Errors in Pharmaceutical analysis.pdf

Errors: Definition, Types and ways to mimimize the errors

1. Dr. Jasmine Chaudhary
Associate Professor
MMCP
[email protected]

2. Introduction
Methods of determination
Classification of errors
a. Determinate Errors
b. Indeterminate Errors
Minimizing Systematic Errors

3. Erroristhedifferencebetweenthetrue(standard/reference)
valueandthemeasured(observed)value.
Error = Measured value-True value

4. Absolute Method
Comparative Method
Absolute method
Sample is synthesized using known quantities of constituent so true value for
amount of different constituents are known.
Sample is analyzed by some method and observed values for
quantities of constituents are noted
Difference in observed value and true value will give error.

5. Comparative method (when sample cant be synthesized)
Data given in reference books, formularies etc. can be taken as
standard value.
Sample is analyzed by some method and observed values for
quantities of constituents are noted
Difference in observed value and standard value will give error.

6. Errorsinanysetofmeasurementscanbedividedintothefollowing
categories:
1.Systematic,determinateorconstanterrors.
2.Random,accidentalorindeterminateerrors.
Systematic Errors
Asthenameindicates,thecauseofthistypeoferrormaybefoundout
&thustheseerrorscanbeeitheravoidedorcorrected.
Determinateerrorsaresystematicerrorsi.e.theyarenotrandom.

7. Types of determinate errors
I.Personalerrors:Theseerrorsarenotconnected
withthemethodorprocedurebuttheindividual
analystisresponsibleforthem.Thistypeoferrors
mayariseduetotheinabilityofindividual,
performingexperimentormeasuringobservationsasa
resultofinexperienceorinsufficienttraining.

8. Someimportantpersonalerrorsare:
A.Inabilityinjudgingcolourchangesharplyinvisualtitrations.
B.Errorinreadingaburette.
C.Mechanicallossofmaterialinvariousstepsofananalysis.
D.Failuretowashanddryaprecipitateproperly.
E.Transcriptionerrors,i.e.copyingthewronginformationintoalabnotebookor
ontoalabel.
F.Usingimpurereagents.
G.Ignitionofprecipitateatincorrecttemperatures.
H.Errorsincalculations.
I.Additionofextratitrantthanactuallyneededforendpoint.
Propertraining,experience,andattentiontodetailonthepartofthe
analystcancorrectthesetypesoferrors.

9. Instrumentsandreagenterrors:Theseerrorsmaybeduetousing
faultyconstructedinstruments,apparatusorimpurereagents.E.g.
A.Balancearmsofunequallengths.
B.Uncalibratedorimproperlycalibratedweights.
C.Incorrectlygraduatedburettes/pipettessoinsteadofmeasuring10
ml;9.8or10.2mlistaken.
D.Attackofforeignmaterialsuponglasswares.
E.Impurereagents.

10. Theseerrorscanbeavoidedby
•usingcalibratedweights,glasswaresandpurereagents.
•followingasystematicproceduretochecktheinstrument
settingsandoperationbeforeuse.Suchproceduresarecalledstd.
operatingprocedures(SOPs).

11. Methodicerrors(ErrorofMethod):Thesearethemostserious
typesoferrorsencounteredinchemicalanalysis.Someexamplesinvolving
methodicerrorsare:
A.Solubilityofprecipitateinmediumandinwashliquid.
B.Decompositionorvolatilizationofweighingformsofprecipitateson
ignitionorheating.
C.Hygroscopicityoftheweighingformsoftheprecipitates.
D.Co-precipitationandPost-precipitation
E.Failureofareactiontoachievecompletion.
F.Occurrenceofsidereactions.
G.Lossofanalyteduringsamplepreparationbyvolatilizationorprecipitation.
Theseerrorscanbeeliminatedorreducedtoasmallmagnitudeby
employingthepropertechnique.

12. AdditiveErrors
Thistypeoferrorisindependentoftheamountoftheconstituentpresentin
thedeterminationandisconstantinaseriesofdetermination.e.g.,
0.1mlextratitranthastobeaddedtoseecolorchangeclearlyatendpoint
soerroris0.1ml.Nowifstdvalueis10ml,observedvaluewillbe10.1
ml.
Ifsampleisdoubled,std.valueis20mlandobservedvaluewillbe20.1ml
duetoendpointerror(0.1ml)soerrorremainssameevensampleis
doubled.
lossinweightofacrucibleaddserrortotheweightofprecipitateignitedin
it.

13. Proportionalerrors
Magnitudeofproportionalerrordependsuponthequantityoftheconstituent/
sample.e.g.,
Impuritypresentinastandardsubstancegivesawrongvalueforthe
normalityofastandardsolutione.gincaseofNaOH.
As10mlof0.1NHClcanbetitratedwith10mlof0.1NNaOH(std.value)
butasNaOHisimpuresoduetoimpurity,observedvaluecanletbe10.2
mlsoerroris10.2-10=0.2ml
Now,if20mlof0.1NHClhastobetitratedthen20mlof0.1NNaOH
shouldbeusedbutobservedvalueis20.4mlsoerroris20.4-20=0.4ml.
Thus,doublingofsample,doubledtheerror.

14. Inderteminateerror
Indeterminateerrorscannotbepin-pointedtoanyspecificwelldefined
reasons.
Theyarerandominnature&takeplaceinseveralsuccessive
measurementsperformedbythesameanalystunderthesameconditions
andidenticalexperimentalparameters.
Theseerrorsareaccidentalandanalysthasnocontroloverthem.
e.g.
Vibration in balance while handling it.
Accidental loss of material during analysis.
Variation in temperature.
Variation in weighing due to air pressure

15. Other examples are the limitations of reading balances, vibrations
caused to the building by heavy vehicular-trafficking , which are
beyond anyone's control.
For e.g. A balance that is capable of measuring only to 0.001 g can not
distinguish between two samples with masses of 1.0151 & 1.0149
g.

16. MINIMIZATION OF ERRORS
Thedeterminateerrormaybeminimizedbyusingfollowingmethods:
1.Runningablankdetermination:Errorsarisingfromtheintroductionof
impuritiesthroughthereagentsandvesselsareaccomplishedbyrunninga
blank.Suchaprocedureinvolvesgoingthroughalltheanalysis,usingthe
samesolventandreagentinthesamequantities,butomittingtheunknown
component.Thus,inmakingablank,sampleisomitted;otherwisethedetails
oftheprocedurearefollowedexactlyasfaraspossible.
2.Calibrationofapparatusandapplicationofcorrections:Allinstruments,
suchasburettes,pipettes,weights,measuringflasks,etc.mustbeproperly
calibratedandtheappropriatecorrectionsmustbeappliedtotheoriginal
measurements.

17. 3.Runningacontrolleddetermination:Itconsistsincarryingouta
determinationunderidenticalexperimentalconditionsasfaraspossible
uponaquantityofastandardsubstance,whichcontainsthesameweightof
theconstituentasitcontainedintheunknownsample.
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18. 4.Useofindependentmethodofanalysis:Determinationismadebysome
othermethodandbothresultsarecompared.e.g.strengthofHClmaybe
determinedbytwomethods:
i.Titratingitwithastandardsolutionofastrongbase.
ii.PrecipitationwithAgNO3andweighingasAgCl.
Iftheresultsobtainedbythetwomethodsareingoodagreement,itmaybe
saidthatthevaluesarecorrectwithinsmalllimitsoferrors.

19. VermaRM.AnalyticalChemistry(TheoryandPractice),3
rd
Edition,2016,CBSPublishers,19-24