Engineering Chemistry S Datta

ELECTROCHEMISTRY 219
Conductance
Volume of AgNO 3
Fig. 9.10 (c) Titration curve of KCl solution by AgNO 3
solution.
Titration of a very weak acid with strong base or a very weak base with strong acids
results in a small initial conductance. The conductance increases as the concentration of the
salt increases. In the region of the equivalence point, due to hydrolysis the titration curve
shows a marked curvature (Fig. 9.10(b)). The equivalence point is located by extrapolating the
straight lines.
SOLVED EXAMPLES
Example 1. A conductivity cell contains two electrodes of area of cross-section 1.25 cm 2 .
They are 10.5 cm. apart. If the cell is filled with an electrolyte and offers a resistance of 1996 ohm,
calculate the cell constant and specific conductivity.
Sol. Cell constant = l a = 10.5
1.25
= 8.4 cm–1
Observed conductivity
1
=
1996 ohm–1 .
Specific conductivity = Cell constant × observed conductivity.
1
= 8.4 ×
1996 = 8.4 × 1
1996
= 4.2 × 10 –3 ohm –1 cm –1 .
Example 2. Specific conductivity of an N 50 KCl solution at 25°C is 0.0002765 mhos cm–1 .
If the resistance of cell containing this solution is 500 ohms, what is the cell constant?
Sol.
Cell constant = l a
and
R = ρ l a
or
l
a = R ρ = R × 1 ρ = R × K
∴
l
a
= 500 × 0.0002765 cm–1
= 0.138 cm –1 .