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CHEM334QuantitativeAnalysisLaboratory

CalibrationofVolumetricGlassware

IntroductionVolumetricglasswareisaclassofglassvesselsthatarecalibratedtocontainordelivercertainvolumesofsubstances.Graduated cylinders, pipettes (alsopipets), burettes (alsoburets) and volumetric flasks areexamplesasarebeakersandErlenmeyerflasks.Itcanbeseenfromthislistthattheterm"calibrated"canmean "to a varying degree". The manufacturer's stated accuracy of beakers and Erlenmeyer flasks istypically5%whilethatforvolumetricflasksisgenerallymuchhigher.

In this laboratory experiment, the three types of glassware typically used by an analytical chemist: avolumetricflask,avolumetricpipetandaburetwillbecalibrated.Thesetoolsareusedextensivelywhenperforming gravimetric and titrimetric Analyses. In order to avoid introducing systematic errors intomeasurements,eachoftheseinstrumentsmustbeproperlycalibrated.And,toreducetherandomerrorsinherentwhenusingtheseinstruments,theirproperusemustbethoroughlyunderstood.Thequalityofthemeasurements obtained from these tools depends heavily on the care taken in calibrating and inusingeachinstrument.

Procedures

In themost precisework it is never safe to assume that the volume delivered by or contained in anyvolumetric instrument is exactly the amount indicatedby the calibrationmark. Instead, the amount ofwater delivered by or contained in the volumetric apparatus is quantified by mass to effect thecalibration.Thismass isthenconvertedtothedesiredvolumeusingthetabulateddensityofwaterandthefollowingstandardrelation:

d=m/v (1)

wherevismeasuredvolume,mismassofwaterusedtofillthevolumeanddisdensityofthewater.

Figure1.Volumetricflask.

Figure2.VolumetricandGraduatedPipettes.

Figure3.Burette.

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AllvolumetricapparatusiseitherpurchasedwithaCalibrationCertificateorcalibratedbyachemistinthisfashion.

SystematicErrorsAffectingVolumetricMeasurements

Thevolumeoccupiedbyagivenmassofliquidvarieswithtemperature,asdoesthevolumeofthedevicethatholdstheliquid.20°Chasbeenchosenasthenormaltemperatureforcalibrationofmuchvolumetricglassware.

Glassisafortunatechoiceforvolumetricwareasithasarelativelysmallcoefficientofthermalexpansion;aglassvesselthatholds1.00000Lat15°Cholds1.00025Lat25°C.Ifdesired,thevolume(V)obtainedatatemperature(T)canbecorrectedto20°Cbyuseof:

V20°C=VT[1+0.000025(20–T)] (2)

Inmostwork,thiscorrectionissmallenoughitmaybeignored.

However, the thermal expansion of the contained liquid is frequently of importance. Dilute aqueoussolutionshaveacoefficientofvolumethermalexpansionofabout0.00025°C-1.Thus,1.000Lofwaterat15°Cwilloccupy1.002Lat25°C.Acorrectionforthisexpansionisoftenappliedinrigorouscalibrationprocedures.

Parallax is another source of error when using volumetric ware. A correction for this expansionmustfrequently be applied during calibration procedures. Frequently, graduationmarks encircle the girth oftheapparatustoaidinpreventingthisartifactbypermittingthechemisttoalignthelineofsight.

Readingsthataretakenfromalineofsightthatiseithertoohighortoolowwillresultotherwise.

TipsforCorrectUseofVolumetricGlassware

Pipettes. The pipette is used to transfer a volume of solution from one container to another. MostvolumetricpipettesarecalibratedTo-Deliver(TD)withacertainamountoftheliquidremaininginthetipandasafilmalongtheinnerbarrelafterdeliveryoftheliquid.Theliquidinthetipshouldnotbeblown-out.Pipetsofthe"blow-out"varietywillusuallyhaveagroundglassringatthetop.And,drainagerates

Figure4.ParallaxerrorfromincorrectLinesofsight.

Figure5.Themeniscusofwater.

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fromthepipetmustbecarefullycontrolledsoastoleaveauniformandreproduciblefilmalongtheinnerglasssurface.MeasuringPipetswillbegradatedinappropriateunits.

Oncethepipetiscleanedandreadytouse,makesuretheoutsideofthetipisdry.Thenrinsethepipetwiththesolutiontobetransferred.Insertthetipintotheliquidtobeusedanddrawenoughoftheliquidintothepipettofillasmallportionofthebulb.Holdtheliquidinthebulbbyplacingyourforefingerovertheendofthestem.

Withdrawthepipetfromtheliquidandgentlyrotateitatananglesoastowetallportions of the bulb. Drain out and discard the rinsing liquid. Repeat this oncemore.

Tofillthepipet,insertitverticallyintheliquid,withthetipnearthebottomofthecontainer. Apply suction to draw the liquid above the graduationmark. Quicklyplacea forefingerover theendof the stem.Withdraw thepipet from the liquidanduseadrypapertowipeoffthestem.Nowplacethetipofthepipetagainstthe container from which the liquid has been withdrawn and drain the excessliquidsuchthatthemeniscusisatthegraduationmark.

Move the pipet to the receiving container and allow the liquid to flow out

(avoidingsplashing)ofthepipetfreely.Whenmostoftheliquidhasdrainedfromthepipet,touchthetiptothewallofthecontaineruntiltheflowstopsandforanadditionalcountoften.

Volumetric Flasks. The volumetric flask is used to prepare Standard Solutions or in diluting a sample.Most of these flasks are calibrated To-Contain (TC) a given volume of liquid.When using a flask, thesolutionorsolid tobediluted isaddedandsolvent isaddeduntil the flask isabout two-thirds full. It isimportant to rinse down any solid or liquid that has adhered to the neck. Swirl the solution until it isthoroughlymixed.Nowaddsolventuntilthemeniscusisatthecalibrationmark.Ifanydropletsofsolventadheretotheneck,useapieceoftissuetoblottheseout.Stoppertheflasksecurelyandinverttheflaskatleasttentimes.

Burets.TheBuretisusedtoaccuratelydeliveravariableamountofliquid.Filltheburettoabovethezeromarkandopenthestopcocktofillthetip.Workairbubblesoutofthetipbyrapidlysquirtingtheliquidthroughthetiportappingthetipwhilesolutionisdraining.

The initialburet reading is takena fewseconds, ten to twenty,after thedrainageof liquidhasceased.Holdingawhitepieceofpaperwithaheavyblackmarkonbehindtheburetcanhighlightthemeniscus.

Place the flask intowhich the liquid is to bedrainedon awhite pieceof paper. (This is doneduring atitrationtohelpvisualizecolorchangesthatoccurduringthetitration.)Theflaskisswirledwiththeright-handwhilethestopcockismanipulatedwiththeleft-hand.

Theburetisopenedandallowedtodrainfreelyuntilnearthepointwhereliquidwillnolongerbeaddedtotheflask.Smalleradditionsaremadeastheend-pointoftheadditionisneared.Allowafewsecondsafter closing the stopcock before making any readings. At the end-point, read the buret in a mannersimilartothatabove.

Figure5.Pipetusage

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Aswith pipets, drainage ratesmust be controlled so as to provide a reproducible liquid film along theinnerbarreloftheburet.

CleaningofVolumetricGlassware.Cleaningofvolumetricglasswareisnecessarytonotonlyremoveanycontaminants,buttoensureitsaccurateuse.Thefilmofwaterthatadherestotheinnerglasswallofacontainerasitisemptiedmustbeuniform.

Two or three rinseswith tapwater, amoderate amount of agitationwith a dilute detergent solution,several rinseswith tapwater,andtwoor threerinseswithdistilledwateraregenerallysufficient if theglasswareisemptiedandcleanedimmediatelyafteruse.

Ifneeded,useawarmdetergentsolution(60-70°C).Aburetortesttubebrushcanbeusedinthecleaningofburetsandtheneckofvolumetricflasks.Volumetricflaskscanbefilledwithcleaningsolutiondirectly.Pipets and burets are filled by inverting them and drawing the cleaning solution into the device usingsuction.Avoidgettingcleaningsolutioninthestopcock.Allowthewarmcleaningsolutiontostandinthedevice for about fifteenminutes; never longer than 20minutes. Drain the cleaning solution and rinsethoroughlywithtapwaterandfinallytwoorthreetimeswithdistilledwater.

Pipetsandburetsshouldberinsedatleastoncewiththesolutiontheyaretofilledwithbeforeuse.

AGeneralCalibrationProcedure

Aswasnotedabove,volumetricglasswareiscalibratedbymeasuringthemassofwaterthatiscontained-inordelivered-bythedevice.

Thismassdataisthenconvertedtovolumedatausingthetabulateddensityofwater(SeeAppendix)atthetemperatureofcalibration.(Inveryaccuratework,thethermometeriscalibratedalsoasanincorrecttemperature reading will lead to the use of an incorrect density for water. This, in turn, will give aninaccuratevolumecalibration.)

Finally, this volume data is corrected to the standard temperature of 20 °C. This can be accomplishedusing(2)todescribethethermalvolumeexpansioncoefficientofwater.

Further details concerning calibration of laboratory glassware can be found in the NIST publicationidentified inthereferences. Inthisexperimentavolumetricflask,ameasuringpipetteandavolumetricpipettewillbeusinggravimetricallydeterminedwater.Ineachcase,themeasuredmassofthecalibratingwaterwillbestandardizedto20°C.

Procedure

Record the laboratory temperature. For this experiment use distilled water contained in one of thecarboys-notdirectlyfromthedistilledwatertap.(Why?)

Cleananddry(ina105°Covenforafewminutes,followedbycoolingtoroomtemperature)aweighingbottleanditscapUsedetergentifnecessaryfollowedbytapwaterthendistilledwater(alsofromitsowntap,why?)rinses.Usea105°Covenforafewminutesfordryingandcooltoroomtemperature.Donothandlethecleanglasswarewithbarehands.(Why?)

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MeasuringPipette.Filla10mLmeasuringpipettewithdeionizedwater,thendrainittoensurethatnodroplets remainonthe innerwalls. Ifdroplets remain,cleanthepipette thoroughlyandrepeat.Fill thepipetteanddeliverapproximatelytwomillilitersintotheweighedbottle.Estimateallvolumereadingstothenearest0.01mL.Cap thebottleand reweigh.Repeat thisprocedure for samplesof4, 6and8mL.Performthreereplicatemeasurementsforthepipette.

Volumetric Pipette. Fill a 10mL volumetric pipettewith deionizedwater and drain to ensure that nodroplets remainonthe innerwalls. Ifdroplets remain,cleanthepipette thoroughlyandrepeat.Fill thepipettetothefilllineanddeliverthetotalvolumeofliquidintoadrybottle.Captheweighingbottleandreweigh.Performthreereplicatemeasurementsforthepipette.VolumetricFlask.Calibratea10mLvolumetricflaskbyweighingitempty(anddry)andfilledtothemarkwithdistilledwater.Performthreereplicatemeasurementsfortheflask.

Results

VolumetricPipette.Use thedensityofwater toconvert themassofwater to thevolumedeliveredbyyourpipette.Determinetheaverageand95%confidenceintervalforthethreereplicatemeasurementsforthepipette.Ifthe95%confidenceintervalsaregreaterthanthetoleranceforthe10mLpipetteandiftimepermitsrepeatthesemeasurements.VolumetricFlask.Convertthemassofwatercontainedinyourvolumetricflasktovolume.Determinetheaveragevolumeand95%confidenceinterval.Iftimepermits,repeatthecalibrationifthe95%confidenceintervalsgreaterthanthetoleranceforthe10mLflask.Inthefuture,usethisaveragevalueastheexactvolumecontainedintheflask.

GraduatedPipette.Convertthemassofwatertothevolumedeliveredbythepipette.Plottheaveragemeasured volume obtained from your replicate measurements delivered versus the expected volumedelivered as read on the pipette for the 10 mL pipette. Plot y-value error bars corresponding to thestandard deviation obtained from your replicatemeasurements. (Can you see the error bars?)Using alinear regression analysis, determine the best-fit equation for the relationship betweenmeasured andexpected volumes. Include both slope and intercept in the analysis. Report the slope, its standarddeviation,they-intercept,itsstandarddeviationandtheR2valueofthecalibrationline.Calculatethe95%confidenceintervalforeachvalueofexpectedvolume.

Discussion

Prepare a LaboratoryReport as describedpreviously. Present allmeasureddata in the report togetherwith the expected data. Describe and explain the relationships between the two sets of data. Includegood-quality figuresand tables to substantiateyourdiscussions. Includeanswers to the several "why?"questionsthatappearinthisdescription.Includeaquantitativeerroranalysisinyourdiscussion.Compareyourassessmentofthevariousvolumetricglasswareaccuracyvaluestothemanufacturer'sstatedvalues.

References

Tufte,E.R.,"EnvisioningInformation"(1990),GraphicPress,CT,Chapters1and6.

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Pierce,C.,Haenisch,E.L. andSawyer,D.T., "QuantitativeAnalysis" (1948)FourthEditionWiley&Sons,severallocations.

Lembeck,J.,“TheCalibrationofSmallVolumetricLaboratoryGlassware”,NBSIR74-461.Availableat:

http://ts.nist.gov/MeasurementServices/Calibrations/upload/74-461.pdf.

Harris,D.C.,"QuantitativeChemicalAnalysis"(2007)7thedition,Freeman&Co.,NY.

Appendix1.Thedensityofwaterasafunctionoftemperature.

Temperature(°C) Density(g/mL)15 0.999102616 0.998946017 0.998777918 0.998598619 0.998408220 0.998207121 0.997995522 0.997773523 0.997541524 0.997299525 0.997047926 0.996786727 0.996516228 0.996236529 0.995947830 0.9956502