Students will be able to undersatnd the concept of Reference Electroede. Working of calomel , Ion Selective and glass electrode.

1. Dr. Swastika Das
Professor, Chemistry
BLDEACET, Vijayapura

2. Reference electrodes:
 The electrodes of known potential are called Reference
electrodes
 The electrode potential of ref electrode is stable and
reproducible.
 Using ref electrodes, the potentials of other
electrodes can be measured.
 Two types: i. Primary. ii. Secondary.

3. Primary Reference Electrode:
Standard Hydrogen Electrode :
The Standard Hydrogen Electrode is often abbreviated to
SHE, and its standard electrode potential is declared to be
0 at a temperature of 298K. This is because it acts as a
reference for comparison with any other electrode.
 The redox half cell of the SHE is where the following
reaction takes place:
 2H+ (aq) + 2e– → H2 (g)
 The reaction given above generally takes place on
a platinum electrode. The pressure of the hydrogen gas
present in this half cell equals to 1 bar.

4. Standard Hydrogen Electrode:

5. Disadvantages of Primary electrode
 It is difficult to maintain H+ ion
concentration as 1M.
 It is difficult to maintain a flow of 1 Atm
of hydrogen at the interface of Pt
electrode.
 Impurities present in hydrogen flow may
poison the Pt electrode which alters the
potential value.

6. Advantages of secondary
electrodes :
 They have constant and stable potential
values at a particular temperature.
 They are calibrated with respect to SHE.
 They have the least potential gradient with
temperature.
 The potential values are reliable and
measurements can be made with great
accuracy.

7. Calomel Electrode

8. Construction :
 Calomel electrode is a metal-sparingly soluble salt electrode.
 The electrode consists of a narrow glass tube provided with two side
tubes.
 A little Hg is placed at the bottom of the glass tube
followed by a layer of Hg2Cl2 (mercurous chloride or
calomel) and Hg.
 The remaining part of the glass tube is filled with KCl solution of
definite concentration.
 A thin glass tube with a platinum wire is then inserted, taking care that
the platinum wire drips into mercury.
 Representation:
Hg| Hg2Cl2 |KCl (Sat)
Hence, three types of Calomel electrodes are available, viz.,
normal (1N), decinormal (0.1N) and saturated.

9. Working :
 The potential of the calomel electrode depends upon the concentration of the
chloride ions in solution. If the electrode is saturated calomel electrode (SCE),
some crystals of KCl are placed over the Hg−Hg2Cl2 paste so as to keep the
solution saturated.
Oxidation : If the electrode serves as anode, then half reaction that occurs on
it will be oxidation. Mercury is first oxidised to mercuric ions.
2Hg(l)⟶ Hg2
2+ + 2e−
 The chloride ions supplied by KCl solution combine with mercuric
ions [Hg2
2+] to form insoluble mercurous chloride.
 Thus,
Hg2
2+ +2Cl−⟶ Hg2Cl2(s)
Overall reaction is,
2Hg(l) + 2Cl - (aq)−⟶ Hg2Cl2(s)+2e−

10.  Reduction : If the electrode is cathode (+) in the galvanic
cell, the half reaction that occurs on it will be reduction :

Hg2Cl2(s)⟶ Hg2
2+ +2Cl−
 Hg2
2+ + 2e− ⟶ 2Hg(l)
Overall reaction:
Hg2Cl2(s)+2e−⟶2Hg(l) + 2Cl(aq)−
Equilibrium :
Hg2Cl2(s)+2e− ⇌ 2Hg(l) + 2Cl(aq)−
Nernst equation :
 E = E0 – 0.0591 log [Cl-]
 The potential of calomel electrode decreases with increase in the concentration of
chloride ions at a given temperature. Thus, electrode is reversible with respect to
concentration of chloride ions.
 [KCl] = 0.1 N 1.0 N Saturated
 E.P. = 0.3335 V 0.2810V 0.2422 V

11. Advantages & Disadvantages:
 Advantages:
 Easy construction
 Easy maintenance
 Reproducible and stable potential
Disadvantages:
Calomel electrode should not be used at above 50 0C
because the mercurous chloride breaks down yielding
unstable readings.
Since mercury is toxic this electrode is losing popularity.

12. Applications:
 Used as a reference electrode in pH meter.
 Used to measure potential of any other electrode.

13. Ion selective electrodes(ISE)
 Ion selective electrodes(ISE) – The membrane of ISE is sensitive to a
particular kind of ions in a solution.
- Ion exchange takes place between the membrane electrode and the
solution containing the specific ions and develops a potential, which
depends on the concentration of the particular ionic species.
- Selectively binds and measures the activity of one ion .
Examples : • (i) pH electrode or glass electrode
• (ii) Calcium (Ca2+) electrode
• Fluoride (F-) electrode
 An ion-selective electrode (ISE), also known as a specific ion electrode
(SIE), is a transducer (or sensor) that converts the activity of a specific
ion dissolved in a solution into an electrical potential, which can be
measured by a voltmeter or pH meter.

14.  An ideal I.S.E. consists of a thin membrane across
which only the intended ion can be transported.
 The transport of ions from a high conc. to a low
one through a selective binding with some sites
within the membrane creates a potential
difference.

15. How ISE work??
 Ion Selective Electrodes (ISE) , a specific ion electrode (SIE), is a
transducer (or sensor) that converts the activity of a specific ion
dissolved in a solution into an electrical potential, which can be
measured by a voltmeter or pH meter .
 Works on the theory that an electrode develops a potential due
to ion-exchange occurring between the sample and the inorganic
membrane .
 This potential is measured against a stable reference electrode of
constant potential .
 The potential difference between the two electrodes will depend
upon the activity of the specific ion in solution. This activity is
related to the concentration of that specific ion.

16. Components of ISE
 1. Ion selective electrode with membrane at the end –
allows ions of interest to pass, but excludes the passage
of the other ions
 2.Internal reference electrode – present within the ion
selective electrode which is made of silver wire coated
with solid silver chloride, embedded in concentrated
potassium chloride solution (filling solution) .
 3.An internal reference solution depending on the
nature of the analytic ion.

17.  The potential developed at the ion selective sensor is a
measure of the concentration of the ionic species of
interest.
 The electrodes cannot only detect but also measure the
concentration of a specific chemical species directly in
a chemical solution are referred to as ion selective
electrode.
 An ion selective membrane is the key component of all
potentiometric ion sensors. The voltage developed
between the sensing and the reference electrodes is a
measure of the concentration of the reactive ion being
measured. As the concentration of the reacting ion at
the sensing electrode varies, so does the voltage
measured between two electrodes.

18. TYPES OF ION SELECTIVE
ELECTRODE (ISE)
 TYPES OF ION SELECTIVE ELECTRODE (ISE)
 • Glass Membrane Electrode
 • Solid State Electrode
 • Liquid Membrane Electrode
 • Gas Sensing Electrode
GLASS MEMBRANE ELECTRODE :
Glass electrode are responsive to univalent cations ( H+ , Na+).
Glass membrane is manufactured from SiO2 glass matrix with
various chemicals. Inside the glass bulb, a dilute HCl solution is
used as imternal reference solution. A silver- silver chloride
electrode is used as internal reference electrode.
The electrode is immersed in the solution and pH to be
measured.

19. Applications:
 1. The most commonly used ISE is the pH probe.
 2. CATIONS: Ammonium (NH4 +), Barium (Ba++), Calcium
(Ca++), Cadmium (Cd++), Copper (Cu++), Lead (Pb++),
Mercury (Hg++), Potassium (K+), Sodium (Na+), Silver
(Ag+).
 3. ANIONS: Bromide (Br-), Chloride (Cl-), Cyanide (CN-)
Fluoride (F-), Iodide (I-), Nitrate (NO3 -), Nitrite (NO2 -),
Perchlorate (ClO4 -) Sulphide (S-), Thiocyanate (SCN-).
 4. Water hardness can be measured with Calcium ISE.
 5. Nitrate monitoring in water sample can be done with
Nitrate ISE
 6. Salinity and chloride monitoring in sea water
 7.Monitoring steams for ammonium.
 8. Determination of Calcium in milk.

20. Glass electrode:
 It is a pH sensitive electrode and is most widely used
for pH determination.
 It consists of a glass bulb made up of SiO2.
 The glass bulb is filled with a solution of constant pH
(0.1N) HCl.
 One Ag-AgCl electrode is used as internal reference
electrode.
 The glass electrode is dipped in a solution which
contains H+ ions or the solution of unknown pH.

21. A glass electrode and Ag-AgCl electrode
combination
Glass electrode used in
laboratories.

23. Electrode potential of glass electrode:
 The potential of glass electrode is given by the
following equation:
 Eglass = E0
glass + 0.0591/n log [H+].
=>E glass = E0
glass – 0.0591 pH.
Since, n=1 ; pH= -log [H+].

24. pH of unknown solution determination:
 The glass electrode is connected to a saturated calomel
electrode.
 Glass electrode is made anode and calomel electrode is made
cathode.
 The complete cell representation:
Ag-AgCl/HCl(0.1N)/glass/analyte solution//KCl(sat),Hg2cl2,Hg(l)
(pH=?)
When a thin glass membrane is placed between two solutions
of different pH values , a potential arises across the
membrane . The potential difference varies as the pH of these
two solutions vary. In practice, pH of internal solution is kept
constant and therefore the electrode potential depends on
the pH of the experimental solutions only.

25.  E cell = Ecathode – E Anode
Ecell = E Calomel – E glass
Ecell = 0.2422 – [E0
glass – 0.0591 pH]
Ecell - 0.2422 + E0
glass = 0.0591 pH.
=> pH = (Ecell - 0.2422 + E0
glass )/ 0.0591

26. Advantages :
 1. It can be used in oxidizing and reducing environment
 2. Electrode does not get poisoned.
 3. Accurate results can be obtained for the pH range 1-14.
Disadvantages :
 It doesn’t work properly in pure alcohol and in some organic
solvents.