84
2 – Methods and techniques
2. The displacement of the interface at the anode is due to the disappearance
of the quantity n + of substance in the form of the species MX, which in turn
requires the species M
z+
to transport the following quantity of charge:
Q + = t c Iτ
Q + may also be expressed as Q + = n + z F
We arrive at
t
zF I
n
c
τ
=
+
n + (or n MX ) can be expressed as a function of the density ρ and the molar
mass M of compound MX
n
M
m
M
S x
MX
MX
ρ
Δ
=
=
#
#
The final expression for the cationic transport number M
z+
in MX is
t
zF M
I
S x
c
ρ
τ
Δ
=
#
#
#
#
3. a. To liberate gaseous iodine, we must ensure that the temperature used
exceeds the sublimation temperature of iodine I 2 under atmospheric
pressure.
b. The following relation should be applied:
t
F M
I
S x
c
ρ
τ
Δ
= #
#
#
#
by using I # τ = Q, we obtain
t
358.13 Q
x
c
Δ
=
#
Δx is expressed in cm and Q in coulombs.
Applying this to the two temperatures under consideration gives
T [°C]
250
300
t c
0.977
1.005
c. The results confirm that, in α-AgI, the cationic transport number is practically equal to unity.
Solution 2.6 – Determination of cationic transport number in CaF 2
by dilatocoulometry
1. We must ensure that the oxygen partial pressure is sufficiently low to minimize the introduction of oxygen into the crystal. Under these conditions, the