EXPERIMENT 9
CONDUCTOMETRIC TITRATIONS
07F
Measurement of the conductivity of a solution (generally aqueous) is perhaps the easiest
way to determine whether the compound is a strong electrolyte, a weak electrolyte, or a non-
electrolyte.
The conductivity (or conductance) of a solution,
L
, is defined as the reciprocal of the
electrical resistance, R, the directly measured quantity:
1
,
1
−
=
ohm
R
L
The resistance of a solution is proportional to the distance between the electrodes,
d
, and
inversely proportional to the size or cross sectional area of the electrodes,
A
. {Compare with the
electrical resistance of wire.} In addition, the resistance of a solution depends on the
concentrations of charge carriers in the solution and how fast they move in solution in an applied
electric field (voltage difference).
For a solution, the following equations hold
ρ
κ
1
1
1
=
⎟
⎠
⎞
⎜
⎝
⎛
=
⎟
⎠
⎞
⎜
⎝
⎛
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
=
=
⎟
⎠
⎞
⎜
⎝
⎛
=
d
A
d
A
R
L
A
d
R
In this equation,
ρ
is the specific resistance of the solution (which depends on the concentrations
and types of ions in solution and not the experimental apparatus) and
κ
is the specific
conductivity of the solution, which is also a function of the solution and not the apparatus.
For aqueous solutions of electrolytes, the specific conductivity (conductance) of the
solution,
κ
, increases with increasing concentration of ions in solution, and is very nearly
proportional to the concentration of ions (for low to moderate concentrations), as shown in the
figure below.
Conductivity vs. Concentration
y = 384.29x
y = 213.18x
y = 103.6x
0
10
20
30
40
50
60
70
80
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
Normality, Eq/L
Specific Conductivity, 10
-3
ohm
-1
HCl
NaOH
NaCl
Linear (HCl)
Linear (NaOH)
Linear (NaCl)
The plots above (data from International Critical Tables) show a nearly linear increase in
specific conductivity with increasing concentration for HCl, NaOH, and NaCl. However, the
1