Engineering Chemistry S Datta

242 ENGINEERING CHEMISTRY
∴
0.045
C 1
=
2.138 × 10 13 = 2.105 × 10 –15 mol l –1 .
∴ K sp
of AgI = [Ag + ] [I – ]
= 2.105 × 10 –15 × 0.045
= 9.472 × 10 –17
Solubility of AgI =
K sp
−
= 9. 472 × 10 17
= 9.732 × 10 –9 mol l –1
= 9.732 × 10 –9 × 143.5 g l –1
= 1.396 × 10 –6 gl –1
Determination of valency
Hg || Hg 2
(NO 3
) 2
0.5(N) || Hg 2
(NO 3
) 2
(0.05) (N)/Hg
C 2
C 1
The e.m.f. of the above cell = 0.029 V
We have, E = 0.059 C 2
log = 0.059 log 0.5
n C1
n 0.05
= 0.059 0.059
log 10 =
n
or 0.029 = 0.059
n
n
∴ n = 0.059
0.029 ≈ 2.
INDICATOR ELECTRODES
Hydrogen Electrode
It consists of a small piece of platinum foil electroplated
with platinum black, over which hydrogen gas is passed. The
platinum black surface exhibits a strong absorption power
towards hydrogen gas, as a result, the metal surface remains
in continuous contact with the hydrogen gas. The electrode
will act as if it were an electrode of metallic hydrogen. When
in use, therefore, only a part of the foil is immersed in 1(N)
HCl; the remainder is surrounded by pure hydrogen gas at
1 atm. pressure. This is standard hydrogen electrode. The
e.m.f. of this electrode has been arbitrarily assigned to be zero.
The pH of a solution can be determined with the
help of a hydrogen electrode in that case, the platinum
electrode is dipped into the solution whose pH is to be
determined, instead of 1(N) HCl and H 2
gas at 1 atm passed
over the platinum electrode.
The electrode reaction is
H + 1
(aq) + e H 2( g)
2
where n is valency.
H-Gas 2
HCl
Fig. 10.6 Hydrogen Electrode.