Influencing policy (training slides from Fast Track Impact)
potentiometry
1. CHM 4102
ELECTROCHEMISTRY
PRINCIPLE OF POTENTIOMETRIC MEASUREMENT
Group 6
Siti Nor Rafidah bt Mohd Ali Jinnah 152025
Ong Hui Jin 152161
Norasheila bt Mohd Saad 152167
Nur Zalikha bt Nasrudin 152843
Fatin Nurul Atiqah bt Osman 153319
Siti Nursyafiqa bt Zainul Abidin 153933
NurAsiqin bt Iskandar 154652
Chan Yun Joo 154663
Nur Syazana bt Abdul Rahman 154794
Nur Amiera bt Azman 155629
2. Introduction
• In potentiometry, the potential of an electrochemical cell
is measured under static conditions because no current
flows while measuring a solution’s potential, its
composition remains unchanged.
• Information on a composition of the sample is obtained
through the potential appearing between two
electrodes.
3. • The first quantitative potentiometric applications appeared
soon after the formulation of Nernst equation.
• In 1889, the Nernst equation relating an electrochemical cell’s
potential to the concentration of electroactive species in the
cell.
• Potentiometric methods of analysis are based upon
measurements of the potential of electrochemical cells under
conditions of zero current, where the Nernst equation
governs the operation of potentiometry.
4. Nernst equation:
Ecell = E0cell - (RT/nF) lnQ
Ecell = cell potential under nonstandard conditions (V)
E0cell = cell potential under standard conditions
R = gas constant, which is 8.31 (volt-coulomb)/(mol-K)
T = temperature (K)
n = number of moles of electrons exchanged in the electrochemical
reaction (mol)
F = Faraday's constant, 96500 coulombs/mol
Q = reaction quotient
5. Basic Principles
Potentiometer
• A device for measuring the potential of an electrochemical
cell without drawing a current or altering the cell’s
composition.
• The potential of an electrochemical cell is measured under
static conditions.
• Because no current, or only a negligible current flows while
measuring a solution’s potential, its composition remains
unchanged.
• For this reason, potentiometry is a useful quantitative
method.
6. Potentiometric measurements
• Made by using a potentiometer to determine the difference in
potential between a indicator electrode and a reference
electrode.
Cathode is the sensing electrode. (right half-cell)
Anode is the reference electrode. (left half-cell)
Ecell = Ec ─ Ea
Where : Ec is the reduction potential at the cathode.
: Ea is the reduction potential at the anode.
7. Potentiometric Electrochemical Cells
• To determine difference in potential between sensing
electrode and reference electrode
• Separation of the 2 electrodes to prevent the redox from
occurring spontaneously on surface of one of electrodes
• Constructed that one of half cells provided a known
reference potential of other half cell indicates the
analyte concentration
9. • Salt bridge contain inert electrolyte such as KCl connects the
two half-cells.
• The ends of the salt bridge are fixed with porous frits (to
allow the ions of electrolyte to move freely between the
half-cells and the salt bridge).
• This movement of ions in the salt bridge completes the
electrical circuit.
• Reference electrode : left electrode (anode) which
undergoes oxidation.
• Sensing electrode : right electrode (cathode) which
undergoes reduction.
• When the potential of an electrochemical cell is measured,
the contribution of the liquid junction potential must be
included;
Ecell = Ec ─ Ea Elj
11. Introduction to ion selective electrode (ISE)
• The cell consists of both an indicator and reference
electrode.
• Since the potential of the reference electrode is constant,
the potential developed at the indicator electrode that
contains information about the amount of analyte in a
sample.
• During the measurement, there is little to no current
flow.
• An electrochemical cell for making a potentiometric
measurement with a membrane electrode also known
as an ion-selective electrode, ISE.
13. Ion selectivity
• A specific ion electrode will only respond to the presence
of one species.
• In reality, ion-selective electrodes can experience
interferences by responding to the presence of other
ions.
• We can account for the lack of 100% specificity by
incorporating the activity of j and a selectivity coefficient
(kij) into this equation:
14. • This new equation is called the Nikolskii-Eisenman
equation:
• The selectivity coefficient is a numerical measure of how
well the membrane can discriminate against the
interfering ion.
• To put this in perspective, if an electrode has equivalent
responses to the two ions, then kij = 1.0.
15. • From the equation, the smaller the kij values, the less
impact the interfering ion will have on the measured
potential.
• When kij values are less than 1, the ISE is more
responsive to the analyte ion
• When kij values are greater than 1, the ISE is more
responsive to the interfering ion. For example, a kij value
of 0.01 means that the electrode is 100 times more
responsive to ion i over j.
16. • The selectivity of the ISE is determined by the
composition of the membrane.
• Ideally the membrane allows the uptake of only one
specific ion into it.
• The three main components of making a
measurement at an ISE are
• an inner reference, or standard solution
• an outer analyte, or sample,
• solution separated by a thin membrane.
• The potential developed at the membrane is the
result of either an ion exchange process or an ion
transport process occurring at each interface
between the membrane and solution.
17. Ion Exchange Process
•Lithium cation displaces a potassium cation from the
organic anion, R-:
KR + Li+ ⇋ LiR + K+
•We can imbed the lipophilic R- in a membrane and place it
in a solution of Li+
KR(mem) + Li+(aq) ⇋ LiR(mem) + K+(aq)
18. • To construct an ion-selective electrode an inner
reference solution added to the other side of the
membrane.
• This solution would contain a fixed concentration of the
ion of interest, Li+ in this example.
• This is typically accomplished by placing a thin
membrane at the end of the plastic tube and filling the
tube with a standard (known concentration) solution of
the analyte.
• A reference electrode is placed in the inner solution and
a second reference electrode is in contact with the
analyte (outer) solution.
19. Ion transport
• A membrane, containing an ionophore, between an
“unknown” analyte solution and a “known” reference
solution .
• The ionophore is a neutral “carrier” molecule
represented by the blue oval.
20. • The ionophore cannot diffuse out of the membrane and
but can “trap” the analyte ion (A+) at the interface
between the solution and membrane.
• Without the ionophore, the analyte would be unable to
partition into the organic membrane.
• As with the ion-exchange process, equilibrium is
established at both solution-membrane interfaces. The
resulting charge separation at each interface leads to a
phase-boundary potential.
• Now potential develop across the membrane.
21. Reference electrode
• It has a standard potential on its own and its potential
does not change to whichever solution it is dipped.
• Always treated as the left-hand electrode (anode)
• Example of reference electrode :
• Standard hydrogen electrode (SHE)
• Saturated calomel electrode
• Silver-silver chloride electrode
22. Standard hydrogen electrode (SHE)
• Defined as the potential that is developed between H2
gas adsorbed on the Pt metal and H+ of the solution .
• It is used for
• determination of electrode potential of metal
electrode system
• determination of pH of the solution
Pt,H2 (g, 1atm) | H+ (aq, a = 1.00) ||2 H+ (aq) + 2 e ─ ↔ H2 (g)
23. Saturated calomel (Hg2Cl2) Electrode (SCE)
• Contains of an inner jacket and outer sleeve.
• Inner jacket has wire contact with Hg and plugged with a mixture of
calomel Hg2Cl2 & KCl.
• Outer sleeve has crystals of KCl & porous plug of asbestos
• Application: pH measurement, cyclic voltammetry and general
aqueous electrochemistry.
• Advantages: ease of construction and stability of potential.
Hg(l) | Hg2Cl2 (sat’d), KCl (aq, sat’d) || Hg2Cl2(s) +2e– ↔2Hg(l ) + 2Cl-(aq)
24. Silver-silver chloride electrode
• Widely used because simple, inexpensive, very stable
and non-toxic.
• Mainly used with saturated potassium chloride (KCl)
electrolyte.
• Advantages : easy to use
• Disadvantage : difficult to prepare
Ag(s) | AgCl (sat’d), KCl (x M) ||AgCl(s) + e– ↔ Ag(s) + Cl- (aq)
25. Sensing Electrodes
• The potential of the sensing electrode in a potentiometric
electrochemical cell is proportional to the concentration of
analyte.
• Two classes of indicator electrodes are used in potentiometry:
• metallic electrodes
• Electrodes of the first kind
• Electrode of the second kind
• Redox electrode
• membrane electrodes (ion-selective electrodes)
• glass pH electrode
26. Metallic electrodes
Electrodes of the first kind
• A metal in contact with a solution containing its cation.
• The potential is a function of concentration of Mn+ in a
Mn+ / M. The most common ones:
• Silver electrode (dipping in a solution of AgNO3)
• Ag+ + e ↔ Ag
• Copper electrode
• Cu+2 + 2e ↔ Cu
• Zn electrode
• Zn+2 + 2e ↔ Zn
27. Electrode of the second kind
• A metal wire that coated with one of its salts precipitate.
• Respond to changes in ion activity through formation of
complex.
• A common example is silver electrode and AgCl as its salt
precipitate.
• This kind of electrode can be used to measure the
activity of chloride ion in a solution.
28. Redox electrode
• An inert metal is in contact with a solution containing the
soluble oxidized and reduced forms of the redox half-
reaction.
• The inert metal is usually is platinum (Pt).
• The potential of such an inert electrode is determined
by the ratio of the reduced and oxidized species in the
half-reaction.
• A very important example of this type is the hydrogen
electrode.
29. Membrane electrodes
Glass pH electrode
• Advantages over other electrodes for pH measurements:
• Its potential is essentially not affected by the presence
of oxidizing or reducing agents.
• It operates over a wide pH range.
• It responds fast and functions well in physiological
systems.
31. Principle:
• For measurement, only the bulb needs to be submerged.
• There is an internal reference electrode and electrolyte
(Ag| AgCl| Cl─) for making electrical contact with the glass
membrane, its potential is necessarily constant and is set by
the concentration of HCl.
• A complete cell, then, can be represented by:
32. Theory of the glass membrane potential
• Both the inside and outside surfaces of the glass membrane in
the GE bulb have SiOH groups.
• The interior surface of the glass membrane is in contact with a
constant concentration of HCl, and so the number of SiO–
groups on the interior surface remains constant.
• By contrast, the number of SiO– groups on the exterior of the
glass membrane will change when the pH of the solution the
glass membrane is immersed in changes.
• The difference in charge on the inside and outside of the glass
membrane results in a membrane potential.
• If we can set up an experiment to measure the membrane
potential, then this corresponds to measuring the pH of the
solution in which the glass electrode is immersed.
33. Alkaline Error
• Systematic error occurs when using glass pH electrode to
measure pH of extremely alkaline solution
• Glass pH electrode responds very selectively to H+ ions,
but, sensitive to alkali metal ions too
• Caused by interference of high concentration of alkaline
metal ions, e.g: Li+, Na+, K+
34. Alkaline Error cont.
• At high pH where [H+] <<< [Na+], electrode begins to
respond to [Na+].
• Ion exchange reaction occurs at membrane surface
• Alkaline ions will replace H+ ions completely/partially in
outer gel layer of glass membrane.
• Result: pH value measured < actual pH
• Usually noticeable: - pH> 12
- [Li+ /Na+] ≥ 0.1 mol/litre
35. Figure 2: Cross-section of glass pH membrane.
Alkaline metal cations will compete with H+for free spaces
in solvated layer.
36. Alkaline Error cont.
Figure 1: Deviation from linear pH
dependence due to alkaline error
• Alkaline error ↑ pH value ↑ alkaline concentration↑
37. Application of Potentiometric Measurement
• Clinical Chemistry
• Ion-selective electrodes are important sensors for clinical samples
because of their selectivity for analytes in complex matricies.
• The most common analytes are electrolytes, such as Na+, K+, Ca2+,H+,
and Cl-, and dissolved gases such as CO2.
• Environmental Chemistry
• For the analysis of of CN-, F-, NH3, and NO3- in water and
wastewater.
• One potential advantage of an ion-selective electrode is the
ability to incorporate it into a flow cell for the continuous
monitoring of wastewater streams.
• Potentiometric Titrations
• Use a pH electrode to monitor the change in pH during the
titration.
• For determining the equivalence point of an acid–base titration.
• Possible for acid–base, complexation, redox, and precipitation
titrations, as well as for titrations in aqueous and nonaqueous
solvents.
38. • Agriculture
• NO3, NH4, Cl, K, Ca, I, CN in soils, plant material, fertilizers and
feedstuffs
• Detergent Manufacture
• Ca, Ba, F for studying effects on water quality
• Food Processing
• NO3, NO2 in meat preservatives
• Salt content of meat, fish, dairy products, fruit juices, brewing
solutions.
• F in drinking water and other drinks.
• Ca in dairy products and beer.
• K in fruit juices and wine making.
• Corrosive effect of NO3 in canned foods
