This document provides an overview of ion selective electrodes (ISEs), including their definition, classification, properties, and applications. ISEs are indicator electrodes that respond selectively to specific ions. They are classified as either membrane or metallic electrodes. Membrane ISEs contain a thin selective membrane and include glass, solid-state, liquid, and molecular electrodes. The document discusses the chemical composition and working principles of common ISEs like glass pH electrodes and fluoride electrodes. It also notes potential applications of ISEs in areas like agriculture, food analysis, industry, and clinical chemistry.

1. Ion selective electrodes and their
applications
By Amare Ayalew (Ph.D)
Under the Guidance of Professor Bhavana. A. Shah
Chemistry department
Veer Narmad South Gujarat University

2. Outline
Definition and classification of Ion selective
electrodes (ISE)
Properties of ion selective membranes
Potential of the cell
Chemical composition and working principle
of each ion selective electrode
Applications

3. Definition- Ion selective electrodes (ISE) are
indicator Electrodes
They are two types
Membrane indicator electrodes
Metallic indicator electrodes
Membrane indicator electrodes
 Responds Selectively to one ion
- Contains a thin membrane capable of selecting the
desired ion
 Does not involve a redox process
 It is a boundary potential that develops across a membrane
that separates the analyte solution from the reference
solution

4. Membrane electrode classified as
Glass membrane (i.e.H+
electrode)
Solid-state electrode (e.g. F-
electrode uses a Eu2+
-
doped LaF3 crystal
Liquid-based electrode (e.g. Ca2+
electrode uses a
liquid chelator)
Molecular/compound electrode (e.g. CO2 gas
sensing electrode)

5. Properties of ion selective membranes
 Minimal solubility-solubility in analyte solutions(usually
aqueous) approaches zero
 Electrical conductivity-the form of migration of singly charged
ions within the membrane
 Selective reactivity with the analyte-the capability of the
membrane selectively binding the analyte ion from the matrix
using:
ion-exchange
Crystalization
 complexation binding technique

6. Potential of the cell
James W. Robinson, Eileen M. Skelly Frame, George M. Frame II
UNDERGRADUATE INSTRUMENTAL ANALYSIS, page 936,6 edition, 2005

7. Most important reference electrodes
 calomel electrode
 Ag/AgCl electrode

8.  Standard hydrogen electrode

9. Chemical composition and working principle of
each ion selective electrode
 Glass membrane electrodes
• Typical electrode system for measuring pH: (a) A glass electrode (indicator)and
SCE (reference) immersed in a solution of unknown pH (b) Combination probe
consisting of an indicator glass electrode and a Silver/silver chloride reference
Ppt of Dr. Kathlia A. De Castro-Cruz school of chemical engineering and chemistry

10. • Cell representation of figure (a)

11. Glass Membrane Structure:
SiO4
2-
framework with charge balancing cations
- SiO2 72 %, Na2O 22 %, CaO 6 %

12. Specific in its response toward hydrogen ions up
to a pH of about 9
In aqueous solution, ion exchange reaction at
surface
H+
+ Na+
Glass-
H+
Glass-
+ Na+
•H+
carries current near surface
•Na+
carries current in interior
• Ca2+
carries no current (immobile)

13. •The concentration difference of H3O+
ion in the sides membrane
results in a potential across the glass membrane

14. Surface where more dissociation occurs becomes
negatively charge with respect to other surface
Boundary potential Eb = E1 - E2
Potential difference determined by
Eref 1 - SCE (constant)
 Eref 2 - Ag/AgCl (constant)
 Eb

15. Eb = E1 - E2 = 0.0592 loga1/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 Eref1and Eref2 are constant
Ecell = constant - 0.0592 pH

16. Interference in Glass Membrane Electrodes:
Sensitive to
H+
 alkali metal ions
Selectivity coefficients (kX/Y) measure sensitivity to
other ions
For k > 1
• ISE responds better to the interfering ion than to the
target ion
For k = 1
• ISE responds similarly to both ions
For k < 1
• ISE responds more selectively to ion of interest

17. • Eind = constant - 0.0592log(aH++ kNa/H× aNa+)
Glass Electrodes for Other Ions:
• Maximize kH/Nafor other ions by modifying glass
surface (usually adding Al2O3 or B2O3)
• Possible to make glass membrane electrodes for
Na+
, K+
, NH4
+
, Cs+
, Rb+
, Li+
, Ag+
...

18. Crystalline Membrane Electrodes:
• Usually ionic compound
• Single crystal
• Crushed powder, melted and formed
• Operation similar to glass membrane
LaF3(s) LaF2
+
(s) + F-
(sol)
Presence of F- analyte pushes equilibrium to left,
reduces +ve charge on electrode surface

19. Figure . Migration of F －
through LaF3 doped with EuF2. Because Eu2+
has less charge than
La3+
, 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

20. • The side of the membrane in contact with lower fluoride
ion concentration becomes positive with respect to other
surface.
• The charge difference produce potential difference
that is the measure of concentration difference of the two
fluoride solutions.
• Eind = L - 0.0592log aF-
= L + 0.0592 pF
.

21. Liquid Membrane Electrodes:
• Based on potential that develops across two
immiscible liquids with different affinities for
analyte
• Porous membrane used to separate liquids
Example: Calcium dialkyl phosphate insoluble
in water, but binds Ca2+
strongly

22. .

23. • Eb = E1 - E2 = 0.0592 / 2 log a1/a2
If a2 is constant
Eb = N+ 0.05922log a1
= N - 0.0592 / 2 pCa

24. Molecule Selective Electrodes:
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

26. • Analyte not in direct contact with either electrode –
dissolved
Mechanism:
in internal solution
CO2 (aq) + H2O H+
+ HCO3
-
• can use glass membrane electrode to sense pH

27. • Overall equation is

28. • activity of neutral unaffected by other ions
aCO2=[CO2]

29. Applications
Potentiometric measurements are often used in routine
analyses because they are simple to achieve and the
detection limit of the electrodes are on the order of 10−5
–10−6
M. However, the observed detection limit is governed by
the presence of interfering ions or impurities. Some routine
applications of the method are:
 in agriculture, the analysis of nitrates in soil samples
 in foodstuffs, the analysis of ions such as NO−3,
F−,
Br−
,Ca2+
,
etc. in drinks, milk, meat, or fruit juices
 in industry, the analyses of chlorides in paper paste,
cyanides in electrolysis baths, chlorides and fluorides in
galvanic processes
 in clinical chemistry, the analysis of certain ions in serum
and other biological fluids.

30. References
1.Douglas A. Skoog, et al, Instrumental analysis,2007 by
Brooks/Cole,a part of Cengage Learning.
2.Francis Rouessac and Annick Rouessac,Chemical Analysis,
Modern Instrumentation Methods and Techniques, Second
Edition, 2007 by John Wiley & Sons Ltd,
3.Daniel C. Harris, Quantitative Chemical Analysis, Eighth
Edition, 2010 by W. H. Freeman and Company.

31. .