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

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

15-7

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

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