17615523-24potentiometry
DESCRIPTION
Potential measurements of electrochemical cells•Ion selective methods❧Reference electrode❧Indicator electrode❧Potential measuring device•Reference electrode•Indicator electrodes•Ion specific electrodes•PotentiometricmeasurementsTRANSCRIPT
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
15-4
15-5
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
15-6
• Not very selective• simple• some metals easily
oxidized (deaeratedsolutions)
• some metals (Zn, Cd) dissolve in acidic solutions
• Ag, Hg, Cu, Zn, Cd, Bi, Tl, Pb
Ion selective electrode
15-7
• 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
2nd kind
15-8
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
15-9
Glass Membrane Electrode
15-10
Glass membrane structure
• H+ carries current near surface
• Na+ carries current in interior
• Ca2+ 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 = 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
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 ➠ Al2O3 or B2O3)
❧ Possible to make glass membrane electrodes for➠ Na+, K+, NH4
+, 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 Ca2+ 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
❧ O2, NH3/NH4+, and
CO2/HCO3-/CO3
2-
15-17
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
➠ pH➠ temperature➠ ionic strength
15-19
Electrode calibration
15-20
NH4 electrode
15-21
Potentiometric titration
15-22
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
15-23
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
15-24
15-25
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
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
➠ 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