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A Potentiometric Titration

This page illustrates the use of the Equilibria package and the ChemEquilibria applet in solving equilibrium problems.

The ChemEquilibria applet will be used to calculate points on a titration curve for the titration of a mixture of Fe2+ and Sn2+ with Cr2O72- in a 1 mole/L HCl solution. The progress of the titration is monitored potentiometrically.

Step 1. Define all Chemical Species

The various chemical species may be defined by the following JavaScript instructions. It is assumed that the counter-ion for the metal ions is chloride ion.

document.chemEquilibria.createChemSpecies("Fe+2",+2.0,55.845);
document.chemEquilibria.createChemSpecies("Fe+3",+3.0,55.845);
document.chemEquilibria.createChemSpecies("Sn+2",+2.0,118.71);
document.chemEquilibria.createChemSpecies("Sn+4",+4.0,118.71);
document.chemEquilibria.createChemSpecies("K+",+1.0,39.098);
document.chemEquilibria.createChemSpecies("Cl-",-1.0,35.453);
document.chemEquilibria.createChemSpecies("Cr2O7-2",-2.0,215.99);
document.chemEquilibria.createChemSpecies("Cr+3",+3.0,51.996);
document.chemEquilibria.createChemSpecies("Pt",0.0,195.078);

The platinum is added, because a platinum indicator electrode is used in the potentiometric titration.

Step 2. Create the Phases

The titration involves two aqueous solutions: the titrant (t) and the sample solution (ss); 100.0 mL of each solution is prepared. The platinum indicator electrode is a solid phase in the system. The first "Pt" argument is the label for the solid and the second is the label for the ChemSpecies the makes up the solid.

document.chemEquilibria.createAqueousPhase("t",0.100);
document.chemEquilibria.createAqueousPhase("ss",0.100);
document.chemEquilibria.createSolidPhase("Pt","Pt",1.00);

Step 3. Add the Species to the Phases

The titrant contains 0.2000 mole/L potassium dichromate solution. The sample solution contains 0.02000 mole/L iron(II) chloride and 0.05000 mole/L tin(II) chloride.

document.chemEquilibria.addSpecies("K+","t",0.004000);
document.chemEquilibria.addSpecies("Cr2O7-2","t",0.002000);
document.chemEquilibria.addSpecies("H+","t",0.1000);
document.chemEquilibria.addSpecies("Cl-","t",0.1000);
document.chemEquilibria.addSpecies("Cr2O7-2","ss",0.0);
document.chemEquilibria.addSpecies("Cr+3","ss",0.0);
document.chemEquilibria.addSpecies("Fe+2","ss",0.001200);
document.chemEquilibria.addSpecies("Fe+3","ss",0.0);
document.chemEquilibria.addSpecies("Sn+2","ss",0.001800);
document.chemEquilibria.addSpecies("Sn+4","ss",0.0);
document.chemEquilibria.addSpecies("H+","ss",0.1000);
document.chemEquilibria.addSpecies("Cl-","ss",0.10600);

It is necessary to define all of the various redox species in the sample solution in order to define the reactions in the next step.

Step 4. Define the Chemical Reactions

The formal potential for the Cr2O72-/Cr+3 couple in 1 mole/L HCl is +1.00 V vs NHE. The formal potential for the Fe+3/Fe2+ couple is 0.732 V vs NHE and that of the Sn+4/Sn2+ couple is +0.139 V vs NHE. These potentials permit calculation of the equilibrium constants for the reactions.

document.chemEquilibria.addReaction("Rxn1","6 Fe+2 (ss) + Cr2O7-2 (ss)  +  14 H+ (ss) = 6 Fe+3 (ss) + 2 Cr+3 (ss) + 7 H2O (ss)", 1.5E27);
document.chemEquilibria.addReaction("Rxn2","2 Fe+3 (ss) + Sn+2 (ss) = 2 Fe+2 (ss) + Sn+4 (ss)", 1.1E20);
document.chemEquilibria.addHalfReaction("Rxn3","Fe+3 (ss) + e- (Pt) = Fe+2 (ss)", 0.491);

The first two reactions are sufficient to describe the reaction between dichromate ion and tin(II) ion. The third reaction is really a half-reaction, and it will be used to determine the potential of the platinum electrode. The formal potential of the half-reaction is reported vs SCE rather than NHE. This envisions an electrochemical cell containing a platinum indicator electrode and SCE reference, such that the half-cell potential (reported vs SCE) is really the measured cell potential. Note that the phase of the electron in the half-reaction is "Pt", indicating the platinum electrode defined above.

Step 5. Create the Titration Curve

In this problem, it is assumed that all reactions behave ideally. This obviously is not true owing to the high ionic strength (1 mole/L HCl), but the use of formal potentials rather than standard potentials accounts for nonideal behavior.

document.chemEquilibria.setIsIdeal(true);

The equilibria package permits a Titration object to be defined. In defining a Titration object, it is necessary to designate one solution as the titrant and a second solution as the sample solution. It is also necessary to identify the property of the sample solution that is being monitored during the titration. One can monitor pX (where X is some species in the sample solution) or one can monitor a cell or half-cell potential. In this case the potential of the platinum electrode will be monitored. Because a half-cell potential is being monitored, the second argument (which indicates the second electrode in the cell) in the setMonitor method is null.

document.chemEquilibria.createTitration("t","ss");
document.chemEquilibria.setMonitor("Pt",null);

Step 6. Calculate the Titration Curve

The calculate method calculates a titration curve for the volume of titrant ranging from zero to the volume (in mL) supplied in the arguement. The instruction below constructs a titration curve where up to 50.0 mL of titrant have been added.

document.chemEquilibria.calculate(50.0);

The calculate method automatically determines how much titrant to add at each step in the titration in order to obtain a smooth curve with well-defined equivalence points.




PotentiometricTitration.html version 2.1
© 2001-2014 David N. Blauch