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