Colorado State University 2018.10.29 Page 1 of 6 CHEM 334 Quantitative Analysis Laboratory Calibration of Volumetric Glassware Introduction Volumetric glassware is a class of glass vessels that are calibrated to contain or deliver certain volumes of substances. Graduated cylinders, pipettes (also pipets), burettes (also burets) and volumetric flasks are examples as are beakers and Erlenmeyer flasks. It can be seen from this list that the term "calibrated" can mean "to a varying degree". The manufacturer's stated accuracy of beakers and Erlenmeyer flasks is typically 5% while that for volumetric flasks is generally much higher. In this laboratory experiment, the three types of glassware typically used by an analytical chemist: a volumetric flask, a volumetric pipet and a buret will be calibrated. These tools are used extensively when performing gravimetric and titrimetric Analyses. In order to avoid introducing systematic errors into measurements, each of these instruments must be properly calibrated. And, to reduce the random errors inherent when using these instruments, their proper use must be thoroughly understood. The quality of the measurements obtained from these tools depends heavily on the care taken in calibrating and in using each instrument. Procedures In the most precise work it is never safe to assume that the volume delivered by or contained in any volumetric instrument is exactly the amount indicated by the calibration mark. Instead, the amount of water delivered by or contained in the volumetric apparatus is quantified by mass to effect the calibration. This mass is then converted to the desired volume using the tabulated density of water and the following standard relation: d=m/v (1) where v is measured volume, m is mass of water used to fill the volume and d is density of the water. Figure 1. Volumetric flask. Figure 2. Volumetric and Measuring Pipettes. Figure 3. Burette.
Transcript
ColoradoStateUniversity 2018.10.29 Page1of6
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:
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.
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
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
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.
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.
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.
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 to three to four significant figures.For thisexperimentusedistilledwatercontainedinoneofthecarboys-notdirectlyfromthedistilledwatertap.(Why?)
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.Filltheflaskandemptyitthreetimestoperformthreereplicatemeasurementsfortheflask.Itisnotnecessarytoovendrytheflaskbetweenthesemeasurements.
Results
Report your experimental results to three figures (nomore, no fewer). Report the uncertainty of theresultsasdescribedbelow.VolumetricPipette.Use thedensityofwater toconvert themassofwater to thevolumedeliveredbyyourpipette.Determinetheaverageand95%confidenceintervalforthethreereplicatemeasurementsforthepipette.Ifthe95%confidenceintervalsaregreaterthanthetoleranceforthe10mLpipetteandiftimepermitsrepeatthesemeasurements.Tabulatetheseresults.VolumetricFlask.Convertthemassofwatercontainedinyourvolumetricflasktovolume.Determinetheaveragevolumeand95%confidenceinterval.Iftimepermits,repeatthecalibrationifthe95%confidenceintervalsgreaterthanthetoleranceforthe10mLflask.Tabulatetheseresults.MeasuringPipette.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 andexpectedvolumes.Includebothslopeandinterceptintheanalysis.
Report the slope, its standarddeviation, the y-intercept, its standarddeviationand theR2 valueof thecalibrationline.Calculatethe95%confidenceintervalforeachvalueofexpectedvolume.Useagraphicalfiguretodisplaytheseresults.
Present all measured data in the report together with the expected data. Describe and explain therelationshipsbetweenthetwosetsofdata. Includegood-qualityfiguresandtablestosubstantiateyourdiscussions. Include answers to the several "why?" questions that appear in this description. Include aquantitative error analysis in your discussion. Compare your assessment of the various volumetricglasswareaccuracyvaluestothemanufacturer'sstatedvalues.
