32–
white precipitate forms,soluble in dilute nitricacid
Pb2+(aq)+CO32–(aq) PbCO3(s)
Pb2+(aq)+ SO32–(aq) PbSO3(s)
Example
State the expected observation in each of the following experiments, and account fortheobservationwith the aidof chemical equation(s).
a) AddingNaOH(aq)dropwise toAl(NO3)3(aq)until in excess. (4marks)
b)Adding excess H2SO4(aq) to K2CrO4(aq), and then excess FeSO4(aq) to the resultingmixture. (4marks)
c) Warming amixtureof (NH4)2SO4(aq) andCuSO4(aq)with excessNaOH(aq). (4marks)
Answer
a) Awhiteprecipitate forms. (1)
Theprecipitatedissolves in excessNaOH(aq) to give a colourless solution. (1)
Al3+(aq) + 3OH–(aq) Al(OH)3(s) (1)
Al(OH)3(s) +OH–(aq) [Al(OH)4]–(aq) (1)
Topic 1610 Analytical Chemistry 11Unit 54 Tests for functional groups, separation and purification of compounds
b)YellowK2CrO4(aq) turnsorangeupon the additionofH2SO4(aq). (1)
2CrO42–(aq) + 2H+(aq) Cr2O7
2–(aq) +H2O(l) (1) yellow orange
A green solution resultswhenFeSO4(aq) is added to theorange solution. (1)
6Fe2+(aq) +Cr2O72–(aq) + 14H+(aq) 6Fe3+(aq) + 2Cr3+(aq) + 7H2O(l) (1)
orange green
c) A gas that turnsmoist red litmuspaperblue is evolved. (1)
NH4+(aq) +OH–(aq) NH3(g) +H2O(l) (1)
Ablueprecipitate forms. (1)
Cu2+(aq) + 2OH–(aq) Cu(OH)2(s) (1)
➤QuestionsoftenaskaboutthecolourchangewhenH2SO4(aq)orHCl(aq) isadded toK2CrO4(aq).
➤Questions often ask about the expected observation when an ammoniumsalt iswarmed withNaOH(aq).
RemarksRemarks*
54.1 Chemical tests for various functional groups in carboncompounds
54.2 Tests for the alkene functional group
54.3 Tests for the alcohol functional group
54.4 Tests for the aldehyde andketone functional groups
54.5 Tests for the carboxylic acid functional group
54.6 Identifying functionalgroup(s) inanunknownsample in thelaboratory
54.7 Separation andpurificationof compounds
54.8 Distillation
54.9 Fractional distillation
54.10 Liquid-liquid extraction
54.11 Re-crystallization
54.12 Chromatography
54.13 Paper chromatography
54.14 Column chromatography
54.15 Thin layer chromatography
54.16 Tests forpurityof aproduct
Tests for functional groups, separation and purification of compoundsUnit 54
Topic 1612 Analytical Chemistry 13Unit 54 Tests for functional groups, separation and purification of compounds
54.1 – 54.6
Summary
1 The following table summarizes results of chemical tests for the alkene functionalgroup.
Test Alkene functional group –C=C–
Aqueousbromine the yellow-brown aqueous bromine becomescolourless rapidly
Cold acidified dilute aqueous solution ofpotassiumpermanganate
the purple solution becomes colourlessrapidly
2 The following table summarizes results of chemical tests for functional groupscontainingoxygen.
* 1°, 2° and3° alcohols canbedistinguishedby treatingwith theLucas reagent.
Functionalgroup
Test
Alcohols
HO
WarmwithK2Cr2O7 /H3O+ 1° and 2° alcohols* — clear orange solution turns green almost
immediately
K2Cr2O7 / H3O+ K2Cr2O7 / H3O
+
RCH2OH CR H CR OH
O O
1° alcohol
2° alcohol
K2Cr2O7 / H3O+
CHOHR
R1
CR R1
O
3° alcohols*—noobservable change
Treatmentwith2,4-dinitrophenylhydrazine
—
WarmwithTollens’ reagent(an aqueous solutionofsilvernitrate in ammonia)
—
Iodoform test (warmwithiodine in an aqueoussolutionof sodiumhydroxide)
alcohols containing the CH3C
H
OH
group gives a bright yellowprecipitate
O–Na++ CHI3
iodoformC
OR
H
C CH3R
OH
I2 / NaOH
Treatmentwith aqueoussodiumhydrogencarbonatesolution
—
Warmwith ethanol in thepresenceof concentratedsulphuric acid, followedbypouring themixture intoan aqueous solutionofsodiumcarbonate
—
Functionalgroup
Test
Aldehydes
HC
O
Ketones
C
O
Carboxylicacids
OHC
O
WarmwithK2Cr2O7 /H3O
+
clear orange solution turns green
K2Cr2O7 / H3O+
CR H CR OH
O O— —
Treatmentwith2,4-dinitro-phenyl-hydrazine
ayellow to redprecipitate forms
(H)R’
RC
H2N
aldehyde or ketone H
NO2
H
NO2 + H2O
NO2
NO2
(H)R’
RC NO +
N N
—
WarmwithTollens’reagent (anaqueoussolutionof silvernitrate inammonia)
silvermirror formson thewall of reactionvessel
R C H + 2[Ag(NH3)2]+ + 3OH–
aldehyde
O
warmR C O–
O
+ 2Ag + 4NH3 + 2H2O
— —
Iodoformtest (warmwith iodinein anaqueoussolutionof sodiumhydroxide)
aldehydesandketoneswiththe CH3C
O
groupgivesabrightyellowprecipitate
R C
O
CH3 + 3I2 + 3OH– R C
O
CI3 + 3I– + 3H2O
R C
O
CI3 + OH– R C
O
O– + CHI3
—
Treatmentwithaqueoussodiumhydrogen-carbonatesolution
— — a colourless gas(CO2) givenoff
RCOOH+NaHCO3
RCOO–Na++CO2+H2O
Warmwithethanolin thepresenceofconcentratedsulphuricacid,followedbypouring themixture intoan aqueoussolutionof sodiumcarbonate
— — a sweet, fruity-smelling estercanbedetected
RCOOH+CH3CH2OH
H+
RCOOCH2CH3
+H2O
Topic 161� Analytical Chemistry 1�Unit 54 Tests for functional groups, separation and purification of compounds
Example
a) At room temperature, acyclic carbon compound X is a liquid. It has the followingcompositionbymass:
C 64.9% H 13.5% O 21.6%
Calculate the empirical formulaof compoundX.
(Relative atomicmasses:H=1.0,C=12.0,O=16.0) (2marks)
b)0.185g of X, upon complete vaporization, occupies 60.0cm3 at room temperatureandpressure.
Deduce themolecular formulaof compoundX.
(Molar volumeof gas at room temperature andpressure=24.0dm3mol–1) (2marks)
c) Compound X gives a yellow precipitate when warmed with iodine in an aqueoussolutionof sodiumhydroxide. It showspositive resultswhen treatedwithacidifiedK2Cr2O7(aq).
i) What is the chemical formulaof the yellowprecipitate. (1mark)
ii) What does its reaction with iodine indicate about the structure of compoundX? (2marks)
iii) Suggest apossible structureof compoundX. Explainyour answer. (2marks)
ExamtipsExamtipsExamtipsExamtips ♦ Students should be able to give the correct structure of 2,4-dinitrophenylhydrazine.
♦ The chemical tests canbeused to distinguish between compounds.
e.g.
The following five colourless liquids
CH3CH2OH CH3CHO (CH3)3COH CH3CH2COOH CH3(CH2)7OH
canbedistinguished in the following way:
– OnlyCH3(CH2)7OH is immiscible withwater.
– OnlyCH3CHOreactswith2,4-dinitrophenylhydrazinetogiveayellowtoredprecipitate /onlyCH3CHOgivesasilvermirrorwhenwarmedwith the Tollens’ reagent.
– Only CH3CH2COOH reacts with NaHCO3(aq) to give a colourlessgas.
– When warmed with K2Cr2O7 / H3O+, CH3CH2OH turns the orange
solution greenwhile (CH3)3COHdoes not.
Answer
a) Suppose we have 100g of compound X, so there are 64.9g of carbon, 13.5g ofhydrogen and21.6gof oxygen.
Carbon Hydrogen Oxygen
Massofelement in thecompound
64.9g 13.5g 21.6g
Numberofmolesof atomsthat combine
64.9g12.0gmol–1 =5.41mol
13.5g1.0gmol–1 =13.5mol
21.6g16.0gmol–1 =1.35mol (1)
Simplest ratioof atoms
5.41mol1.35mol
=4.0113.5mol1.35mol
=10.01.35mol1.35mol
=1.00 (1)
∴ the empirical formulaof compoundX isC4H10O.
b)Let (C4H10O)n be themolecular formulaof compoundX.
Numberofmolesof compoundXvaporized= 60.0cm3
24000cm3mol–1
= 0.00250mol
Molarmassof compoundX = 0.185g0.00250mol
= 74.0gmol–1 (1)
Molarmassof compoundX = n(4 x 12.0+10 x 1.0 +16.0) = 74 n = 1 (1)
∴ the molecular formulaof compoundX isC4H10O.
c) i) CHI3 (1)
ii) CompoundX contains a CH3
O
C (1) /
OH
CH3C
H
(1) group.
iii)As compoundX showspositive resultswhen treatedwith acidifiedK2Cr2O7(aq), it is not a ketone. Thus, it is probably a secondary alcohol containing a
OH
CH3C
H
group. (1)
Apossible structureof compoundX:
(1)
OH
CH3CH3CH2 C
H
➤The chemical formula of iodoform is CHI3,NOTCH3I. ✔ ✘
RemarksRemarks*
Topic 161� Analytical Chemistry 1�Unit 54 Tests for functional groups, separation and purification of compounds
54.7 – 54.16
Summary
1 The following table summarizes common separation and purification methods forliquid and solidproducts.
Typeofproduct Separationandpurificationmethod to employ
Liquidproduct
• simpledistillation
• fractionaldistillation
• liquid-liquid extraction
Solidproduct• re-crystallization
• chromatography
2 During liquid-liquid extraction, place the aqueous solution containing the liquidproductinaseparatingfunnel.Thenaddanorganicsolventthatisimmisciblewithwater, such as ethoxyethane.
ring with pieces of rubbertubing to cushion funnel
ethoxyethane layer
separating funnel
aqueous layer containing the product
3 A re-crystallizationprocess involves threemain stages:
Stage1 Choose a suitable solvent.
Stage2 Use the minimum amount of hot solvent to dissolve the crude solidproduct. Filter to remove the insoluble impurities.
Stage3 Allowthefiltratetocoolslowlyforcrystalstore-form.Collectthecrystalsby filtrationunder reducedpressure anddry them.
4 The following table summarizes the working principles of three chromatographicmethods.
Chromatographicmethod
Stationaryphase
Mobilephase Working principle
Paper water in thepaper fibres
liquidsolvent
Eachcomponentinamixturedistributes itselfbetween thewater in thepaper fibresand theliquid solvent.
Compoundsthataremoresolubleintheliquidsolvent travel up more quickly. Thus, thecomponents separate.
Columnadsorbent invertical glasscolumn
liquidsolvent
Each component in a mixture has its ownequilibrium between adsorption onto thesurface of the adsorbent and solubility in thesolvent.
Compounds that are more soluble in theliquid solvent move more quickly. Thus, thecomponents separate.
Thin layer
fine layer ofa l u m i n a o rsilicagelcoatedonto a glassplate
liquidsolvent
The component in amixture canbe identified from its retention ratio Rf.
Rf = distance travelledby the componentdistance travelledby the solvent
5 The purity of a solid product may be checked by its melting point and that of aliquidproductby its boilingpoint.
ExamtipsExamtipsExamtipsExamtips ♦ Remember the importance of releasing the pressure in the separatingfunnel regularly when shaking a liquid mixture in a separating funnel.
♦ Ethoxyethane is frequently used in liquid-liquid extraction due to thefollowing reasons:
– itischemicallyinertandunlikelytoreactwiththecarboncompounddissolved in it;
– it dissolves most carbon compounds;
– it is immiscible withwater;
– itcanbeeasilyremovedafterextractionbysimpledistillation,owingto its great volatility (boiling point 37°C).
♦ Paper chromatography can be used to separate Cu2+(aq) ions froma mixture containing Cu2+(aq) ions and Cr3+(aq) ions. This is becauseCu2+(aq) ions and Cr3+(aq) ions havedifferent colours.
Topic 161� Analytical Chemistry 1�Unit 54 Tests for functional groups, separation and purification of compounds
Example
Benzoincanbeobtainedbyheatingbenzaldehydewithpotassiumcyanideinasolvent-free condition.
OH
C
O
C
H
KCN
benzoin (m.p. 137 °C)
CHO2
a) What is the functionofpotassiumcyanide in this conversion? (1mark)
b)Give TWO reasons why this reaction can be considered as an example of greenchemistry. (2marks)
c) Describehow thepotassiumcyanide canbe removed from the crudeproduct. (2marks)
d)Thebenzoinobtained canbepurifiedby re-crystallization.
i) SuggestTHREE criteria for an appropriate solvent for the re-crystallization. (3marks)
ii) Outline theprocedure in the re-crystallizationprocess. (3marks)
iii) Suggestamethodtoverifywhetherornot there-crystallizedsampleofbenzoinis pure. (1mark)
Answer
a) As a catalyst (1)
b)Any twoof the following:
• The reactiondoesnot involve theuseof solvents. (1)
• The reactionhas ahigh atomeconomy. (1)
• A catalyst (KCN) is used. (1)
c) Extract benzoinwith an appropriateorganic solvent (e.g. ethoxyethane). (1)
Wash theorganic layerwithwater to remove thepotassiumcyanide. (1)
d)i) Any threeof the following:
• Benzoin should have a high solubility in the solvent while the impuritiesshouldnot. (1)
• The solubility of benzoin in the solvent should be high at high temperaturesbut lowat room temperature. (1)
• The solvent shouldbe volatile. (1)
• The solvent shouldnot reactwithbenzoin. (1)
ii) Dissolve the crudebenzoin in theminimumamountofhot solvent. (1)
Filter themixturewhilehot. (1)
Allow the filtrate to cool and collect the crystals by filtration. (1)
Wash the crystals anddry. (1)
➤Questionsoftenaskstudentstodescribethere-crystallizationprocedureforthepurification of a crude solid product.
– Use activated charcoal to remove coloured impurities.
– Use a short-stem funnel when filtering thehot solution.
➤Studentsshouldbeabletodrawadiagramoftheexperimentalset-upusedfor filtration under reduced pressure.
DrawaBuchner funnel (NOT a filter funnel). ✔ ✘
to aspirator
water trap Buchnerflask
Buchner funnelfilter paper
RemarksRemarks*
iii)Determinethemeltingpointoftheproductandcomparetheresultwithliteraturedata. (1)
Topic 1620 Analytical Chemistry 21Unit 55 Quantitative methods of analysis
55.1 Quantitative analysis
55.2 Steps of theprecipitationmethod in gravimetric analysis
55.3 Possible major sourcesof error
55.4 Gravimetricdeterminationofphosphoruscontentinasampleof fertilizer
55.5 Gravimetric determination of calcium content in a samplesolution
55.6 Precipitation titrations in volumetric analysis
55.7 Redox titrations in volumetric analysis
55.8 Redox titrations involving an aqueous solution of potassiumpermanganate
55.9 Determining the iron content in commercial iron tablets
55.10 Analyzingthequalityofwaterbydeterminingitspermanganateindex
55.11 DeterminingthevitaminCcontentinasampleusingaredoxtitration involving iodine
55.12 Determining the concentration of sodium hypochlorite in ahouseholdbleachusing an iodine / thiosulphate titration
Quantitative methods of analysisUnit 55
ExamtipsExamtipsExamtipsExamtips ♦ Todeterminethepercentagebymassof iron inasample,H2SO4(aq) isusedtoconvert the irontoFe2+(aq) ions.Thenthe resultingsolution istitrated against KMnO4(aq).
HCl(aq) is NOT used in this case because MnO4–(aq) ions may oxidize
theCl–(aq) ions.
55.1 – 55.10
Summary
1 Examplesof gravimetric analysis:
a) determiningthephosphoruscontentinasampleoffertilizerbyprecipitatingthephosphorus asmagnesiumammoniumphosphate (MgNH4PO4•6H2O);
b)determiningthecalciumcontentinasamplesolutionbyprecipitatingthecalciumas calciumoxalate (CaC2O4•H2O).
2 The concentration of chloride ions in a sample solution can be determined byprecipitation titrationwith a standard aqueous solutionof silvernitrate.
Ag+(aq) + Cl–(aq) AgCl(s)
Useachromateindicatorinthetitration.Whenallthechlorideionsareprecipitated,thefirstexcesssilvernitratesolutiongivesareddishbrownsilverchromateprecipitatewith the chromate indicator. This signals the endpointof the titration.
2Ag+(aq) + CrO42–(aq) Ag2CrO4(s)
reddishbrown
3 Redox titration involving an aqueous solution of potassium permanganate can beused todetermine the iron content in commercial iron tablets.
MnO4–(aq) + 8H+(aq) + 5Fe2+(aq) Mn2+(aq) + 5Fe3+(aq) + 4H2O(l)
Example
Awater-soluble solid sampleXcontains tin(II) ions.Thepercentagebymassof tin inX canbedetermined from the experimental data listedbelow:
Step 1 25.0cm3of0.0326moldm–3Na2C2O4(aq),afteracidification, requires26.5cm3of a certainKMnO4(aq) for complete reaction.
Step 2 An aqueous solution of 1.05g of X, after acidified, requires 32.2cm3 of thesameKMnO4(aq) for complete reaction.
a) i) Write an equation for the reaction thatoccurs in Step 1. (1mark)
ii) Describehow the titration endpoint canbedetected in Step 1. (1mark)
iii)Calculate the concentrationof theKMnO4(aq)used. (2marks)
b)The equation for the reaction involved in Step 2 is as follows:
5Sn2+(aq) + 2MnO4–(aq) + 16H+(aq) 5Sn4+(aq) + 2Mn2+(aq) + 8H2O(l)
Calculate thepercentagebymassof tin in solid sampleX.
(Relative atomicmass: Sn=118.7) (3marks)
Topic 1622 Analytical Chemistry 23Unit 55 Quantitative methods of analysis
c) StateONEassumption for the experiment. (1mark)
Answer
a) i) 2MnO4–(aq) + 5C2O4
2–(aq) + 16H+(aq) 2Mn2+(aq) + 10CO2(g) + 8H2O (1)
ii) KMnO4(aq)servesasitsownindicator.AddKMnO4(aq)untilthefirstappearanceof apersistentpalepink colour. (1)
iii)2MnO4–(aq) + 5C2O4
2–(aq) + 16H+(aq) 2Mn2+(aq) + 10CO2(g) + 8H2O(l)
?moldm–3 0.0326moldm–3
26.5cm3 25.0cm3
NumberofmolesofC2O42– ions in25.0cm3 solution
= 0.0326moldm–3 x 25.01000
dm3
= 8.15 x 10–4mol (1)
According to the equation, 2molesofMnO4– ions reactwith5molesofC2O4
2–ions.
i.e. numberofmolesofMnO4– ions in26.5cm3 solution
= 25
x 8.15 x 10–4mol
= 3.26 x 10–4mol
ConcentrationofKMnO4(aq)=3.26 x 10–4mol
26.51000
dm3
= 0.0123moldm–3 (1)
b)5Sn2+(aq) + 2MnO4–(aq) + 16H+(aq) 5Sn4+(aq) + 2Mn2+(aq) + 8H2O(l)
1.05g 0.0123moldm–3
32.2cm3
NumberofmolesofMnO4– ions in32.2cm3 solution
=0.0123moldm–3 x 32.21000
dm3
= 3.96 x 10–4mol (1)
According to the equation, 5molesof Sn2+ ions reactwith2molesofMnO4– ions.
i.e. numberofmolesof Sn2+ ions = 52
x 3.96 x 10–4mol
= 9.90 x 10–4mol
Massof Sn in solid sample= 9.90 x 10–4mol x 118.7gmol–1
= 0.118g (1)
Percentagebymassof Sn in solid sample= 0.118g1.05g
x 100%
= 11.2% (1)
c) Xdoesnot contain anyother species that can reactwithKMnO4(aq). (1)
➤It is necessary to standardize KMnO4(aq) prior to its use for titrimetricanalysis.
➤StudentsshouldbeabletodescribehowtheendpointinatitrationinvolvingKMnO4(aq) canbedetected.
RemarksRemarks*
ExamtipsExamtipsExamtipsExamtips ♦ Questions often ask about titrations involving iodine.
♦ The reaction below can be used to determine the concentration of asodium thiosulphate solution:
IO3–(aq)+5I–(aq)+6H+(aq) 3I2(aq)+3H2O(l)
– FirstpreparestandardIO3–(aq)bydissolvingaknownmassofKIO3(s)
in distilled water and diluting the solution to a known volume.
– AddexcessKI(aq)andH2SO4(aq)toaknownvolumeofthestandardIO3
–(aq) to obtain standard I2(aq).
– Titrate the I2(aq) against theNa2S2O3(aq).
Studentsshouldknowthatstandardiodinesolutioncanbepreparedbyadding excess KI(aq) and H2SO4(aq) into a known volume of standardIO3
–(aq) (refer to the example in Unit 57).
55.11 – 55.12
Summary
1 The concentration of a reducing agent in a sample solution can be determined bymeasuring the amount of iodine required to undergo a redox reaction with thereducing agent in a knownvolumeof the sample.
reducing agent+ I2 I– +otherproducts
Asaqueousiodinesolutionisunstable,iodinecanbegeneratedin situbythereactionof iodate ions (IO3
–)with iodide ions in an acidic solution:
IO3–(aq) + 5I–(aq) + 6H+(aq) 3I2(aq) + 3H2O(l)
2 To determine the concentration of an oxidizing agent in a sample solution, allowtheoxidizingagent inaknownvolumeof thesamplesolutionto reactwithexcessiodide ions to liberate iodine. Then titrate the iodine liberated against standardNa2S2O3(aq).
oxidizing agent Stage 1 being analyzed + excess I– I2 + otherproducts in sample solution
Stage 2 I2 + 2S2O32– 2I– + S4O6
2–
knownvolume and concentration
Topic 162� Analytical Chemistry 2�Unit 55 Quantitative methods of analysis
Example
Inanexperimenttodeterminethepercentagebymassofcopperinasampleofcopperore, 4.20g of the sample were warmed with excess HNO3(aq) to convert all copper-containing substances to Cu2+(aq) ions. The resulting solution was boiled to removeallnitrogenoxides formed.
Uponcooling,thesolutionwasdilutedto250.0cm3.25.0cm3ofthedilutedsolutionwerewithdrawnandtreatedwithexcessKI(aq)toliberateCuI(s)andI2(aq).Thetitrationof the I2(aq) liberated required27.7cm3of 0.112moldm–3Na2S2O3(aq).
a) Write equations for (2marks)
i) the reactionofCu2+(aq)with I–(aq); and
ii)the reactionof I2(aq)withNa2S2O3(aq).
b)SuggesthowtheendpointofthetitrationbetweentheI2(aq)liberatedandNa2S2O3(aq)canbedetected. (2marks)
c) Calculate thepercentagebymassof copper in the sampleof copperore.
(Relative atomicmass:Cu=63.5) (4marks)
Answer
a) i) 2Cu2+(aq) + 4I–(aq) 2CuI(s) + I2(aq) (1)
ii)I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O6
2–(aq) (1)
b)TitratetheiodineliberatedagainstNa2S2O3(aq)untilthecolourofthemixturechangestopale yellow. (1)
Atthispoint,addafewdropsofstarchsolution,producingadarkbluecolour.AddNa2S2O3(aq)dropwiseuntil themixturebecomes colourless. (1)
c) I2(aq) + 2S2O32–(aq) 2I–(aq) + S4O6
2–(aq)
?mol 0.112moldm–3
27.7cm3
Numberofmolesof S2O32– ions reactedwith iodine liberated
=0.112moldm–3 x 27.71000
dm3
=3.10 x 10–3mol (1)
According to the equation, 1moleof I2 reactswith2molesof S2O32– ions.
i.e. numberofmolesof I2 reactedwith S2O32– ions
= 3.10 x 10–3
2mol
= 1.55 x 10–3mol
=numberofmolesofI2liberatedinthereactionbetweenCu2+ionsin25.0cm3ofdiluted solution andKI(aq)
2Cu2+(aq) + 4I–(aq) 2CuI(s) + I2(aq)
4.20g 1.55 x 10–3mol 250.0cm3
(used) 25.0cm3
➤The titrationbetween the I2(aq) liberatedandNa2S2O3(aq) is carriedoutbyaddingNa2S2O3(aq) to I2(aq),NOT the reverse order.
➤Questionsoften askhow the endpointof the titrationbetween the I2(aq)liberated and Na2S2O3(aq) can be detected (refer to Fig.55.23 of thetextbook).
RemarksRemarks*
According to the equation, 2molesofCu2+ ions react to give1moleof I2.
i.e. numberofmolesofCu2+ ions in25.0cm3diluted solution = 2 x 1.55 x 10–3mol = 3.10 x 10–3mol (1)
NumberofmolesofCu2+ ions in250.0cm3dilute solution =10 x 3.10 x 10–3mol =3.10 x 10–2mol (1) =numberofmolesofCu in the sampleof copperore
MassofCu in the sampleof copperore= 3.10 x 10–2mol x 63.5gmol–1
= 1.97g
PercentagebymassofCu in the sampleof copperore = 1.97g4.20g
x 100%
= 46.9% (1)
Topic 162� Analytical Chemistry 2�Unit 56 Instrumental analytical methods
56.1 The useof instruments in analytical chemistry
56.2 Electromagnetic radiation
56.3 The interactionof radiationwithmatter
56.4 Colorimetry
56.5 Infrared spectroscopy
56.6 Basic featuresof an infrared spectrum
56.7 Characteristicabsorptionwavenumberrangesfordifferentbondsin carbon compounds
56.8 Using infrared spectrum in the identification of bonds (orfunctional groups) in carbon compounds
56.9 Interpreting infrared spectra
56.10 Mass spectrometry
56.11 Finding relative atomic masses and relative molecular massesfrommass spectra
56.12 Using fragmentation patterns to determine the structures ofmolecules of carbon compounds
56.13 Fragmentation patternsof alkanes
56.14 Fragmentation patternsof aromatic compounds
56.15 Fragmentation patternsof aldehydes andketones
56.16 Differences inmass/charge ratiobetweenpeaks
56.17 Interpreting mass spectra
Instrumental analytical methodsUnit 56 56.1 – 56.17
Summary
1 a) Colorimetry determines the concentration of a substance based on its ability toabsorb certainparts of radiationof the visible spectrum.
b)The concentration of a species being analyzed in a sample solution can bedetermined froma calibration curve.
2 a) Infrared (IR) spectroscopy is theabsorptionmeasurementof infraredradiationofdifferentwavenumbersby a sample.
b)The main goal of infrared spectroscopy is to determine the functional groupspresentincarboncompoundsasdifferentcovalentbondsabsorbinfraredradiationof characteristicwavenumbers.
c) The following table* shows the characteristic infrared absorption wavenumberranges for some covalentbonds (stretchingmodeof vibration).
Bond Compound type Wavenumber range(cm–1)
Intensity(s = strong,m=medium)
C–Cl — 700 – 800 s
C–O alcohols, esters 1000 – 1300 s
C=C alkenes 1610 – 1680 m
C=O aldehydes, ketones, acids,esters 1680 – 1750 s
C C alkynes 2070 – 2250 m
C N nitriles 2200 – 2280 m
O–H acids (hydrogen-bonded) 2500 – 3300 m (broad)
C–H alkanes, alkenes 2840 – 3095 m– s
O–H alcohols (hydrogen-bonded) 3230 – 3670 s (broad)
N–H amines 3350 – 3500 m
* Youneednotrememberthesecharacteristicwavenumberranges.Thistablewillbeprovidedin examinations.
3 a) Massspectrometryisaformofanalysisthatseparatesandidentifiessubstancesonthe basis of mass-to-charge ratios of positive ions formed from substances whenthey arebombardedbyhigh energy electrons in a vacuum.
b)Massspectracanbeusedtomeasurerelativeatomicmassesandrelativemolecularmasses.
c) Structural information of a compound can be determined from fragmentationpatterns of the molecular ion. The following table shows typical fragment ionsproduced in amass spectrometer.
Ion Mass/charge ratio (m/e) Ion Mass/charge ratio (m/e)
CH3+ 15 C4H9
+ 57
C2H5+ 29 CH3CH2CO+ 57
CH3CO+ 43 C6H5+ 77
C3H7+ 43 C6H5CH2
+ 91
Topic 162� Analytical Chemistry 2�Unit 56 Instrumental analytical methods
Example
Compound X contains carbon, hydrogen and oxygen only. It shows negative resultswhentreatedwithacidifiedK2Cr2O7(aq).Theinfraredspectrumandmassspectrumareshownbelow.Deduceonepossible structureofX. (7marks)
Tran
smit
tan
ce (
%)
wavenumber (cm–1)
100
50
04 000 3 000 2 000 1 500 1 000 500
Rel
ativ
e in
ten
sity
(%
)
Mass/charge ratio (m/e)
10 20 30 40 50 60 70 80 90
0
50
100
ExamtipsExamtipsExamtipsExamtips ♦ Infrared spectroscopy can be used to follow the progress of areaction.
e.g.
Addition polymerization is involved when Superglueworks.
CN n
COOCH2CH3
CC
H
H
CN
COOCH2CH3
CCn
H
H
additionpolymerization
Theprogressof thepolymerizationcanbemonitoredbyobservingthechangeofIRabsorptionofthepeakat1614cm–1duetothestretchingof theC=Cbond.
As the polymerization proceeds, the concentration of the monomerdecreases. Thus, the intensity of theC=Cabsorption reduces.
50
60
70
80
90
100
1 614 cm–1
t = 600 st = 400 st = 200 s
t = 0 s
Tran
smitt
ance
(%)
♦ Questionsoftengivethemassspectraofaromaticcompoundsandaskabout possible structures of the compounds. Pay special attention tothe following peaks:
– m/e =77due toC6H5+ ion; and
– m/e =91due toC6H5CH2+ ion.
Topic 1630 Analytical Chemistry 31Unit 57 Contribution of analytical chemistry to our society
➤Questions may give the IR spectra of carbon compounds and ask aboutpossiblestructuresofthecompounds.Payspecialattentiontothefollowingcharacteristic infrared absorptions that often appear.
RemarksRemarks*
Bond Compound typeWavelength range
(cm–1)Intensity
C=O aldehydes, ketones, acids, esters 1680 – 1750 strong
O–H acids (hydrogen-bonded) 2500 – 3300 medium (broad)
O–H alcohols (hydrogen-bonded) 3230 – 3670 strong (broad)
57.1 Introduction
57.2 Food analysis
57.3 Gas chromatography
57.4 Carbon monoxide in the air
57.5 Dioxins in the air
57.6 Formaldehyde —a common indoor air pollutant
57.7 Forensic chemistry
57.8 Drink driving
57.9 Fingerprint analysis
57.10 The roleof analytical chemistry in clinical diagnoses
Contribution of analytical chemistry to our societyUnit 57
Answer
The infrared absorption at 1680 – 1750cm–1 indicates the presence of a C=O bondinX.
HenceXmaybe an aldehyde, a ketone, an acidor an ester. (1)
Theabsenceofstronginfraredabsorptionat1000–1300cm–1and2500–3300cm–1suggests thatXdoesnot containC–OandO–Hbonds.
HenceXmaybe an aldehydeor a ketone. (1)
AsXshowsnegativeresultswhentreatedwithacidifiedK2Cr2O7(aq),thusXisprobablya ketone. (1)
The peak at m/e = 86 in the mass spectrum suggests that the relative molecular massofX is 86. (1)
Molecular ions of a ketone (RCOR’) tend to undergo fragmentation to give the RCO+ion andR’CO+ ion.Thepeak atm/e=57 is likely tobedue to theC2H5CO+ ion. (1)
Thepeakatm/e=29islikelytobeduetotheC2H5+ionwhichformedwhenC2H5CO+
ion loses aCO fragment. (1)
X is likely tobeC2H5COC2H5. (1)
Topic 1632 Analytical Chemistry 33Unit 57 Contribution of analytical chemistry to our society
57.1 – 57.10
Summary
1 Volumetricanalysisorgaschromatographycanbeemployedtodeterminetheethanolcontentof a spirit.
2 Ingas-liquidchromatography,themobilephase isagasandthestationaryphase isanon-volatile liquid.
3 Carbonmonoxide level in the airmaybedeterminedby infrared spectrometry.
4 The level of dioxins in the air may be determined by gas chromatograph-massspectrometry (GC-MS).
5 Formaldehyde is a common indoor pollutant. The level of formaldehyde in indoorairmaybedeterminedbyhighperformance liquid chromatography (HPLC).
6 The working of an alcohol breathalyzer is based on the redox reaction involvingpotassiumdichromate.
7 Latent fingerprints canbedevelopedwith iodine fumes.
ExamtipsExamtipsExamtipsExamtips ♦ When orange dichromate ions are brought into contact with ethanol,the following redox reactionoccurs.
2Cr2O72–(aq)+3CH3CH2OH(aq)+16H+(aq)
4Cr3+(aq)+3CH3COOH(aq)+11H2O(l)
Green chromium(III) ions form.
Example
Anexperiment,consistingof the twostages,wascarriedout todeterminethesulphurdioxide content in awhitewine.
Stage 1 Excess KI(aq) and H2SO4(aq) were added to 25.0cm3 of 0.00360moldm–3KIO3(aq). The mixture was then diluted to 250.0cm3 using distilled water togive a standard aqueous iodine solution.
Stage 2 25.0cm3portionsof thewinewere titratedwith the standard aqueous iodinesolution,using starch solution as an indicator. Themean titrewas11.4cm3.
a) The reaction in Stage 1 canbe representedby the following equation:
IO3–(aq) + 5I–(aq) + 6H+(aq) 3I2(aq) + 3H2O(l)
Calculate themolarityof the standard aqueous iodine solutionprepared.(2marks)
b)State the colour change at the titration endpoint in Stage 2. (1mark)
c) i) Write a chemical equation for the reaction involved in the titration. (1 mark)
ii)Calculate the concentration, inmoldm–3, of sulphurdioxide in thewhitewine. (2marks)
Answer
a) IO3–(aq) + 5I–(aq) + 6H+(aq) 3I2(aq) + 3H2O(l)
0.00360moldm–3 ?moldm–3
25.0cm3 250.0cm3
NumberofmolesofKIO3 in25.0cm3 solution
=0.00360moldm–3 x 25.01000
dm3
=9.00 x 10–5mol (1)
According to theequation,1moleof IO3– ions reactswith I– andH+ ions togive3
molesof I2.
i.e. numberofmolesof I2 formed =3 x 9.00 x 10–5mol = 2.70 x 10–4mol
Concentrationof I2(aq) =2.70 x 10–4mol
250.01000
dm3
= 1.08 x 10–3moldm–3 (1)
b)Fromcolourless toblue (1)
c) i) I2(aq) + SO2(aq) + 2H2O(l) 2I–(aq) + SO42–(aq) + 4H+(aq) (1)
ii) I2(aq) + SO2(aq) + 2H2O(l) 2I–(aq) + SO42–(aq) + 4H+(aq)
1.08 x 10–3moldm–3 ?moldm–3
11.4cm3 25.0cm3
Numberofmolesof I2 in11.4cm3 solution
=1.08 x 10–3moldm–3 x 11.41000
dm3
=1.23 x 10–5mol (1)
According to the equation, 1moleof I2 reactswith1moleof SO2.
i.e. numberofmolesof SO2 in25.0cm3whitewine=1.23 x 10–5mol
Concentrationof SO2 inwhitewine =1.23 x 10–5mol
25.01000
dm3
= 4.92 x 10–4moldm–3 (1)
➤Whenstandard I2(aq) isused todetermine theconcentrationofa reducingagentinasample(e.g.sulphurdioxidecontentinwine)usingstarchsolutionas an indicator, the colour change at the end point is from colourless toblue.
Distinguishthisfromthecasewhenusingstarchsolutionasanindicator inthetitrationbetween I2(aq) liberatedandNa2S2O3(aq) (refer totheexampleinUnit 55).
RemarksRemarks*