§7.10 Application of EMF and electrode potential Levine pp. 431--443 14.8 Concentration cell 14.9 Liquid-junction potential 14.10 Applications of EMF measurements 14.12 ion-selective membrane electrodes

Our Goals Ability: Compare the oxidation ability of species; Ability: determine the reaction direction through potential; Ability: determine activity coefficient of electrolyte; Knowledge: principle of electroanalysis. Knowledge: principle of ion-selective electrode.

7.10.1 Computation of emf For cell with single solution: Cd(s)|CdSO4(a±) |Hg2SO4(s)|Hg(l) Because a is a measurable quantity, E of the cell with single electrolyte can be calculated exactly.

Zn(s)|ZnSO4(m1) ||CuSO4(m1) |Cu(s) For cell with two electrolytic solutions: Zn(s)|ZnSO4(m1) ||CuSO4(m1) |Cu(s) we have to use mean activity coefficient () which is measurable in stead of the activity coefficient of individual ion (+ or -) which is unmeasurable.

7.10.2. Judge the strength of the oxidizing and reducing agents Oxidative form: Fe3+, I2 Reductive form: Fe2+, I- ⊖ (Fe3+/Fe2+) = 0.771 V ⊖ (I2/I) = 0.5362 V The oxidative form with higher (standard) electrode potential is stronger oxidizing species, while the reductive form with lower (standard) electrode potential is stronger reducing agent. Why? E > 0 criterion

(Ox)1 + (Red)2 = (Red)1+ (Ox)2 7.10.3 Determination of the reaction direction Stronger oxidizing species oxidizes stronger reducing species to produce weaker reducing and weaker oxidizing species. ⊖ (Fe3+/Fe2+) = 0.771 V; ⊖ (I2/I) = 0.5362 V (Ox)1 + (Red)2 = (Red)1+ (Ox)2 Fe3+ + I = Fe2+ + 1/2I2 When concentration differs far from the standard concentration,  should be used in stead of ⊖.

Application of Pourbaix diagram Cu2+ Cu(OH)2 Cu pH  / V 2 4 6 8 10 12 14 CuO22 Cu2O 0.0 0.5 1.0 1.5

Example In order to make Au in mine dissolve in alkaline solution with the aid of oxygen, people usually add some coordinating agent into the solution. Which coordination agent is favorable? Please answer this question based on simple calculation.

Cl2 + 2NaCl = NaCl + NaClO + H2O Divergent /Disproportionation reaction Cl2 + 2NaCl = NaCl + NaClO + H2O which species can undergo divergent reaction? Divergent reaction occur when R > L HIO  IO3 + I2

Exercise Can what species undergo divergent reaction?

7.10.4. Advance of reaction (equilibrium constants) Fe3+ + I¯  Fe2+ + ½ I2 At equilibrium 1 mol dm-3 iodine solution + Fe2+ (2 mol dm-3)

Standard emf and standard equilibrium constant For any reaction that can be designed to take place in an electrochemical cell, its equilibrium constant can be measured electrochemically. Four equilibria in solution 1) Dissolution equilibrium 2) Reaction equilibrium 3) Dissociation equilibrium 4) Coordination equilibrium

Example AgCl(s) Ag+ + Cl¯ The designed cell is Determine the solubility products of AgCl(s). AgCl(s) Ag+ + Cl¯ The designed cell is Ag(s)|AgNO3(a1)||KCl(a2)|AgCl(s)|Ag(s)

7.10.5 Potentiometric titrations GEH+(mx)SCE automatic potential titrator

Differential plot HCl-NaOH E / V HAc-NaOH inflexion point 0.00 10.00 20.00 30.00 40.00 50.00 0.300 0.100 0.500 0.700 E / V HAc-NaOH HCl-NaOH 0.4 0.2 inflexion point

7.10.6 Determination of mean ion activity coefficients Pt(s), H2 (g, p⊖)|HCl(m)|AgCl(s)-Ag(s) 1/2 H2 (g, p⊖) + AgCl(s) = Ag(s) + H+(m) + Cl(m) For combined concentration cell Using one electrolytic solution with known mean activity coefficient, the mean activity coefficient of another unknown solution can be determined.

Example: Pt(s), H2 (g, p) |HBr(m) | AgBr(s)-Ag(s) Given E = 0.0714 V, m = 1.262  10-4 mol·kg-1, E = 0.5330 V, calculate . Answer:  = 0.9946

Zn(s)|ZnSO4(a,1)|Hg2SO4(s)-Hg(l)-Hg2SO4(s)|ZnSO4(a,2)|Zn(s) 7.10.7 Determination of transference number The relationship between transference number and liquid junction potential can be made use of to determine the transference number of ions. Zn|ZnSO4(a,1) |ZnSO4(a,2) |Zn Zn(s)|ZnSO4(a,1)|Hg2SO4(s)-Hg(l)-Hg2SO4(s)|ZnSO4(a,2)|Zn(s) Electromotive forces of cell with and without liquid junction potential gives liquid junction potential.

7.10.8 Measurement of pH 1909, Sorensen defined: pH =  log [H+] Non-operational definition present definition: The way to determine pH 1) Hydrogen electrode Pt(s), H2 (g, p⊖)|H +(x) |SCE poison of platinized platinum

1:1 quinone: hydroquinone 2) Quinhydrone electrode Q + 2H + + 2e-  H2Q supramolecule : 1:1 quinone: hydroquinone Pt(s)|Q, H2Q, H+(mx) |SCE Equal concentrations of both species in the solution. Being nonelectrolytes, activity coefficients of dilute Q and H2Q is unity.

3) Glass electrode 内充液 离子选择性膜 0.1 molkg-1 HCl

Ag(s) AgCl(s) HCl(as)  GM H+(ax)(SCE) Outer Variable Inner fixed 2 4 6 8 10 12 14 16 -2 GE / mV pH Linear relation of GE and pH exists within pH range from 0 to 14. membrane potential  GE = ⊖ GE - 0.05915 pH Test cell: GE H+(mx)(SCE) Ag(s) AgCl(s) HCl(as)  GM H+(ax)(SCE) Reference-2 Reference-1

4) Operational definition of pH Es = ⊖SCE –(⊖GE - 0.05915 pHs ) Calibration Ex = ⊖SCE –(⊖GE - 0.05915 pHx ) Measurement pH of standard buffer solutions at 25 oC Buffer A B C D E pH 3.557 4.008 6.865 7.413 9.180 pH meter with standard buffer solution

What is the concentration of hydrogen ion in this solution? Composite electrode: with reference electrode, usually AgCl/Ag electrode embedded on the side of glass electrode.

7.10.9. Determination of ion concentration Ion-selective electrode For F- electrode, thin film of LaF3 single crystal is used as ion selective membrane. Cutaway view of an ion selective electrode For S2- electrode, compressed thin film of AgCl-Ag2S mixture is used as ion-selective membrane.

7.10.10 Electrochemical sensor Electrochemical nose Electroanalytical chip antigen antibody electrochemical sensor of potential type PbO2 Ion-exchange membrane amplifier annunciator Pt electrode Gas-permeable membrane electrochemical sensor of current type