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CORNELL CEE 453 - Measurement of Acid Neutralizing Capacity

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Figure . Gran plot from titration of a weak base with 0.05 N acid. Ct = 0.001 moles of carbonate and sample volume is 48 mL. The equivalent volume is 4.8 mL. From equation  the ANC is 5 meq/L.Measurement of Acid Neutralizing CapacityIntroductionTheoryGran PlotpH MeasurementsANC Determination for Samples with pH < 4Titration TechniquesProcedureCalibrate the pH ProbeDetermine ANC of a Known StandardDetermine ANC of Acid Rain SamplesPrelab QuestionsQuestionsWriting a High Performance ReportReferencesLab Prep NotesTable . Reagent list.Table . Equipment listSetupMeasurement of Acid Neutralizing CapacityIntroductionAcid neutralizing capacity (ANC) is a measure of the ability of water to neutralize acidinputs. Lakes with high ANC (such as Cayuga Lake) can maintain a neutral pH even withsome acid rain input whereas lakes with an ANC less than the acid input will not maintaina neutral pH. In the Adirondack region of New York State, lakes typically receive largeinputs of acids during the spring thaw when the accumulated winter snow melts and runsoff into the lakes. The ANC of Adirondack lakes is not always sufficient to neutralizethese inputs.TheoryThe ANC for a typical carbonate-containing sample is defined as:- -2 -3 3ANC [HCO ] 2[CO ] [OH ] - [H ]+= + +This equation can be derived from a charge balance if ANC is considered to be the cationcontributed by a strong base titrant and if other ions present do not contributesignificantly. Determination of ANC or Alkalinity involves determination of an equivalence point.The equivalence point is defined as the point in the titration where titrant volume that hasbeen added equals the "equivalent" volume (Ve). The equivalent volume is defined as:· =s setV NVNwhere:Ns = normality (in this case Alkalinity or ANC) of sample, equivalents/LVs = volume of sample, litersNt = normality of titrant, equivalents/L.The titration procedure involves incrementally adding known volumes of standardizednormality strong acid (or base) to a known volume of unknown normality base (or acid).When enough acid (or base) has been added to equal the amount of base (or acid) in theunknown solution we are at the "equivalence" point. (Note: the point at which we addexactly an equivalent or stoichiometric amount of titrant is the equivalence point.Experimentally, the point at which we estimate to be the equivalence point is called thetitration endpoint).There are several methods for determining Ve (or the equivalence point pH) fromtitration data (titrant volume versus pH). The shape of the titration curve (Vt versus pH)can reveal Ve. It can be shown that one inflection point occurs att eV V=. In the case ofmonoprotic acids, there is only one inflection in the pH range of interest. Therefore, aneffective method to find the equivalence volume is to plot the titration curve and find theinflection point. Alternately, plot the first derivative of the titration plot and look for amaximum.Gran PlotAnother method to find the ANC of an unknown solution is the Gran plot technique.When an ANC determination is being made, titration with a strong acid is used to"cancel" the initial ANC so that at the equivalence point the sample ANC is zero. TheGran plot technique is based on the fact that further titration will result in an increase inthe number of moles of H+ equal to the number of moles of H+ added. Thus after theequivalence point has been reach the number of moles of H+ added equals the number ofmoles of H+ in solution.( ) ( )t t e s tN V V V V H+� �- = +� �Solving for the hydrogen ion concentration:( )( )t t es tN V VHV V+-� �=� �+Equation can be solved directly for the equivalent volume. ( )s te ttH V VV VN+� �+� �= -Equation is valid if enough titrant has been added to neutralize the ANC. A bettermeasure of the equivalent volume can be obtained by rearranging equation so that linearregression on multiple titrant volume - pH data pairs can be used. ( )s tt t t es s sV VN V N VHV V V++� �= -� �We define F1 (First Gran function) as:y = 9.57E-04x - 4.62E-03R2 = 9.99E-0100.00010.00020.00030.00040.00050.00060.00070.00080.00090 1 2 3 4 5 6Volume of Titrant (mL)First Gran FunctionGran FunctionGran Function Linear RegionLinear (Gran Function LinearRegion)Figure . Gran plot from titration of a weak base with 0.05 N acid. Ct = 0.001 moles ofcarbonate and sample volume is 48 mL. The equivalent volume is 4.8 mL. Fromequation the ANC is 5 meq/L.1F [H ]s tsV VV++=If F1 is plotted as a function of Vt the result is a straight line with slope = tsNV and abscissaintercept of Ve (Figure ). The ANC is readily obtained given the equivalent volume. At the equivalence pt: s e tV ANC V N=Equation can be rearranged to obtain ANC as a function of the equivalent volume. ANC e tsV NV=pH MeasurementsThe pH can be measured either as activity ({H+} as measured approximately by pHmeter) or molar concentration ([H+]). The choice only affects the slope of F1 since [H+] ={H+}/.1{ }F [H ] s t s t t ets s sV V V V V VHV V Vg+++ + -= = = Nwhere  is the activity correction factor and the slope is Nt/V0. If H+ concentration is usedthen1 tF {H } Ns t t es sV V V VV Vg++ -= =where the slope is tsVg �N.(This analysis assumes that the activity correction factor doesn't change appreciablyduring the titration).There are many other Gran functions that can be derived. For example, one can bederived for Acidity or the concentration of a single weak or strong acid or base.To facilitate data generation and subsequent Gran plot construction and analysis pHversus titrant volume can be read directly into a computer, that can be programmed toanalyze the data using the Gran, plot theory. The program generates the Gran function forall data and then systematically eliminates data until the Gran function (plot) is as linearas possible. The line is then extrapolated to the abscissa to find the equivalent volume. ANC Determination for Samples with pH < 4After the equivalence point has been reached (adding more acid than ANC = 0) theonly significant terms in equation are H+� �� � and ANC.- -2 -3 3H HCO 2 CO OH +� � � � � � � �>> + +� � � � � � � �When the pH is 2 pH units or more below the pKs of the bases in the system the onlyspecies contributing significantly to ANC is the hydrogen ion (equation) and


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