SCI3023 ELECTROCHEMISTRY Chapter 8e: Potentiometry

Potentiometry Potential measurements of electrochemical cells Ion selective methods Reference electrode Indicator electrode Potential measuring device Indicator electrodes Ion specific electrodes Potentiometric measurements

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

Calomel electrode

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

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

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

Metallic Redox Indicators 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-

Metallic Redox Indicators 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

Glass Membrane Electrode

Glass membrane structure H+ carries current near surface Na+ carries current in interior Ca2+ carries no current (immobile)

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

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+

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

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

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-/CO32-

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

Electrode calibration

NH4 electrode

Potentiometric titration