Potentiometry

1. Potentiometry • Potential measurements of electrochemical cells • Ion selective methods • Reference electrode • Indicator electrode • Potential measuring device • Reference electrode • Indicator electrodes • Ion specific electrodes • Potentiometric measurements

2. Reference electrode • Known half-cell • Insensitive to solution under examination • Reversible and obeys Nernst equation • Constant potential • Returns to original potential • Calomel electrode • Hg in contact with Hg(I) chloride • Ag/AgCl

3. Calomel electrode

5. Indicator electrode • Ecell=Eindicator-Ereference • Metallic • 1st kind, 2nd kind, 3rd kind, redox • 1st kind • respond directly to changing activity of electrode ion • Direct equilibrium with solution

6. Ion selective electrode • Not very selective • simple • some metals easily oxidized (deaerated solutions) • some metals (Zn, Cd) dissolve in acidic solutions • Ag, Hg, Cu, Zn, Cd, Bi, Tl, Pb

7. 2nd kind • Precipitate or stable complex of ion • Ag for halides • Ag wire in AgCl saturated surface • Complexes with organic ligands • EDTA • 3rd kind • Electrode responds to different cation • Competition with ligand complex

8. Metallic Redox Indictors • Inert metals • Pt, Au, Pd • Electron source or sink • Redox of metal ion evaluated • May not be reversible • Membrane Indicator electrodes • Non-crystalline membranes: • Glass - silicate glasses for H+, Na+ • Liquid - liquid ion exchanger for Ca2+ • Immobilized liquid - liquid/PVC matrix for Ca2+ and NO3- • Crystalline membranes: • Single crystal - LaF3 for FPolycrystalline • or mixed crystal - AgS for S2- and Ag+ • Properties • Low solubility - solids, semi-solids and polymers • Some electrical conductivity - often by doping • Selectivity - part of membrane binds/reacts with analyte

9. Glass Membrane Electrode

10. Glass membrane structure • H+ carries current near surface • Na+ carries current in interior • Ca2+ carries no current (immobile)

11. Boundary Potential • Difference in potentials at a surface • Potential difference determined by • Eref 1 - SCE (constant) • Eref 2 - Ag/AgCl (constant) • Eb • Eb = E1 - E2 = 0.0592 log(a1/a2) • a1=analyte • a2=inside ref electrode 2 • If a2 is constant then • Eb = L + 0.0592log a1 • = L - 0.0592 pH • where L = -0.0592log a2 • Since Eref 1 and Eref2 are constant • Ecell = constant - 0.0592 pH

12. Alkaline error • Electrodes respond to H+ and cation • pH differential • Glass Electrodes for Other Ions: • Maximize kH/Na for other ions by modifying glass surface • Al2O3 or B2O3) • Possible to make glass membrane electrodes for • Na+, K+, NH4+, Cs+, Rb+, Li+, Ag+

13. Crystalline membrane electrode • Usually ionic compound • Single crystal • Crushed powder, melted and formed • Sometimes doped (Li+) to increase conductivity • Operation similar to glass membrane • F electrode

14. Liquid membrane electrodes • Based on potential that develops across two immiscible liquids with different affinities for analyte • Porous membrane used to separate liquids • Selectively bond certain ions • Activities of different cations • Calcium dialkyl phosphate insoluble in water, but binds Ca2+ strongly

16. Molecular Selective electrodes • Response towards molecules • Gas Sensing Probes • Simple electrochemical cell with two reference electrodes and gas permeable PTFE membrane • allows small gas molecules to pass and dissolve into internal solution • O2, NH3/NH4+, and CO2/HCO3-/CO32-

18. Biocatalytic Membrane Electrodes • Immobilized enzyme bound to gas permeable membrane • Catalytic enzyme reaction produces small gaseous molecule (H+, NH3, CO2) • gas sensing probe measures change in gas concentration in internal solution • Fast • Very selective • Used in vivo • Expensive • Only few enzymes immobilized • Immobilization changes activity • Limited operating conditions • pH • temperature • ionic strength

19. Electrode calibration

20. NH4 electrode

21. Potentiometric titration

22. Coulometry • Quantitative conversion of ion to new oxidation state • Constant potential coulometry • Constant current coulometry • Coulometric titrations • Electricity needed to complete electrolysis measured • Electrogravimetry • Mass of deposit on electrode

23. Constant voltage coulometry • Electrolysis performed different ways • Applied cell potential constant • Electrolysis current constant • Working electrode held constant • ECell=Ecathode-Eanode +(cathode polarization)+(anode polarization)-IR • Constant potential, decrease in current • 1st order • It=Ioe-kt • Constant current change in potential • Variation in electrochemical reaction • Metal ion, then water

25. Analysis • Measurement of electricity needed to convert ion to different oxidation state • Coulomb (C) • Charge transported in 1 second by current of 1 ampere • Q=It I= ampere, t in seconds • Faraday (F) • Charge in coulombs associated with mole of electrons • 1.602E-19 C for electron • F=96485 C/mole e- • Q=nFN • Find amount of Cu2+ deposited at cathode • Current = 0.8 A, t=1000 s • Q=0.8(1000)=800 C • n=2 • N=800/(2*96485)=4.1 mM

26. Coulometric methods • Two types of methods • Potentiostatic coulometry • maintains potential of working electrode at a constant so oxidation or reduction can be quantifiably measured without involvement of other components in the solution • Current initially high but decreases • Measure electricity needed for redox • arsenic determined oxidation of arsenous acid (H3AsO3) to arsenic acid (H3AsO4) at a platinum electrode. • Coulometric titration • titrant is generated electrochemically by constant current • concentration of the titrant is equivalent to the generating current • volume of the titrant is equivalent to the generating time • Indicator used to determined endpoint