Experiment 8

Experiment 8Potentiometric Determination of the Ionization Constant of a Weak Acid or a Weak BaseObjectivesTo determine the ionization constant of a weak acid or a weak base by potentiometric titrationTo determine whether the sample is monoprotic or polyproticTo identify (name) the acid or baseIntroductionPotentiometryPotentiometry is the field of electroanalytical chemistry in which potential is measured under the conditions of no current flow. The potential that develops in the electrochemical cell is the result of the free energy change that would occur if the chemical phenomena were to proceed until the equilibrium condition has been satisfied.

Potentiometric TitrationA titrimetric method involving measurement of the potential between a reference electrode and an indicator electrode as a function of titrant volume.Potentiometric titrations are particularly useful with colored or turbid solutions and for detecting the presence of unsuspected species.The potentiometric instrument merely signals the end point and thus behaves in an identical fashion to a chemical indicator.Potentiometric TitrationThe pH of a solution is measured as a function of the amount of titrant added. The change in pH is small until the end point where there is a sharp change. The strength of an acid or base determines the sharpness of the change.The end point is found by graphing the points and determining the location of the sudden change in pH. Types of Potentiometric TitrationAcid-baseRedox

PrecipitationComplexometricIntroductionThe strength of an acid is defined by its ability to donate a proton to a base.For many common weak acids, we can quantify acid strength by expressing it as the equilibrium constant for the reaction in which the acid donates a proton to the standard base, water, as shown in the equations below:HA (aq) + H2O (l) H3O+(aq)+ A- (aq)

IntroductionThe same can be applied to a weak base and the formula for the base dissociation constant is given by the formula below:A- (aq) + H2O (l) OH-(aq) + HA (aq)

IntroductionA typical procedure in experimentally determining pKi would be as follows: -- acidification with strong acid -- titration with strong base -- at each point, pH is measured using a glass electrode and a pH meter -- equilibrium constants are found by plotting the recorded pH against the volume of the titrant used -- first derivative and second derivative of the graph are also constructed to determine the correct end pointA Typical Cell For Potentiometric AnalysisEindEjErefReference ElectrodeHas an accurately known electrode potential independent of the concentration of the solution.Must have a potential that is unchanged by the passage of the small amount of current required of the instrument.Treated as the anodeSalt BridgePrevents the components of the analyte solution from mixing with those of the reference electrode.Indicator ElectrodeImmersed in a solution of the analyte, develops a potential which depends on the activity of the analyte.Treated as the cathode.

Glass ElectrodeComposed of: A thin hydrogen-ion-responsive glass membrane sealed to a stem of high resistance, nonresponsive glassAn internal reference electrode with a constant internal hydrogen-ion concentrationGlass ElectrodeThere is no formal electron exchange involved in their functioningThus, they are uninfluenced by oxidizing and reducing agents in a solution unlike other pH electrodes.However, they must be regularly calibrated with buffers with about a pH unit of the pH to be measured.Glass ElectrodeThese consists of a thin glass bulb at the bottom that is selective to H+.Two reference electrodes (usually Ag/AgCl) measure the potential difference across the membrane.

IntroductionDerivativey/x vs. xaveHenderson- Hasselbach EquationpH = pKa + log [A-] / [HA]

IntroductionStandard Hydrogen Electrode (SHE)2H+ + 2e- H2(g)Calomel electrodeHg2Cl2 + 2e- 2Hg + 2Cl-Ag/AgCl electrodeAgCl + e- Ag +Cl-MethodologyMethodologyThe total volume of added strong base should be small compared to the initial volume of analyte solution in order to keep the ionic strength nearly constant. This will ensure that pKaremains constant during the titration.MethodologyFor the experiment, a combination electrode that incorporates both electrodes (glass indicator-calomel electrode) surrounded by a protective plastic sleeve was used.The pH meter, a high input impedance millivolt meter, is calibrated by setting the pH to that of a known standard buffer.

unknowntest

Litmus paperIf acidicIf basic

0.1 M NaOH

0.1 M HCl

Warm up the pH meter for 5 minutescalibratepH 4pH 7pH 9rinsedry

0.20.01 g or 5ml of the unknown sample diluted with 25 ml water stirMeasure pHRecord the initial pHRinse and dryRepeat until there is a sharp difference in pH1 ml incrementtitrate2 ml increment

titrateRepeat until the pH becomes constantErrors in pH MeasurementStandards: The pH measurement cannot be any more accurate than the standards (typically +0.01 pH unit).Junction potential: Changing the ionic composition of the analyte (compared to standard), changes the junction potential that exists at the porous plug. Gives an uncertainty of at least 0.01 pH unit.Errors in pH MeasurementJunction potential drift: The presence of a reducing agent in the analyte can causes Ag(s) to be precipitated inside the plug, changing the junction potential.Alkaline error: the electrode also responds to alkaline ions (Li+, Na+). Having high concentrations of these causes the apparent pH to be lower than the true pH.

Errors in pH MeasurementAcid error: The measured pH is always higher than the actual pH in strong acid solutions because the glass surface becomes saturated with H+ and cannot be protonated at anymore sites.Equilibration time: Electrode must equilibrate with the solution.Hydration of glass: A dry electrode dont work.Errors in pH MeasurementTemperature: This affects ion mobility and, consequently, the pH measurement.Errors in Low Ionic Strength Solutions: Partial clogging of the fritted plug or porous fiber which is used to restrict the flow of liquid from the salt bridge to the analyte solution

ResultsResults (Unknown A)VolumepH03.6414.2324.6434.8845.154.55.3255.525.55.7766.236.26.736.510.21710.927.511.158.511.379.511.5311.511.63Unknown AUnknown AUnknown AResults (Unknown B)VolumepH08.7418.2127.962.57.8637.793.67.6747.6257.5367.4377.337.57.387.268.57.2297.2107.11117126.94136.86146.79156.6816.16.4917.16.3518.16.2119.16.0120.15.8721.15.6821.65.3922.14.5622.6423.13.6623.63.4824.63.2125.63.126.62.9628.62.8Unknown BUnknown BUnknown BResults (Unknown C)VolumepH08.370.58.2217.951.57.3324.162.23.162.42.862.62.712.82.6232.53.22.453.42.37Unknown CUnknown CUnknown CResults (Unknown D)VolumepH010.10.510.0219.871.59.6529.342.58.9738.123.57.5147.424.57.2557.135.56.9766.876.56.7276.587.56.2684.988.52.9592.69.52.42102.3610.52.26112.17Unknown DUnknown DUnknown DDiscussionWhat makes up a Titration Curve?Initial pHPreequivalenceregion where moles of analyte remaining> moles of titrantBuffering Regionthe start of the titration where there is a gradual change in pHboth the acid (or base) and its conjugate are in significant amounts Equivalence Point- the point where there is a drastic change in pHPost-equivalenceOnly excess titrant remains

Monoprotic AcidsPolyprotic AcidsDetecting EndpointsThe endpoint occurs at the volume, V', where pH/ml has the maximum value:pH2 - pH1 = pH V2 - V1 = mlDetecting the EndpointUsing the pH curveVisually estimate the inflection point in the steeply rising portion of the curve, and take it as the end pointUsing the First DerivativeCalculate the change in potential per unit volume of the titrantPlot of the first derivative data as a function of the average volume produces a curve with a maximum that corresponds to the point of inflection

Detecting EndpointsUsing the Second DerivativeThe data changes sign at the point of inflectionThe point at which the second derivative crosses zero is the inflection point, which is taken as the end pointThis point can be located quite precisely

CASE 1: Monoprotic Weak AcidFor a monoprotic weak acid, HA, it undergoes the dissociation reaction:HA(aq) + H2O(l) H3O+(aq) + A-(aq)

Figure 7. Titration curve for a monoprotic weak acidMonoprotic Weak AcidAt equivalence point, all the moles of the analyte HA are all used up and converted to A-. This just implies that at half equivalence point, half of the acid has only reacted at the half equivalence point, therefore forming of the possible conjugate base that can be formed with the analyte. Therefore at this point, pKa = pH as the Henderson-Hasselbach equation sayspH = pKa + log [A-] / [HA]Monoprotic Weak Acidif [A-] = [HA], then the expression is:pH = pKa + log 1 ,where log 1 = 0thus, pH = pKaHalf of the volume at equivalence point that will be obtained will give the value for the pKa since at this volume, pH = pKa.Unknown AUnknown AEquivalence Point Volume: 6.55 mlHalf Equivalence Volume: 3.275 mlpH at Half Equivalence: 4.95425Ka : 1.11x10-5 CASE 2: Monoequivalent Weak BaseB (aq) + H2O (l) HB+ (aq) + OH-(aq)Figure 8. Titration Curve for a Monoequivalent Weak baseMonoequivalent Weak BaseWeak monoequivalent base and the case presented earlier use almost the same principle. At half equivalence point half of the analyte base has reacted and has formed of the total possible conjugate acid that can be formed.

At half-equivalence point, pOH=pKb.

Unknown BUnknown CUnknown BEquivalence Point Volume: 22.1 mlHalf Equivalence Volume: 11.05 mlpOH at Half Equivalence: 7Kb : 1x10-7Unknown CEquivalence Point Volume: 1.925 mlHalf Equivalence Volume: 0.9625 mlpOH at Half Equivalence: 6.08Kb : 8.32x10-7CASE 3: Polyprotic Weak Acid(1) H2X(aq) H+(aq) + HX-(aq)Ka1(2) HX-(aq) H+(aq) + X2-(aq)Ka2

Figure 3. Titration Curve for a Diprotic AcidPolyprotic Weak AcidSo for the first and second half equivalence points respectively:pH = pKa1 + log [HX-] / [H2X]pH = pKa2 + log [X-] / [HX-]

CASE 4: Polyequivalent BaseFor example, Na2CO3, which is a diequivalent base, has two ionizations:

Polyequivalent BaseFigure 9. Titration Curve of Na2CO3, a diequivalent basePolyequivalent BaseFigure 9.Plot of the First Derivative of the Titration Curve of Na2CO3Polyequivalent BasepOH = pKb1 + log [HCO3-] / [CO3]first half-equivalence point, pOH = pKb1pOH = pKb2 + log [H2CO3] / [HCO3-]second half-equivalence point, pOH = pKb2.

Unknown DUnknown DEquivalence Point Volume: 3.00 mlHalf Equivalence Volume: 1.50 mlpOH2 at Half Equivalence: 4.35Kb2 : 4.47x10-5 Equivalence Point Volume: 8.50 mlHalf Equivalence Volume: 5.75 mlpOH1 at Half Equivalence: 7.08Kb1 : 8.31x10-8

ConclusionConclusionPotentiometric titration is a method that can be used to determine the ionization constant of a weak acid or base; to classify the said weak acid or as either monoprotic or diprotic, the weak base as polyfunctional; and to identify the name of the acid or baseObtaining the volume of the titrant at equivalence point enabled the determination of the volume of titrant at half-equivalence pointConclusionThe corresponding pH of the volume of the latter can also be determined.Finding the corresponding pH led to the determination of the dissociation constant, Ka or Kb.ConclusionThe Equivalence Point occurs at the point of maximum deflection in titration curvethe maximum/minimum in a 1st Derivative Plot the point where the line crosses zero (change of sign) in a 2nd Derivative Plot pKi is equal to pH EP for acids and to pOH EP for bases

Recommendations For most cases, it may be necessary to consider adjustments as regards the computation of ionization constants given the fact that they are highly dependent to temperature.Therefore, it is wise to remember that printed literature (eg. Skoogs ionization constants) are not at all times applicable.RecommendationsBe very careful when using computer programs for the graphs of titration curves and its derivatives.It is advisable to use as little plot increments as possible to make the detection of the endpoints as accurately as possible.Recommendations With regards to the pH meter, it is almost completely dependent to luck for the instrument to produce precise results.However, the calibration of the pH meter is one major factor for the proper functioning of the pH meter. One should also remember to rinse the electrode before each measurement to minimize possible contaminants.