15-1 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

15-2 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

15-3 Calomel electrode

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15-5 Indicator electrode E cell =E indicator -E reference Metallic §1 st kind, 2 nd kind, 3 rd kind, redox 1 st kind §respond directly to changing activity of electrode ion §Direct equilibrium with solution

15-6 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

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

15-8 Metallic Redox Indictors Inert metals §Pt, Au, Pd àElectron source or sink àRedox of metal ion evaluated §May not be reversible Membrane Indicator electrodes §Non-crystalline membranes: àGlass - silicate glasses for H+, Na+ àLiquid - liquid ion exchanger for Ca2+ àImmobilized liquid - liquid/PVC matrix for Ca2+ and NO3- §Crystalline membranes: àSingle crystal - LaF3 for FPolycrystalline àor 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

15-9 Glass Membrane Electrode

15-10 Glass membrane structure H+ carries current near surface Na+ carries current in interior Ca 2+ carries no current (immobile)

15-11 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 = log(a1/a2) a1=analyte a2=inside ref electrode 2 If a2 is constant then Eb = L log a1 = L pH where L = log a2 Since Eref 1 and Eref2 are constant Ecell = constant pH

15-12 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 àAl 2 O 3 or B 2 O 3 ) §Possible to make glass membrane electrodes for àNa +, K +, NH 4 +, Cs +, Rb +, Li +, Ag +

15-13 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

15-14 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 Ca 2+ strongly

15-15

15-16 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 §O 2, NH 3 /NH 4 +, and CO 2 /HCO 3 - /CO 3 2-

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15-18 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 àpH àtemperature àionic strength

15-19 Electrode calibration

15-20 NH 4 electrode

15-21 Potentiometric titration

15-22 Coulometry Quantitative conversion of ion to new oxidation state §Constant potential coulometry §Constant current coulometry àCoulometric titrations *Electricity needed to complete electrolysis measured §Electrogravimetry àMass of deposit on electrode

15-23 Constant voltage coulometry Electrolysis performed different ways §Applied cell potential constant §Electrolysis current constant §Working electrode held constant àE Cell =E cathode -E anode +(cathode polarization)+(anode polarization)-IR Constant potential, decrease in current §1 st order àI t =I o e -kt Constant current change in potential §Variation in electrochemical reaction àMetal ion, then water

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15-25 Analysis Measurement of electricity needed to convert ion to different oxidation state §Coulomb (C) àCharge transported in 1 second by current of 1 ampere *Q=It I= ampere, t in seconds §Faraday (F) àCharge in coulombs associated with mole of electrons *1.602E-19 C for electron *F=96485 C/mole e - Q=nFN Find amount of Cu 2+ 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

15-26 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 àarsenic determined oxidation of arsenous acid (H 3 AsO 3 ) to arsenic acid (H 3 AsO 4 ) 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