Potentiometry introduction

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Electrochemical Methods of Analysis
Chapter 5.1
Compiled By Million M 2016
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Learning Objectives

• After learning this topic students will be able
  to
• Identify the different classes of ECMA
• Define potentiometry
• Explain the working principle of Potentiometry
• Types of Electrodes involved in Potentiometry
• ISE working principle
• Application of Potentiometry

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Electroanalytical methods Electroanalytical
methods involve the measurement of electrical
parameter such as - potential developed between
a pair of electrodes immersed in the solution
tested (potentiometric methods) - or the
electrical current flowing between a pair of
electrodes immersed in the solution tested
(voltammetric and amperometric methods). In
either case, the measured parameter (potential or
current) is proportional to the concentration of
analyte.
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• Classification of electrochemical methods
• POTENTIOMETRYMeasure electrical potential
  developed by an electrode in an electrolyte
  solution at zero current flow. Use NERNST
  EQUATION relating potential to concentration of
  some ion in solution.
• VOLTAMMETRYDetermine concentration of ion in
  dilute solutions from current flow as a function
  of voltage when POLARIZATION of ion occurs around
  the electrode.POLARIZATION depletion of
  concentration caused by electrolysis.If using a
  dropping mercury electrode, method is termed
  POLAROGRAPHY.
• COULOMETRYElectrolysis of a solution and use of
  Faraday's law relating quantity of electrical
  charge to amount of chemical change.
  essentially states that it takes 9.65 x 104
  Coulombs of electrical charge to cause
  electrolysis of 1 mole of a univalent electrolyte
  species.
• CONDUCTIMETRYMeasure conductance of a solution,
  using INERT ELECTRODES, ALTERNATING CURRENT, AND
  AN ELECTRICAL NULL CIRCUIT - thereby ensure no
  net current flow and no electrolysis. The
  concentration of ions in the solution is
  estimated from the conductance.

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1. Potentiometry

• The potentiometry cell consists of two
  electrodes.

• The potential of one of the electrodes is
  sensitive to the analytes concentration and is
  called the working, or indicator electrode.
• The second electrode, which is called the
  reference electrode.

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• In potentiometry
• The potential of an electrochemical cell is
  measured under static conditions. (no current, or
  only a negligible current flows in the cell)
• 2. The measured potential in potentiometric cell
  is related to the concentration of the
  electroactive species (analyte) by Nernst equation

?????- ???? ???? ??????
E is the measured potential, Eo is the standard
electrode potential, R is the general gas
constant, T is the absolute temperature, n is the
number of electrons involved in the
electrochemical process, F is the Faradays
constant and Q is activity of the analyte
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General Principles Reference electrode salt
bridge analyte solution indicator electrode
Eref Ej
Eind Ecell
Eind Eref Ej Reference cell a half
cell having a known electrode potential Indicator
electrode has a potential that varies in a
known way with variations in the concentration of
an analyte
A cell for potentiometric determinations.
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1. Reference Electrode

• The ideal reference electrode must be of a stable
  potential so that any change in Ecell is
  attributed to the indicator electrode, and
  therefore, to a change in the analytes
  concentration.
• In addition, the ideal reference electrode should
  be easy to make and to use.

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1. Saturated calomel Electrode (S.C.E.) Calomel
reference electrodes are based on the redox
couple between Hg2Cl2 (calomel salt) and Hg
Hg2Cl2(s) 2e 2Hg(l)
2Cl(aq) Eo 0.268V The potential of
calomel electrode according to Nernst equation
is
E Eo (0.05916/2) logCl2 0.2682-0.0592/2
logCl2 0.244 V
A calomel electrode saturated with KCl is called
a saturated calomel electrode, abbreviated S.C.E.
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SCE is constructed using an aqueous solution
saturated with KCl, has a potential at 25 C of
0.2444 V. The shorthand notation for this cell
is Hg(l) Hg2Cl2 (satd), KCl (aq, satd)
Advantage The potential of the electrode
remains constant even if the KCl solution
partially evaporates. Disadvantage The
solubility of KCl is sensitive to a change in
temperature. At higher temperatures the
concentration of Cl increases, and the
electrodes potential decreases.
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2. Silver/Silver Chloride Electrodes It based
on the redox couple between AgCl and Ag. AgCl(s)
e Ag(s) Cl(aq) Eo
0.244V
The potential of the Ag/AgCl electrode is
determined by the concentration of Cl used in
its preparation. E EAgCl/Ag 0.05916 log
Cl 0.2223 0.05916 log Cl
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Potential of Ag/AgCl electrode Electrode
containing saturated solution of KCl has a
potential of 0.197 V at 25 C. Electrode with
3.5 M KCl has a potential of 0.205 at 25 C.
The shorthand notation for the cell is Ag(s)
AgCl (satd), KCl (x M)
where x is the concentration of
KCl. Advantage Useful at higher temperatures.
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3. Standard hydrogen electrode (SHE)
Refer to your handout on Introduction to
Electrochemistry The most fundamental reference
electrode in electrochemistry. The shorthand
notation for the cell is Pt H2(g, 1.0
atm)H(aq, A 1.0M) ½ H2(g, 1.0 atm)
H(aq, A 1.0M) e Eo 0.000 V
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Example The potential for an Fe3/Fe2
half-cell is 0.750 V relative to the standard
hydrogen electrode. What is its potential when
using a saturated calomel electrode or a
saturated silver/silver chloride
electrode? Solution When using a standard
hydrogen electrode the potential of the
electrochemical cell is EcellEFe3/Fe2-ESHE0.75
0V-0.000V0.750V We can use the same equation to
calculate the potential when using a saturated
calomel electrode EcellEFe3/Fe2-ESCE0.750V-0.2
444V0.506V or a saturated silver/silver
chloride electrode EcellEFe3/Fe2-EAg/AgCl0.750
V-0.197V0.553V
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Q1 What is the general feature of reference
electrode? Q2 Mention two common reference
electrodes Q3 Reference electrode is based on
certain redox couple, mention the redox
couple for each of the following
electrodes - Calomel electrode -
Silver/silver chloride electrode Q4 Sketch a
diagram to show the structure of calomel
electrode Q5 Compare between calomel and Ag/AgCl
electrodes regarding to the advantages and
disadvantages of each
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Practice Exercise The potential of a UO2/U4
half-cell is 0.0190 V relative to a saturated
calomel electrode. What is its potential when
using a saturated silver/silver chloride
electrode or a standard hydrogen electrode? ANS

• When using a saturated calomel electrode, the
  potential of the electrochemical cell is
• Ecell EUO2/U4-ESCE
• Substituting in known values
• -0.0190VEUO2/U4- 0.2444V
• and solving for EUO2/U4 gives its value as
  0.2254 V. The potential relative to the Ag/AgCl
  electrode is
• Ecell EUO2/U4- EAg/AgCl0.2254V-0.197V
  0.028V
• and the potential relative to the standard
  hydrogen electrode is
• Ecell EUO2/U4 -ESHE0.2254V-0.0000V0.2254V

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Junction potential A junction potential
develops at the interface between two ionic
solution if there difference in the concentration
and mobility of the ions.
Development of the junction potential caused by
unequal mobilities of ions. Mobilties of ions in
water at 25oC Na 5.19 10 8 m2/sV K
7.62 10 8 Cl 7.91 10 8
When we measure the potential of an
electrochemical cell the junction potential also
contributes to Ecell thus, we rewrite the
equation EcellEc-EaEj
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2. Indicator Electrodes
The reference electrode represents half of the
complete system for potentiometric measurements.
The other half is the electrode at which the
potential that is related to the concentration of
the analyte is developed and measured. This
half-cell is called the indicator electrode. Two
classes of indicator electrodes are used in
potentiometry Metallic electrodes and
Ion-selective electrodes
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Indicator electrodes Metallic indicator electrode
responds to analyte activity. Electrode of the
first type Direct equilibrium with
analyte Ag for Ag, Au for
Au3, etc Potential described by
Nernst equation. As M , E
Note potential linearly related
to log of the concentration !
Remember - indicator BY DEFINITION cathode
measurement theoretically under
zero-current (steady state) Electrode of the
second type Indirect equilibrium with
analyte M/MX/X
Silver/Silver chloride for chloride
also Nernstian response
as X , E Inert Metallic electrode
for Redox systems Provides a surface
for the electrochemistry to occur Pt,
Au, Pd, C e.g. Ag(s) AgClsatd,
KClxM Fe2,Fe3) Pt
Xn(aq) ne X(s) Eind Eo (0.5916/n) log
(1/Xn)
A plot for an electrode of the first kind.
AgCl(s) e Ag(s) Cl(aq) Eind Eo
0.5916 log Cl
A plot for an electrode of the second kind for
Cl.
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• If we place a copper electrode in a solution
  containing Cu2, the electrodes potential due to
  the reaction
• Cu2(aq)2e-?Cu(aq)
• is determined by the activity of Cu2.
• E ECu2/Cu - 0.05916 2 log 1 aCu2
  0.3419V- 0.05916 2 log 1 aCu2
• If copper is the indicator electrode in a
  potentiometric electrochemical cell that also
  includes a saturated calomel reference electrode
• SCECu2(aq,aCu2x)Cu(s)
• then we can use the cell potential to determine
  an unknown activity of Cu2 in the indicator
  electrodes half-cell
• Ecell Eind - ESCE Ej 0.3419V- 0.05916 2
  log 1 aCu2 - 0.2444V Ej
• In general, if a metal, M, is in a solution of
  Mn, the cell potential is
• Ecell K - 0.05916 n log 1 aMn K 0.05916
  n log aMn

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Ion Selective Electrodes
Ion selective electrodes (ISEs) are
electrochemical sensors that allow potentiometric
determination of the activity of certain ions in
the presence of other ions in the sample
solution.
Membrane Electrodes and membrane
potentials Membrane potential is the potential
developing across a conductive membrane whose
opposite sides are in contact with solutions of
different composition.
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The Figure shows a typical potentiometric
electrochemical cell equipped with an
ion-selective electrode. The short hand
notation for this cell is reference(sample)Asa
mp(aq,aAx)Aint(aq,aAy)reference(internal)
ion-selective membrane
The potential of the cell is given by
EcellEref(int)-Eref(samp)EmemEj
Potential across the membrane
Constant
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Emem Easym - RT zF ln (aA)int (aA)samp
Asymmetric Potential
Substituting this eqn in the above eqn _at_ 25?C
results
Ecell K 0.05916 z log (aA)samp
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Examples of Ion-selective electrodes
1. The glass membrane electrode Glass electrode
is considered the first known ion-selective
membrane electrode. Its membrane is manufactured
from special type of glass and used as a hydrogen
ion-selective electrode. The membrane
potential of this electrode is a function of the
concentration of hydrogen ion in solution. The
glass membrane electrode, function by using a
membrane that reacts selectively with a single
ion.
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Glass membrane H-Selective Electrodes (pH
electrode) Glass composition approximately 22
Na2O, 6 CaO, and 72 SiO2.
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Mechanism of action and selectivity to H in
glass membrane
1. When immersed in an aqueous solution, the
outer approximately 10 nm of the membrane becomes
hydrated.
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2. Hydration of the glass membrane results in the
formation of O-Na sites, that are part of
the glass membranes silica framework. 3.
Hydrogen ions from solution diffuse into the
membrane and, since they bind more strongly
to the glass than does Na, displace the
sodium ions (ion-exchange process). H(aq)
O-Na(s) O-H(s) Na(aq)
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• The potential of glass electrodes obeys the
  equation
• Ecell K 0.05916 log H
• over a pH range of approximately 0.5 12.
• Above a pH of 12, the glass membrane may become
  more responsive to other cations, such as Na
  and K. (Alkaline error)

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The response of the glass membrane to monovalent
cations other than H at high pH led to the
development of glass membranes possessing a
greater selectivity for other cations.
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2. Crystalline Solid-State Ion-Selective
Electrodes
Membrane polycrystalline or single-crystal
inorganic salts
a) Ag ion selective electrode
The membrane potential for a Ag2S pellet develops
as the result of a difference in the equilibrium
position of the solubility reaction Ag2S(s)
2Ag(aq) S2(aq) on the two sides of the
membrane. When used to monitor the concentration
of Ag ions, the cell potential is
Ag2S pellet
Ecell K 0.05916 log Ag
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b) Fluoride ion-selective Electrode
The membrane of a F ion-selective electrode is
fashioned from a single crystal of LaF3 that is
usually doped with a small amount of EuF2
The membrane potential for a F ion-selective
electrode results from a difference in the
solubility of LaF3 on opposite sides of the
membrane, with the potential given by
Ecell K 0.05916 log F
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3. Liquid-membrane Ion-Selective Electrodes
Liquid-Based Ion-Selective Electrode is an
ion-selective electrode in which a chelating
agent is incorporated into a hydrophobic
membrane.
Example Ca2-selective electrode The electrode is
based on porous plastic membrane saturated with
di)-n-decyl) phosphate
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Ca2 ions-selective Electrode
A membrane potential develops as the result of a
difference in concentration s of Ca2 ions on the
two sides of the membrane is and the equilibrium
position of the following complexation reaction
Ca2(aq) 2(C10H21O)2PO2 (m)
Ca(C10H21O)2PO22(m)
(PVC membrane in which the di-(n-decyl) phosphate
is immobilized)
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Examples of Liquid-Based Ion-Selective
Electrodes.