Engineering Chemistry S Datta

258 ENGINEERING CHEMISTRY
Q. 11. What do you mean by potentiometry?
Ans. Potentiometry is an electrochemical method of analysis based on measuring the
potential difference (e.m.f.) between two half cells, one of which is an indicating electrode and
the other is a reference electrode.
Q. 12. What is an indicating electrode?
Ans. It is an electrode in balance with an redox couple, the potential of which is given by
Nernst equation.
Q. 13. Give the hemistry the different indicating electrodes.
Ans. (a) First Class ⇒ Metallic electrodes in contact with a solution of their own ion
(an electrode which is reversible in relation to the cation)
M n+ + ne – M; where, [M n+ ] ⇒ conc. of cation in moles dm –3
0.
059
+
n
∈ = ∈° n + log [ M ]
MM /
+
n
(b) Second class ⇒ Metallic electrodes in contact with one of their not readily soluble
salts and with the anion of this salt:
M | MA(s), A –n
where,
M ⇒ Metal
MA(s) ⇒ the salt which is not readily soluble.
[A –n ] = Conc. of the anion in moles dm –3 .
0.
059 0.
059
And, ∈ = ∈° + log K − K [ A−
n]
MM / n+ sp sp
n
n
where K sp
= solubility product of the salt which is not readily soluble.
(c) Redox = Electrodes which are inert in contact with a redox couple in solution.
Pt/Ox, Red
And, ∈ = ∈° Redox
+ 0. 059 log [Ox]/[Red]
n
Pt ⇒ Platinum
[Ox] = Conc. of the oxidised form in moles dm –3 .
[Red] = Conc. of the reduced form in moles dm –3 .
(d) Membrane type = Selective electrodes specific to certain ions. They consist of a
membrane which in presence of particular ions assumes a potential which is dependent on the
activity of the ions in relation to those of which the electrode is selective. The best known of
this type is glass electrode.
∈ = ∈ r
+ 0. 059 log a
n
where, ∈ r
⇒ Constant potential linked to the choice of the reference electrode and the internal
solution, a ⇒ activity of the ion in relation to which the electrode is selective.
Q. 14. What are reference electrodes?
Ans. These are the electrodes whose potential is constant and independent of the
composition of the contacting solution.
The most widely used are saturated calomel electrodes (∈ = + 0.246 V) and the silver-silver
chloride electrode (∈ = + 0.222 V).