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 15-1
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 15-2
Calomel electrode 15-3
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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 15-5
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 Ion selective electrode 15-6
2 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 15-7
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 15-8
Glass Membrane Electrode 15-9
Glass membrane structure H+ carries current near surface Na+ carries current in interior Ca 2+ carries no current (immobile) 15-10
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 Boundary Potential 15-11
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 Al 2 O 3 or B 2 O 3 ) Possible to make glass membrane electrodes for Na +, K +, NH 4+, Cs +, Rb +, Li +, Ag + 15-12
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 15-13
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 15-14
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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 2-3 15-16
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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 15-18
Electrode calibration 15-19
NH 4 electrode 15-20
Potentiometric titration 15-21
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 15-22
Constant voltage coulometry Electrolysis performed different ways Applied cell potential constant Electrolysis current constant Working electrode held constant E Cell =E cathode -E anode +(cathode polarization)+(anode polarization)-ir Constant potential, decrease in current 1 st order I t =I o e -kt Constant current change in potential Variation in electrochemical reaction Metal ion, then water 15-23
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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 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 15-25
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 (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 15-26