Introduction to Volumetric Analysis

1 Introduction to Volumetric Semester IVDr. :burettes,micro-burettes,volumetricpipet tes,graduatedpipettes, Principles&TypesIntroduction Chemicalanalysisplaysanimportantroleinst udyofcompositionorconstituentsofsubstanc esormaterial. Thechemicalanalysisisbroadlydividedintot wotypes Qualitativeanalysis Quantitativeanalysis Thevolumetricmethodinvolvesmeasurementof volumesofthereactingsubstances. Theweightoftherequiredconstituentsisindi rectlyobtainedbymeasuringthevolumeofasol utionofknowncompositionrequiredtoreactwi thaknownvolumeofasolutioncontaininganunk nownweightofthedesiredconstituent. reductionmethods Thequantitativeanalysisisfurtherclassifi edintotwobranchesnamely Volumetricanalysis GravimetricanalysisVolumetric Analysis Volumetricanalysisisalsoknownastitrimetr icanalysis.

2 Thereagent(thetitrant)isaddedgraduallyor stepwisetotheanalytefromaburette. Thekeytoperformingasuccessfultitrimetric analysisistorecognizetheequivalencepoint ofthetitration(thepointatwhichthequantit iesofthetworeactingspeciesareequivalent) ,typicallyobservedasacolourchange. Ifnospontaneouscolourchangeoccursduringt hetitration,asmallamountofachemicalindic atorisaddedtotheanalytepriortothetitrati on. Chemicalindicatorsareavailablethatchange colouratorneartheequivalencepointofacid- base,oxidation-reduction,complexation,an dprecipitationtitrations. Thevolumeofaddedtitrantcorrespondingtoth eindicatorcolourchangeistheendpointofthe titration. Theendpointisusedasanapproximationofthee quivalencepointandisemployed,withtheknow nconcentrationofthetitrant, Involumetricanalysis,concentrationofsubs tancesarefoundoutbyvolumedetermination.

3 Theunitusedformeasurementofvolumeofaliqu idorsolutionis litre . Thelitreisdefinedasthe volumeoccupiedbyonekilogramofwateratthet emperatureofitsmaximumdensity(4oC)andatn ormalatmosphericpressure . Themillilitreisthethousandthpartofalitre . Thecubiccentimeter(cm3)isthevolumeoccupi edbyacubesuchthateachedgeofthecubeisonec entimeterinlength. Therelationshipbetweenmillilitreandcubic centimeteris1000ml= Thetermmlismorecorrect Theprecisionofvolumetricworkdependsupona ccuracywithwhichvolumeofliquidscanbemeas ured. Therearecertainsourcesoferrorswhichmustb ecarefullyconsidered. Sucherrorscanbeeliminatedonlybyrecalibra tingtheapparatusMethods of Expressing Concentration of solutionsTherearemanysystemsofexpressing thecontractionofsolutions.

4 Weightofsolutepresentinagivenvolumeofsol ution Molesofsolutepresentinonelitreofsolution ,calledmolarity Gramequivalentsofsolutepresentinonelitre ofsolution,callednormality of Solute per Unit Volume of SolutionInpreparationofsolutionusingthis system, ,thesoluteisweighedandtransferredtoavolu metricflask, , :Themoleisverylargeunit,henceasmalleruni twhichisonethousandthofit, of solute in gramsNumber of Moles = --------------------------------Molecula r weight of the of moles of the soluteMolarity= --------------------------------------Vo lume of the solution in litreNumber of moles = Molarityx LitreNumber of millimolesof the solute Molarity= ---------------------------------------- --Volume of the solution in millilitreNumber of millimoles= Molarityx MilliLitres3.

5 Weight :Theequivalentweightofasubstance(element orcompound)as: Thenumberofpartsbyweightofit, ,8partsbyweightofoxygen, . MilliequivalentWeight : It is obtained by dividing the equivalent weight by 1000. Gram Equivalent Weight : It is the equivalent weight expressed in gramsEquivalentweightofasubstance=1equiv alentofasubstance=Equivalentweightofsubs tanceingrams of equivalents of the solute Normality= ---------------------------------------- ---Volume of titration in litreNumber of milliequivalentsof the solute = ---------------------------------------- ---------Volume of the solution in millilitreNumber of gram equivalents = Normality x Litres (molaritymade easy) (normality and equivalent weight)

6 Primary and Secondary Standard Solutions Thestandardsolutionispreparedbydissolvin ganaccuratelyweighedquantityofahighlypur ematerialcalledaprimarystandardandthendi lutingtoanaccuratelyknownvolumeinavolume tricflask Ifmaterialisnotsufficientlypurethensolut ionispreparedtogiveapproximatelythedesir edconcentrationandthisstandardizedbytitr atingaweighedquantityofaprimarystandard. Theprimarystandardshouldfulfillthebelowr equirements Itshouldbe100%pure Itshouldbeeasilyandreadilyavailable Althoughnotnecessaryitshouldhaveahighfor mulaweight. Thisissothatarelativelylargeamountofitwi llhavetobeweighedtogetenoughtotitrate. Therelativeerrorinweighingagreateramount ofmaterialwillbesmallerthanthatforsmalla mount Preparation of Standard Solution Incaseofvolumetricanalysis,thefinalresul tdependsupontheaccuracywithwhichthestand ardsolutionisprepared.

7 Theconcentrationofstandardsolutionisesta blishedbydirectandindirectmethod. DirectMethodofPreparationofStandardSolut ion(PrimaryStandard) Inordertoprepareastandardsolution,thesub stanceisweighedaccuratelyequaltoanequiva lent,oradefinitefractionormultipleofaneq uivalent,dissolvedinwateranddilutedtoakn ownvolume. IndirectMethodofPreparationofStandardSol ution(SecondaryStandard) Theprocessofdeterminingtheconcentrationo fasolutionbytitratingitagainstasolutiono fapuresubstanceofknownconcentrationiscal ledasstandardization. Thustheexactconcentrationofthesecondarys tandardsolutionisdeterminedbytitratingit againstaprimarystandardsolution. weight in different types of reactions Samecompoundsmayhavedifferentequivalentw eightsindifferentchemicalreactions.

8 Hencenormalityofaparticlesolutionmayalso bedifferentindifferentchemicalreactions. Equivalentweightindifferenttypesofchemic alreactionsisdiscussedasshownbelow NeutralizationReactions ComplexFormationandPrecipitationReaction s Oxidation Reductionreactions Neutralization reactions Inreactionsbetweenacidsandbases,oneequiv alentistheweightofthereagentwhichcontain sorreactswithonegramatomofreplaceablehyd rogen( )orwithonegrammoleculeofhydroxyl( ).Molecular weight of acid Equivalent weight of acid = ----------------------------Basicityof that acid Thebasicityisdefinedasthenumberofreplace ablehydrogenatomspresentinonemoleculeoft hatacid. Thetermacidityisdefinedasthenumberofrepl aceablehydroxylionspresentinonemoleculeo fthatbaseMolecular weight of base Equivalent weight of acid = ----------------------------Acidity of that base Complex Formation and Precipitation Reactions Theequivalentweightofcomplexformationand precipitationreactionsistheweightofthesu bstancethatcontainsorreactswith1gatomofa monovalentcationM+, gatomofbivalentcationM2+,1/3gatomofatriv alentcationM3+andsoon.

9 Theequivalentweightofcationisequaltoitsa tomicweightdividedbyvalency. The equivalent weight of a substance in complex formation reaction can be determined by writing down the ionic equation of the reaction2 KCN + AgNO3 K [Ag(CN)2] + + Ag+ [Ag(CN)2]- Calibration Calibration of measuring instruments Analysis Instrumentalanalysisisafieldofanalytical chemistrythatinvestigatesanalytesusingsc ientificinstruments. Instrumentalchemistryinvolvestheuseofcom plexmachineswithinthefieldofanalyticalch emistry. ,whilemassspectrometryusesmagneticandele ctricfields. Instrumentalchemistryisutilizedwithinthe food,pharmaceutical,medicalandenvironmen talindustries.

10 Theinstrumentalmethodsofchemicalanalysis aredividedintocategoriesaccordingtothepr opertyoftheanalytethatistobemeasured. Manyofthemethodscanbeusedforbothqualitat iveandquantitativeanalysis. Themajorcategoriesofinstrumentalmethodsa rethespectral,electroanalyticalandsepara tory. Thepossibilitytorealiseanalysesonsmallan dverysmallsamplesisoneofadvantagesofinst rumentalmethodsincomparisonwithchemicalm ethods. Bothchemicalandinstrumentalanalysismetho dshaveaseriesoflimitation,everyofthemhav ingadvantagesanddisadvantagesAdvantages of Instrumental methods of Analysis UVvisiblespectrometryasmallamountofthesa mpleisrequiredforanalysis Determinationbyinstrumentalmethodisconsi derablyfast Complexmixturecanbeanalyzedeitherwithorw ithouttheirseparation Reliabilityandaccuracyofresultsareobtain edbyinstrumentalmethod Whennon-instrumentalmethodisnotpossible.