3. Potentiometric Analysis
• Based on potential measurement of
electrochemical cells without any
appreciable current
• The use of electrodes to measure
voltages from chemical reactions
5. Components of a
Potentiometric Cell
1. Reference electrode
2. Salt bridge
3. Analyte
4. Indicator electrode
RE SB A IE
– Eref + Ej + Eind
6. Reference electrode
• Half-cell with known potential (Eref)
• Left hand electrode (by convention)
•Easily assembled
•Rugged
• Insensitive to analyte concentration
▫ Reversible and obeys Nernst equation
▫ Constant potential
▫ Returns to original potential
7. Indicator electrode
•Generates a potential (Eind)
that depends on analyte
concentration
•Selective
•Rapid and reproducible
response
9. Liquid Junction Potential
• Liquid junction - interface between
two solutions containing different
electrolytes or different
concentrations of the same
electrolyte
• A junction potential occurs at every
liquid junction.
▫ Caused by unequal mobilities of the +
and - ions.
10.
11. The role of the R.E. is to
*provide a fixed potential not affected by sample composition
which does not vary during the experiment.
*Follows Nernst equation
.
Aqueous
SCE Ag/AgCl Hg/HgO SHE
Nonaqueous
Ag+/Ag Pseudoreferences
Pt, Ag wires Ferrocene/ferricinium couple
Reference Electrode (RE)
12. The Standard Hydrogen Electrode (SHE)
A universal reference, but is really a hypothetical
electrode (not used in practice)
– Uses a platinum electrode, which at its surface
reduces 2H+ to H2 gas.
– Very sensitive to temperature, pressure, and
H+ ion activity
Because the SHE is difficult to make, the
saturated calomel electrode (SCE) is used
instead.
– Calomel = mercury (I) chloride
17. 3-Silver/silver chloride reference electrode Ag/AgCl
# Ag wire coated with AgCl(s), immersed in NaCl or KCl solution
# Ag+ + e- = Ag(s)
E0 = 0.22 V vs. SHE @ 20C
Advantages
chemical
processing
industry has
standardized on this
electrode
convenient
rugged/durable
Disadvantages
solubility of
KCl/NaCl
temperature
dependent
dE/dT = -0.73 mV/K
(must quote
temperature)
19. Silver/silver ion reference electrode Ag+/Ag
Ag+ + e-= Ag(s)
Requires use of internal
potential standard
Advantages
Most widely used
Easily prepared
Works well in all
aprotic solvents:
THF, AN, DMSO,
DMF
Disadvantages
Potential depends on
solvent
electrolyte (LiCl,
TBAClO4, TBAPF6,
TBABF4
Care must be taken to
minimize junction
potentials
20. Mercury-Mercuric oxide reference electrode Hg/HgO
Metal/metal oxide reference electrode
Used in particular for electrochemical studies in aqueous alkaline solution.1.0 M NaOH
usually used in inner electrolyte compartment which is separated from Test electrolyte
solution via porous polymeric frit. Hence reference electrode in like SHE system in that it
is pH independent.
23. We can initially ignore the fact that the electrode contains AgI and find E for
the silver ion reduction.
Electrode Potentials
24. Indicator Electrodes
I. Metallic IE
A. Electrodes of the First Kind
B. Electrodes of the Second Kind
C. Inert Metallic Electrodes (for Redox Systems)
II. Membrane IE
A. Glass pH IE
B. Glass IE for other cations
C. Liquid Membrane IE
D. Crystalline-Membrane IE
III. Gas Sensing Probes
26. Electrodes of the First Kind
Pure metal electrode in direct equilibrium with its cation
• Metal is in contact with a solution containing its cation.
M+n(aq) + ne- M(s)
27. Disadvantages of First Kind Electrodes
• Not very selective
▫ Ag+ interferes with Cu+2
• May be pH dependent
▫ Zn and Cd dissolve in acidic solutions
• Easily oxidized (de-aeration required)
• Non-reproducible response
28. Electrodes of the Second Kind
• Respond to anions by forming precipitates
or stable complex
• Examples:
1. Ag electrode for Cl- determination
2. Hg electrode for EDTA determination
29. Inert Metallic (Redox) Electrodes
• Inert conductors that respond to redox systems
• Electron source or sink
• An inert metal in contact with a solution
containing the soluble oxidized and reduced
forms of the redox half-reaction.
• May not be reversible
• Examples:
▫ Pt, Au, Pd, C
30. MEMBRANE
ELECTRODES
• Aka p-ion electrodes
• Consist of a thin membrane separating 2 solutions of
different ion concentrations
• Most common: pH Glass electrode
32. Properties of Glass pH electrode
• Potential not affected by the presence
of oxidizing or reducing agents
• Operates over a wide pH range
• Fast response
• Functions well in physiological
systems
• Very selective
• Long lifespan
33. Theory of the glass membrane potential
• For the electrode to become operative, it must be soaked in water.
• During this process, the outer surface of the membrane becomes
hydrated.
• When it is so, the sodium ions are exchanged for protons in the
solution:
• The protons are free to move and exchange with other ions.
Charge is slowly carried
by migration of Na+
across glass membrane
Potential is determined
by external [H+]
34. Several complications for using pH
• Confusion over the meaning of pH.6 (The activity
and the concentration of H+ are not the same in 0.1 M HCl
because the activity coefficient for H+ is not 1.00 in this
matrix.)
• The uncertainty (For this reason, before using a pH
electrode we calibrate it using two standard buffer of
known pH, one is pH = 7)
35. Alkaline error
• Exhibited at pH > 9
• Electrodes respond to
H+ and alkali cations
• C,D,E and F:
measured value is <
true value
▫ Electrode also
responds to other
cations
• Higher pH at lower
[Na+]
36. Acid error
• Exhibited at pH
< 0.5
• pH readings are
higher (curves
A and B)
▫ Saturation effect
with respect to
H+
37. Selectivity Coefficient
• No electrode responds exclusively to one kind of ion.
▫ The glass pH electrode is among the most selective, but it
also responds to high concentration of Na+.
• When an electrode used to measure ion A, also
responds to ion X, the selectivity coefficient gives
the relative response of the electrode to the two
different species.
▫ The smaller the selectivity coefficient, the less interference
by X.
A
to
response
X
to
response
,
X
A
k
38. Selectivity Coefficient
• Measure of the response of an ISE to other ions
Eb = L’ + 0.0592 log (a1 + kHBb1)
• kA,I = 0 means no interference
• kA,I 1 means there is interference
• kA,I < 1 means negligible interference
• kA,I provided by the manufacturer
39. Ex: Sokalski and co-workers were prepared ion-
selective electrodes with significantly improved
selectivities.3 For example, a conventional Pb2+
ISE has a logKPb2+/Mg2+ of –3.6. If the potential
for a solution in which the activity of Pb2+is
4.1×10–12 is identical to that for a solution in
which the activity of Mg2+ is 0.01025, what is the
value of logKPb2+/Mg2+ /?
SOLUTION
Making substitutions into eq., we find that
The value of logKPb2+/Mg2+, therefore, is –9.40.
41. Liquid Membrane Electrodes
• Potential develops across the interface between
the analyte solution and a liquid ion exchanger
(that bonds with analyte)
• Similar to a pH electrode except that the
membrane is an organic polymer saturated with a
liquid ion exchanger
• Used for polyvalent ions as well as some anions
• Example:
• Calcium dialkyl phosphate insoluble in water, but
binds Ca2+ strongly
47. Crystalline-Membrane Electrodes
• Solid state electrodes
• Usually ionic compound
• Crushed powder, melted and formed
• Sometimes doped to increase
conductivity
• Operation similar to glass membrane
49. F- Selective Electrode
• A LaF3 is doped with EuF2.
• Eu2+ has less charge than the La3+, so an
anion vacancy occurs for every Eu2+.
• A neighboring F- can jump into the vacancy,
thereby moving the vacancy to another site.
• Repetition of this process moves F- through
the lattice.
52. Gas Sensing Probes
• A galvanic cell whose potential is related to
the concentration of a gas in solution
• Consist of RE, ISE and electrolyte solution
• A thin gas-permeable membrane (PTFE)
serves as a barrier between internal and
analyte solutions
• Allows small gas molecules to pass and
dissolve into internal solution
• O2, NH3/NH4
+, and CO2/HCO3
-/CO3
2-
54. DIRECT POTENTIOMETRY
• A rapid and convenient method of
determining the activity of cations/anions
55. Potentiometric Measurement
• Ionic composition of standards must be
the same as that of analyte to avoid
discrepancies
• Swamp sample and standard with inert
electrolyte to keep ionic strength
constant
• TISAB (Total Ionic Strength Adjustment
Buffer) = controls ionic strength and pH
of samples and standards in ISE
measurements
57. Special Applications:
Potentiometric pH Measurement
using Glass electrode
• One drop of solution
• Tooth cavity
• Sweat on skin
• pH inside a living cell
• Flowing liquid stream
• Acidity of stomach
58. Potentiometric Titration
• Involves measurement of the potential
of a suitable indicator electrode as a
function of titrant volume
• Provides MORE RELIABLE data than
the usual titration method
• Useful with colored/turbid solutions
• May be automated
• More time consuming
61. SCALE OF OPERATION
• The working range is from 1.0 M–10–11 M.
• This broad working range is significantly greater
than many other analytical techniques
62. SENSITIVITY
• The sensitivity is determined by the term RT/nF
or RT/zF in the Nernst equation.
• Sensitivity is best for smaller values of n or z.
63. ACCURACY
• The accuracy is limited by the error in measuring
Ecell.
• Several factors contribute to this error,
• (1) the potential from interfering ions,
(by including a separation step before the potentiometric analysis)
• (2) current passing through the cell,
(use high impedance potentiometers to minimize the current passing)
• (3) differences between the analyte’s activity in the
samples and the standard solutions, and
• (4) junction potentials
(To overcome 3 and 4: by matching the matrix of the standards to that of
the sample to evaluate the effect of uncertainty on the accuracy measurement.)
64. SELECTIVITY
• Most ion-selective electrodes respond to more
than one analyte;
• So, the selectivity is often significantly greater
than for the interfering ions.
• The manufacturer provides an ISE’s selectivity
coefficients.
65. TIME, COST, AND EQUIPMENT
• In comparison to other techniques, potentiometry
provides:
• Rapid,
• Relatively low-cost means for analyzing samples.
• The limiting factor is the need to rinse the
electrode between samples.
• The use of inexpensive, disposable ion-selective
electrodes can increase a lab’s sample throughput.
66. PRECISION
• Precision is limited by variations in temperature
and the sensitivity of the potentiometer
we can measure with of ±0.1 mV
Corresponds to an uncertainty of ±0.4% for
monovalent and ±0.8% for divalent analytes
67. Types of Electrodes/Reference electrodes
I
▫ Provide constant potential – not affected by sample
composition
▫ Follows Nernst equation
▫ Two common reference electrodes:
1. Saturated calomel electrode (SCE)
Mercury in contact with a solution saturated with mercury
chloride
|| KCl (saturated), Hg2Cl2(s)|Hg(s) (E0 = +0.214 vs.
SHE at 25 ºC)
2. Silver wire coated with silver chloride in a saturated KCl
solution
|| KCl (saturated), AgCl(s)|Ag(s) (E0 = +0.197 vs. SHE at 25 ºC)
E0 above is calculated based on reference to the standard
