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Properties of Umass Boston
Potentiometric Electrode
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Properties of Umass Boston
Reference Electrode
Standard Hydrogen Electrode Ag/AgCl reference electrode
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Properties of Umass Boston
Reference electrode • Reversible and obey Nernst equation • Exhibit a potential that is constant with
time • Return to its original potential after
being subjected to small currents (high exchange current)
• Little change in potential with temperature
• Behave as an ideal nonpolarizable electrode
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Properties of Umass Boston
Configuration of a typical reference electrode
Silver-Silver Oxide Reference Electrode:
Ag + Cl- → AgCl + e
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Properties of Umass Boston
Configuration of a typical reference electrode
Calomel Reference Electrode:
Hg2Cl2 (s) + 2e → 2Hg (l) + 2Cl-
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Properties of Umass Boston
Voltage conversions between different reference electrode
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Properties of Umass Boston
Liquid Junction Potential • Due to the concentration gradient
– Difference on chemical potential – Difference on the mobility of ions
• NOT a equilibrium process, but when it react steady state, can be treated as a constant
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Properties of Umass Boston
KCl is normally used in the salt bridge to minimize the junction potential due to the fact that K+ and Cl+ have similar mobility
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Thermodynamics
• Under the conditions of constant P and T (∆G)T,P = -Wnon-expansion = -nF∆E Chemical energy Electric Energy (∆G=∑µI ; µ is the chemical potential)
• For Reaction aA + cC ↔ bB + dD ∆G= ∆Go + RTln([B]b[D]d/[A]a[c]c) -nF∆E=-nF∆Eo + RTln([B]b[D]d/[A]a[c]c) ∆E = ∆Eo – (RT/nF)ln ([B]b[D]d/[A]a[c]c) Nernst Equation
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Nernst Equation
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Equilibrium Constant
KnFRTE
nFEKRTGGmequilibriuat
KRTGCADBRTGG
dDbBcCaA
o
oo
oca
dbo
ln
ln0:
ln)][][][][ln(
=
−=−=∆
=∆
+∆=+∆=∆
+⇔+
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Properties of Umass Boston
Oxidation/reduction power
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Properties of Umass Boston
The system for Potentiometric methods
• Reference Electrode – Maintains a fixed potential
• Indicator electrode – Response to analyte activity
• Potential measuring device – Volt meter with high input impedance.
• E(cell)=E(indicator)-E(reference)+E(junction) E (cell)=Klog[ion]
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Properties of Umass Boston
Redox Electrode
• Electrochemically inert electrode e.g. Pt, Ag, Au, carbon etc.
• Function as an electrocatalyst and conductor for the redox reaction
• Fe(CN)63- + e ⇔ Fe(CN)6
4-
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Properties of Umass Boston
Indicator electrode
• Metal electrode – first kind e.g. Cu (Solid) ⇔ Cu2+ (aqueous) + 2e
E = Eo – RT/nF ln(1/[Cu2+]) • Not very selective
Cu2+ ⇔ Cu+
Cu2+ + I- + e ⇔ CuI (solid) etc
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Properties of Umass Boston
Indicator electrode • Second kind: ions forms a precipitate and
stable complex ion on metal AgCl (s) + e ⇔ Ag (s) + Cl- E=0.222 – 0.0591log[Cl]
• Good selectivity • Microelectrode array for multiple
detections
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Properties of Umass Boston
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Properties of Umass Boston
Ion selective membranes Electrode
• Is capable of binding only the intended ion. • Minimal solubility. • Electrical conductivity through ionic migration. • Selective reactivity:
selectively binding with target ion, three types of binding, ion-exchange, crystallization and complexation.
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Properties of Umass Boston
L is call ionophone, which is a ligand, only binds with C+. L is soluble in the membrane but insoluble in aqueous solution, so it will stay in the membrane
R- is hydrophobic organic species, which is hardly soluble in water, so will stay inside the membrane
There are sufficient amount of C+ inside membrane and excess amount of L, so almost all C+ is attached with L. The concentration of C+ inside the membrane is constant. Why?
Very little amount of C+ diffuse out of the membrane and leave R+ behind. The space charge region will be created, the potential field is against the diffusion of C+. When it becomes equilibrium, a potential different will be created at the interface, which is proportional to the concentration of C+ in the solution.
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Properties of Umass Boston
Thermodynamics for ion selective membrane
)ln()()ln()()ln()(
)ln()(
)ln()(0)()ln(
)ln(
int
minnermosolvation
innero
msolvationinnerouter
o
msolvationoutout
o
msolvation
out
o
msolvation
CnFRTConstESoEC
nFRTC
nFRT
nFGE
ECC
nFRT
nFGEEE
CC
nFRT
nFGESonFE
CCRTG
nFEGmequilibriuatCCRTGG
erfaceoutertheat
+=−−+∆
=
−−∆
=−=
−∆
==−+−∆
−=∆
−∆=∆
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Properties of Umass Boston
Configuration of ion selective membrane electrodes
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Properties of Umass Boston
Requirement of indicator electrode
• Sensitivity • Selectivity
–K=response to X/response to A
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Properties of Umass Boston
pH Electrode – an example
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Properties of Umass Boston
Glass membrane – a ion selective membrane
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Properties of Umass Boston
Redox Titration
• Redox titration is based on the redox reaction (oxidation-reduction) between analyte and titrant.
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Properties of Umass Boston
Electrochemical reactions • Two electrochemical reactions
– Titration reaction: (titrate ferrous with ceric) Ce4+ + Fe2+ → Ce3+ + Fe3+
– Indication reaction: (determine end point) Reference (calomel): 2Hg(l)+2Cl-⇄ Hg2Cl2+2e Indicator 1 half reaction: Fe3++e ⇄ Fe2+ Indicator 2 half reaction: Ce4++e⇄ Ce3+
2Fe3++2Hg(l)+2Cl-⇄2Fe2++Hg2Cl2(s)…1 2Ce4++2Hg(l)+2Cl-⇄2Ce3++Hg2Cl2(s)…2 E1=Eo-(RT/nF)ln([Fe2+]2/[Fe3+]2[Cl-]2) E2=Eo -(RT/nF)ln([Ce3+]2/[Ce4+]2[Cl-]2) E1 = E2 =constant - constant ln ([Fe2+]2/[Fe3+]2) =constant –constant ln ([Ce3+]2/[Ce4+]2)
Different reaction!!
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Properties of Umass Boston
The Shape of the titration curve • Before the equivalent Point:
Added Ce4+ will react with Fe2+ to form Fe3+ and Ce3+. The indication potential on Pt electrode can be calculated by E=0.526-0.059log[Fe2+]/[Fe3+] when [Fe3+]=[Fe2+]; E=Eo (half way)
• At equivalent point: Stoichiometry amount of Fe2+ react with Ce4+, so [Fe3+]=[Ce3+] and [Ce4+]=[Fe2+], combine them with E1=E2,, one should get E=0.99 V (end point potential)
• After the equivalent point: the potential can be calculated from 2Ce4++2Hg(l)+2Cl-⇄2Ce3++Hg2Cl2(s), since there will be excess amount of Ce4+ in the solution
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Properties of Umass Boston
Dilution factor • The indication potential is determined by
the ratio of the redox couple e.g. E1 =constant- constant ln([Fe2+]2/[Fe3+]2)
• The shape of the titration curve will be independent of dilution
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Properties of Umass Boston
Position of the end point • The position of the end point is related to the reaction stoichiometry
• Since the potential change near the end point is so steep, the position of end point will not introduce much error