5.10 Total Alkalinity | Monitoring & Assessment | US EPA

Total alkalinity is measured by measuring the amount of acid (e.g., sulfuric acid) needed to bring the sample to a pH of 4.2. At this pH all the alkaline ... Jumptomaincontentorareanavigation. LearntheIssues Science&Technology Laws&Regulations AboutEPA ContactUs Water:Monitoring&Assessment You are here: WaterOurWatersRivers&StreamsMonitoring&Assessment5.10TotalAlkalinity 5.10TotalAlkalinity Whatistotalalkalinityandwhyisitimportant? Alkalinityisameasureofthecapacityofwatertoneutralizeacids(seepHdescription).Alkalinecompoundsinthewatersuchasbicarbonates(bakingsodaisonetype),carbonates,andhydroxidesremoveH+ionsandlowertheacidityofthewater(whichmeansincreasedpH).TheyusuallydothisbycombiningwiththeH+ionstomakenewcompounds.Withoutthisacid-neutralizingcapacity,anyacidaddedtoastreamwouldcauseanimmediatechangeinthepH.Measuringalkalinityisimportantindeterminingastream'sabilitytoneutralizeacidicpollutionfromrainfallorwastewater.It'soneofthebestmeasuresofthesensitivityofthestreamtoacidinputs. Alkalinityinstreamsisinfluencedbyrocksandsoils,salts,certainplantactivities,andcertainindustrialwastewaterdischarges. Totalalkalinityismeasuredbymeasuringtheamountofacid(e.g.,sulfuricacid)neededtobringthesampletoapHof4.2.AtthispHallthealkalinecompoundsinthesampleare"usedup."Theresultisreportedasmilligramsperliterofcalciumcarbonate(mg/LCaCO3). Analyticalandequipmentconsiderations Fortotalalkalinity,adoubleendpointtitrationusingapHmeter(orpH"pocketpal")andadigitaltitratororburetisrecommended.Thiscanbedoneinthefieldorinthelab.Ifyouwillanalyzealkalinityinthefield,itisrecommendedthatyouuseadigitaltitratorinsteadofaburetbecausetheburetisfragileandmoredifficulttosetupanduseinthefield.ThealkalinitymethoddescribedbelowwasdevelopedbytheAcidRainMonitoringProjectoftheUniversityofMassachusettsWaterResourcesResearchCenter. Burets,titrators,anddigitaltitratorsformeasuringalkalinity Thetotalalkalinityanalysisinvolvestitration.Inthistest,titrationistheadditionofsmall,precisequantitiesofsulfuricacid(thereagent)tothesampleuntilthesamplereachesacertainpH(knownasanendpoint).Theamountofacidusedcorrespondstothetotalalkalinityofthesample.Alkalinitycanbemeasuredusingaburet,titrator,ordigitaltitrator(describedbelow). Aburetisalong,graduatedglasstubewithataperedtiplikeapipetandavalvethatisopenedtoallowthereagenttodripoutofthetube.Theamountofreagentusediscalculatedbysubtractingtheoriginalvolumeintheburetfromthevolumeleftaftertheendpointhasbeenreached.Alkalinityiscalculatedbasedontheamountused. Titratorsforcefullyexpelthereagentbyusingamanualormechanicalplunger.Theamountofreagentusediscalculatedbysubtractingtheoriginalvolumeinthetitratorfromthevolumeleftaftertheendpointhasbeenreached.Alkalinityisthencalculatedbasedontheamountusedorisreaddirectlyfromthetitrator. Digitaltitratorshavecountersthatdisplaynumbers.Aplungerisforcedintoacartridgecontainingthereagentbyturningaknobonthetitrator.Astheknobturns,thecounterchangesinproportiontotheamountofreagentused.Alkalinityisthencalculatedbasedontheamountused.Digitaltitratorscostapproximately$90. Digitaltitratorsandburetsallowformuchmoreprecisionanduniformityintheamountoftitrantthatisused. Howtocollectandanalyzesamples ThefieldproceduresforcollectingandanalyzingsamplesforpHandtotalalkalinityconsistofthefollowingtasks: TASK1Preparethesamplecontainers Samplecontainers(andallglasswareusedinthisprocedure)mustbecleanedandrinsedbeforethefirstrunandaftereachsamplingrunbyfollowingtheproceduredescribedunderMethodAinChapter5-WaterQualityConditions.Remembertowearlatexgloves. TASK2Preparebeforeleavingforthesamplingsite Refertosection2.3-SafetyConsiderationsfordetailsonconfirmingsamplingdateandtime,safetyconsiderations,checkingsupplies,andcheckingweatheranddirections.Inadditiontothestandardsamplingequipmentandapparel,whensamplingforpHandalkalinityincludethefollowingequipment: Digitaltitrator 100-mLgraduatedcylinder 250-mLbeaker pHmeterwithcombinationtemperatureandreferenceelectrodeorpH"pocketpal" Sulfuricacidtitrationcartridge,0.16N DatasheetforpHandtotalalkalinitytorecordresults Alkalinityvoluetteampulesstandard,0.500N,foraccuracycheck WashbottlewithdeionizedwatertorinsepHmeterelectrode Magneticstirrer,iftitratedinthelab BesuretocalibratethepHmeterbeforeyouanalyzeasample.ThepHmetershouldbecalibratedpriortosampleanalysisandafterevery25samplesaccordingtotheinstructionsinthemetermanual.UsetwopHstandardbuffersolutions:4.01and7.0.Followingarenotesregardingbuffers: Thebuffersolutionsshouldbeatroomtemperaturewhenyoucalibratethemeter. Donotuseabufferafteritsexpirationdate. Alwayscapthebuffersduringstoragetopreventcontamination. BecausebufferpHvalueschangewithtemperature,themetermusthaveabuilt-intemperaturesensorthatautomaticallystandardizesthepHwhenthemeteriscalibrated. Donotreusebuffersolutions! Besuretoletsomeoneknowwhereyouaregoingandwhenyouexpecttoreturn. TASK3Collectthesample RefertoTask2inChapter5-WaterQualityConditionsfordetailsonhowtocollectwatersamplesusingscrew-capbottlesorWhirl-pak®bags. TASK4Measuretotalalkalinity(fieldorlab) Thefollowingstepsareforuseofadigitaltitratorinthefieldorthelab.Ifyouareusingaburet,consultStandardMethods(APHA,1992). Alkalinityisusuallymeasuredusingsulfuricacidwithadigitaltitrator.Sulfuricacidisaddedtothewatersampleinmeasuredamountsuntilthethreemainformsofalkalinity(bicarbonate,carbonate,andhydroxide)areconvertedtocarbonicacid.AtpH10,hydroxide(ifpresent)reactstoformwater.AtpH8.3,carbonateisconvertedtobicarbonate.AtpH4.5,itiscertainthatallcarbonateandbicarbonateareconvertedtocarbonicacid.BelowthispH,thewaterisunabletoneutralizethesulfuricacidandthereisalinearrelationshipbetweentheamountofsulfuricacidaddedtothesampleandthechangeinthepHofthesample.So,additionalsulfuricacidisaddedtothesampletoreducethepHof4.5byexactly0.3pHunits(whichcorrespondstoanexactdoublingofthepH)toapHof4.2.However,theexactpHatwhichtheconversionofthesebasesmighthavehappened,ortotalalkalinity,isstillunknown.Thisprocedureusesanequationderivedfromtheslopeofthelinedescribedabovetoextrapolatebacktotheamountofsulfuricacidthatwasaddedtoactuallyconvertallthebasestocarbonicacid.Themultiplier(0.1)thenconvertsthistototalalkalinityasmg/LCaCO3.Thefollowingstepsoutlinetheproceduresnecessarytodeterminethealkalinityofyoursample. Insertacleandeliverytubeintothe0.16Nsulfuricacidtitrationcartridgeandattachthecartridgetothetitratorbody. Holdthetitrator,withthecartridgetippointingup,overasink.Turnthedeliveryknobtoejectairandafewdropsoftitrant.Resetthecounterto0andwipethetip. MeasurethepHofthesample(seepH,section5.4).Ifitislessthan4.5,gotostep9below. Insertthedeliverytubeintothebeakercontainingthesample.TurnthedeliveryknobwhilemagneticallystirringthebeakeruntilthepHmeterreads4.5.RecordthenumberofdigitsusedtoachievethispH.Donotresetthecounter. ContinuetitratingtoapHof4.2andrecordthenumberofdigits. Applythefollowingequation:Alkalinity(asmg/LCaCO3)=(2a-b)x0.1 Where: a = digitsoftitranttoreachpH4.5 b = digitsoftitranttoreachpH4.2(includingdigitsrequiredtogettopH4.5) 0.1 = digitmultiplierfora0.16titrationcartridgeanda100-mLsample Example: InitialpHofsampleis6.5. Ittakes108turnstogettoapHof4.5. Ittakesanother5turnstogettopH4.2,foratotalof113turns. Alkalinity = = ((2x108)-113)x0.1 10.3mg/L Recordtheresultsasmg/Lalkalinityonthelabsheet. Rinsethebeakerwithdistilledwaterbeforethenextsample. IfthepHofyourwatersample,priortotitration,islessthan4.5,proceedasfollows: Insertthedeliverytubeintothebeakercontainingthesample. TurnthedeliveryknobwhileswirlingthebeakeruntilthepHmeterreadsexactly0.3pHunitslessthantheinitialpHofthesample. RecordthenumberofdigitsusedtoachievethispH. Applytheequationasinstep6,buta=0andb=thenumberofdigitsrequiredtoreducetheinitialpHexactly0.3pHunits. Example: InitialpHofsampleis4.3. Enter"0"inthe4.5columnonthelabsheet. TitratetoapHof0.3unitslessthantheinitialpHinthiscas4.0. Ittakes10digitstogetto4.0. Enterthisinthe4.2columnonthelabsheetandnotethatthepHendpointis4.0.Alkalinity=(0-10)x0.1=-1.0. Recordtheresultsasmg/Lalkalinityonthelabsheet. Performanaccuracycheckonthefirstfieldsample,halfwaythroughtherun,andafteranalysisofthelastsampleasdescribedbelow.CheckthepHmeteragainstpH7.0and4.01buffersafterevery10samples. TASK5Performanaccuracycheck Thisaccuracycheckshouldbeperformedonthefirstfieldsampletitrated,againabouthalfwaythroughthefieldsamples,andatthefinalfieldsample. Snaptheneckoffanalkalinityvoluetteampulestandard,0.500N.Orifusingastandardsolutionfromabottle,pourafewmillilitersofthestandardintoacleanbeaker. Pipet0.1mLofthestandardtothetitratedsample(seeabove).ResumetitrationbacktothepH4.2endpoint.Recordthenumberofdigitsneeded. Repeatusingtwomoreadditionsof0.1mLofstandard.TitratetothepH4.2aftereachaddition. Each0.1-mLadditionofstandardshouldrequire250additionaldigitsof0.16Ntitrant. TASK6Returnthefielddatasheetsandsamplestothelabordrop-offpoint Alkalinitysamplesmustbeanalyzedwithin24hoursoftheircollection.Ifthesamplescannotbeanalyzedinthefield,keepthesamplesoniceandtakethemtothelabordrop-offpointassoonaspossible. References APHA.1992.Standardmethodsfortheexaminationofwaterandwastewater.18thed.AmericanPublicHealthAssociation,Washington,DC. Godfrey,P.J.1988.AcidraininMassachusetts.UniversityofMassachusettsWaterResourcesResearchCenter,Amherst,MA. RiverWatchNetwork.1992.TotalalkalinityandpHfieldandlaboratoryprocedures(basedonUniversityofMassachusettsAcidRainMonitoringProject).July1. AreaNavigation WaterHome DrinkingWater Education&Training Grants&Funding Laws&Regulations OurWaters DrinkingWater GroundWater Lakes Oceans,Coasts,Estuaries&Beaches Rivers&Streams Stormwater Wastewater Watersheds Wetlands WhereYouLive PollutionPrevention&Control Resources&Performance Science&Technology WaterInfrastructure WhatYouCanDo NewsFeeds Podcasts EPAMobile NewsbyEmail Widgets EPAHomePrivacyandSecurityNoticeContactUs LastupdatedonTuesday,March06,2012 TheSealoftheUnitedStatesEnvironmentalProtectionAgency Jumptomaincontent.